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Posts Tagged ‘Roger’

Roger Cicala: Understanding field curvature for fun and profit

24 Dec
When deciding which 35mm lens to buy, what do you want to know? How sharp it is? How it handles? How much it costs? I want to know what its field curvature looks like. (Spoiler: the plot on the right is from the little guy.)

I’m not a fan of lens testing purely for the purpose of winning pissing contests. I am, however, a great fan of lens testing for learning how to best use a lens. There are a few tests I find particularly useful, and the single most important one is field curvature.

Field Curvature (in metrology speak MTF v Field v Focus) tells me a lot about how to use a lens. It is also the most complete way to test a lens because it’s three-dimensional. Shooting a brick wall or test chart the way most people do is 2-Dimensional. The 2-D chart test below says the lens is sharp in the center and soft at the edges. How nice.

This is a test image of a lens’ MTF, basically what you’d see shooting a test chart except with color representing sharpness. This one is sharp in the center, really soft at the edges, and a tiny bit softer on one side than the other. But what does that tell you about the lens? Not much.

I’ve spent over a decade developing fast, sensitive optical tests. My gold standard is a modified $ 250,000 optical bench that quickly analyzes field curvature. That test (the graph below) tells me that this lens is actually amazingly sharp at the edges, but that field curvature causes the area of maximum sharpness to be further forward at the edges than at the center. In a 2-D test, the edges look soft because they are out of focus when the center is in focus.

A 3-D (Field curvature) MTF graph. The center focus is along the black horizontal line. The Y axis represents focusing distance, the X axis edge-to-edge sharpness, and the MTF is the color (red is sharpest). So the edges of the lens are very sharp, but not at the same focusing distance as the center.

The 2-D test chart images, like the first graph, are taken right along the black line of best center focus. They show the center is sharp and the edges soft. The 3-D graph shows, the edges are very sharp, but not in the same plane of focus as the center. That’s very, very different than the edges are soft.

Think about that for a second. Photographer #1 gets that lens, knows how to frame with it, and posts about how awesomely sharp the edges are in his photographs, which are 3-D. Photographer #2 buys it, tests it on a 2-D chart and sees the edges suck so he sends it back because it’s supposed to have sharp edges. Again and again.

Inexperienced photographers think a curved field is bad and a flat field good. But a designer may have chosen to let the field curve so the lens has other, wonderful attributes. Not to mention a curved field is a tool that can be useful. Many great portrait lenses are great portrait lenses because of their curved field, for example.

knowing your lens’s field curvature will help you take better pictures

I showed how to check field curvature with just a photo in a previous post. Today I’ll show a slightly different method using a test chart or brick wall. But field curvature isn’t really about better testing; knowing your lens’s field curvature will help you take better pictures.

Take the lens above as an example. I saw a group photo taken with that lens. The photographer positioned everyone in a slight crescent rather than a line because he knew the lens’ field curvature and placed his subjects so they were all in best focus. Someone else (someone without that information) would probably have said the lens was ‘too soft at the edges’ to use in a group shot.

Field Curvature graphs (clockwise from top left) showing overall curvature (this lens doesn’t have much); astigmatism, tangential field, and sagittal field.

A Quick Word About the Graphs

The shape of the field is different for sagittal and tangential rays (the two lower graphs above), which many people don’t realize. Where the fields don’t overlap, there is astigmatism (upper right graph above). The overall curvature (upper left) is what you see at home if you do my not-patented ‘field of grass’ test. Most of the time I’ll just show the sagittal and tangential fields; you can eyeball whether they overlap or not and what the overall curvature would be like.

Testing Field Curvature at Home

If you follow my grass-photo-with-find-edges-filter technique, you get a nice image showing the field curvature. You’ll also know if the field is tilted and if it is, how badly. Here’s the grass test for two copies of the Sigma 24mm f.14 Art, a lens with a bit of field curvature. One copy has tilt problems and it’s pretty easy to see which one.

One copy is good, one is pretty tilted. Can you tell which one is which? I thought that you could. This two-copy test took 60 seconds, was shot hand held, and required no home testing lab.

If you’ve already got a home testing setup and want to put some numbers to your lens, that’s easy, too. First, mount the lens on a tripod and manually focus on your 2-D target of choice: test target, brick wall, treeline, whatever. (If you don’t use a tripod and you don’t manually focus, you should be filled with shame and delete all your test posts because you did NOT test the lens. I never, ever, take a single AF image of a test chart. It’s a waste of time. But you can do the find-edges technique with a hand-held AF shot even if you don’t own a tripod and don’t know how to manually focus.)

Where was I before the rant? Oh, yeah. Take your first image past (distant) to best center focus, then take a series of 6-10 images while manually moving the focus back a bit after each shot until you’ve gone out of focus to the near side.

I never, ever, take a single image of a test chart – it’s a waste of time

Next, you take that set of six or 10 through-focused images, find the one with best center sharpness, the one with best right edge sharpness, and the one with best left edge sharpness. If they are all the same image (it happens sometimes), congratulations – you have a very good lens with a flat field. Most of the time, though, you will get one of three other possibilities:

  • Both edges are sharpest in the same image, and the center is sharpest in another. Which means: The field is curved but not tilted.
  • The edges are sharpest in different images: The field is tilted.
  • One edge never gets as sharp as the other: The lens is optically abnormal.

For example, let’s say you take six images. Images #1 and #6 from the sequence shown below were way out of focus, so I’m only showing you images #2-5. The center is sharpest in image #3, the right edge sharpest in #4, and the left in #5.

What this tells me is that I’ve got a lens with a field that is both curved towards the camera and tilted to the left.

Taking a series of images from far focus (2) through near focus (5) lets you evaluate field curvature and tilt.

Let’s all take just a moment to think about all those threads that started with someone posting just image #3 and asking “do you think this lens is OK??” You’ll see 57 or so responses with no definitive conclusion because the OP didn’t give enough information from which to draw a proper conclusion. If they had done a through-focus test, they probably wouldn’t need to ask the question; the answer would be obvious.

Why Should I Bother?

If the field is badly tilted (scroll back up to the first grass images) you’ll know to exchange it for another copy, or if a little tilted you’ll have that information for framing your shots. I had a favorite landscape lens which had a field that was slightly curved and slightly tilted. It gave me great images, usually with a subject of interest closer and on the left side in sharp focus. It was a great lens for me because I knew how to frame my shots with it and I liked the different look that gave.

If the field is markedly curved, you can use that knowledge to better frame your shots. Or perhaps you’ll decide that this lens isn’t for you. Personally, I often prefer a curved field because it’s a tool I can use, but some people want flat fields all the time. I might choose one lens over another for certain shots because of the field curvature. That lens I showed at the beginning is going to focus the edges closer than the center, for example. It might be great for isolating the subject for center-framed portraits. Or to frame shots so the center point of interest is further away than the edge points of interest. I would prefer a different lens with a flatter field for an architectural shot. You might prefer flat fields for all of your shots, for that matter. I find field curvature a fun tool, but some people are flat lensers.

As an alternative, if the field is really curved, focusing slightly away from center gives an overall sharper image. Here’s an example. The Zeiss 50mm T/1.5 has big-time curvature with the edges towards the camera as shown in the top-half of the image below.

Field curvature of the Zeiss 50mm T1.5 showing that if you place the focus point to the left or right of center you get maximum edge-to-edge sharpness. The calculations show the best off-axis point is 9mm from center (about halfway to the edge) but you could eyeball this pretty accurately.

I love a curved field for just this reason. Center focus can isolate my subject but off-axis focus brings good edge-to-edge sharpness. I get to choose. I love getting to choose.

I have some cool software (bottom half of the image) that tells me exactly where to focus to get the best edge-to-edge sharpness (the black line across the field curvature graph) but you can eyeball your homemade field curvature graph and know where it should go – about halfway to the edge in this case. This can serve as an alternative to stopping down for edge-to-edge sharpness, or let you get edge-to-edge sharpness when stopping down isn’t enough.

The big takeaway is you can often get excellent get excellent edge sharpness in lenses with field curvatures if you know how to use them. Many lenses with flatter fields sacrifice edge sharpness to get flat fields, and you can’t find edge sharpness that just isn’t there.

Do you know the focal length at which your zoom lens has the flattest field, or at which focal length the field curvature changes? That’s useful information, and I want to know this kind of thing for every zoom I carry (pro tip, the flattest field is rarely at the center of the zoom range; it’s often 1/3 of the way from one extreme). Some zooms have massive curve at an extreme, but if you zoom just a few mm away from the extreme the field is much flatter. That’s another useful thing to know.

Very often your 24-70mm is curved one way at 70, while your 70-200mm is curved the another (ditto at 24mm, etc.). Knowing that helps choose which lens best frames the shot. (I should also mention that one zoom is always sharper than the other at 70mm. Of course, I probably should also mention neither one is really 70mm. Most 24-70mm lenses are actually 26-67mm; most 70-200s are about 73mm to 190mm.)

Sagittal field of Canon 70-200mm f/2.8 L III and24-70mm f/4 L IS, both set at 70mm.

The 70-200mm has a very slight curve back towards the camera and is pretty sharp (red) even at the edges at 70mm. The 24-70mm has a more significant curve and is not as sharp at the edges. Depending on what you are shooting, those differences could be important.

At least a few of you, I hope, have read this far and are now interested in field curvature. This article is already long enough, so I’ll stop here for today. For the next article though, I’ll show example field curvatures from various kinds of lenses. To be clear, I’m not going to put out 6,342 field curvature graphs for all the lenses at all the focal lengths. I’m showing you how to fish, not hosting a fish fry.

Since everyone tells me I should click-bait tease the next article, here you go: Next time I’ll show how field curvature explains ‘3-D pop’ and ‘microcontrast’. (Spoiler: No, no I won’t. Field curvature explains a lot of things and is a useful tool, but it’s not magic.)

Until Next Time…

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Roger Cicala: the difference between sample variation and a ‘bad copy’ (Part 2)

13 Nov
I compare a lot of lenses. They aren’t all exactly the same.

In today’s article we’ll look at variation versus bad copies a bit differently to last time. Plus, I’ll explain how people get three ‘bad copies’ of a lens in a row.

Variation versus bad copy frequency

Imatest type graphs are easier to visualize so I’m going to use those today. These graphs allow us to visualize center resolution (toward the top on the y-axis of the graph) and overall resolution (toward the right on the x-axis), with individual lenses plotted as dots. Don’t worry about the numbers on the X and Y axes, all you need to know is that the sharpest lenses are plotted up and to the right, and the softest are lower and to the left.

The graph below shows plots from multiple copies of two prime lenses. Let’s call them ‘Red’ and ‘Green’. The Green lens is a fairly expensive, pro-grade optic. The Red lens is a cheaper, consumer-level prime. You’ll see that there’s one copy of each in roughly the middle of this graph, away from the main cluster at upper-right. I’d return both of these samples to the manufacturer. So would you – they’re awful.

Multiple copies of two lenses, the ‘Red’ lens and the ‘Green’ lens, plotted by center and overall sharpness. Two bad copies of each are obvious at the lower left.

But could you tell the difference between the best and the worst of the other copies, in that big cluster at upper-right? That would depend on the resolution of your camera, how carefully you pixel-peeped, which lens we are talking about, and honestly, how much you cared.

The Green lens shows less variation, which is about what we expect (but don’t always get) from a fairly expensive, high-quality lens. A perfectionist with a high resolution camera, some testing skill and enough time could tell the top third from the bottom third, but it would take effort.

The Red lens has more variation, which is typical for a consumer-grade lens. A reasonably picky photographer could tell the difference from the top third and the bottom third. None of the bottom third are awful; they’re a little fuzzier, a little more tilted, not quite as good when viewed at 100% magnification, and you might see issues if you made a large print.

With more variation, you get more ‘not as good’ lenses, but they’re still not ‘bad copies’

If you look carefully, though, the top third of the Green and Red samples are about the same. With more variation, you get more ‘not as good’ lenses, but they’re still clearly not ‘bad copies’; they’re just ‘not quite as good’ copies.

So why would we argue about these two lenses on the Internet? Because based on a graph like this, a lot of testing sites might say “Red is as good as Green and costs a lot less.” The truth is simply that the Red lens has more variation. Sure – a good copy of the Red lens might match a good copy of the Green lens. But you’re not guaranteed to get one.

A word about that yellow line and worse variation

There’s obviously a point when large variation means the lower end of the ‘acceptable group’ is unacceptable. Where that line lies is of course arbitrary, so I put an arbitrary yellow line in the graph above, to illustrate the point. Where the yellow line is for you depends on your expectations and your requirements.

The Subjective Quality Factor can theoretically decide when the low end of variation is not OK, and it can be used as a guide to where to place the yellow line. The key words, though, are ‘subjective quality’. Things like print size, camera resolution, even subject matter are variables when it comes to deciding when SQF is not OK. For example, the SQF needed for online display or 4K video is a lot lower than for a 24″ print of a detailed landscape taken with a 40 megapixel camera.

Every one of us has our own SQF; call it your PQF (Personal Quality Factor) and your yellow line might be higher or lower than the one in the graph above. Manufacturers have a Manufacturer’s Quality Factor (MQF) for each of their lenses, which is the famous ‘in spec’.

When your PQF is higher than the MQF, those lower lenses are not OK for you. They might be fine for someone else. Wherever a person’s yellow line is, that’s their demarkation line. These days, if they get a lens below the line, they go on an Internet rant. So now, as promised, I have explained the cause of 8.2% of Ranting On Online Forums (ROOFing). It’s the difference between MQF and PQF.

Put another way, it’s the difference between expectations and reality.

If you test a set of $ 5,000 lenses carefully enough, you may find some differences in image quality. The technical term for this phenomenon is ‘reality’.

It should be pretty obvious that people could screen three or four copies of the Red lens and end up with a copy that’s as good as any Green lens. I don’t find it worth my time, but I’m not judging; testing lenses is what I do.

Unfortunately, though, people don’t post online “I was willing to spend a lot of time to save some money, so I spent 20 hours comparing three copies and got a really good Red lens.” They say “I went through three bad copies before I got a good one.”

The frequency of bad copies and variation

Just so we get it out of the way, the actual, genuine ‘bad copy’ rate is way lower than I showed in the graph above. For high-quality lenses it’s about 1% out-of-the-box. This explains why I roll my eyes every time I hear “I’ve owned 14 Wonderbar lenses and they’re all perfect.” Statistics suggest you’d need to buy over 50 lenses to get a single bad one. The worst lenses we’ve ever seen have a bad copy rate of maybe 3% so even then, the chances are good you wouldn’t get a bad one out of 14.

Most of these ‘those lenses suck / I’ve never had a bad copy’ arguments are just a different way of saying ‘I have different standards than you’

What about the forum warrior ROOFing about getting several bad copies in a row? He’s probably screening his way through sample variation looking for a better than average copy. If he exchanges it, there’s a good chance he won’t get a better one, but after two or three, he’ll get a good one. So he’s really saying “I had to try three copies to find one that was better than average.” Or close to average. Something like that.

Semantics are important. Most of these “those lenses suck / I’ve never had a bad copy” arguments are just a different way of saying “I have different standards than you”. I get asked all the time what happens to the two lenses John Doe returned when he kept the third? Well, they got re-sold, and the new owners are probably happy with them.

Why are there actual bad copies?

In short – inadequate testing. Most photographers greatly overestimate the amount and quality of testing that’s actually done at the factory, particularly at the end of the assembly line.

Many companies use a test target of thick bars to set AF and give a cursory pass-fail evaluation. A target of thick bars is low-resolution; equivalent to the 10 lp/mm on an MTF bench. Some use a 20 lp/mm target to test, and 20 is higher than 10, so that’s good. The trouble is that most modern sensors with a good lens can resolve 50 lp/mm easily. This is what I mean when I say (as I do often) that you and your camera are testing to a higher standard than most manufacturers.

Why is there high variation?

Usually, it’s the manufacturer’s choice, and usually for cost reasons. Occasionally it’s because the manufacturer is living on the cutting edge of technology. I know of a couple cases where a lens had high variation because the manufacturer wanted it to be spectacularly good. They designed-in tolerances that turned out to be too tight to practically produce, but convinced themselves they could produce it. Lenses like this tend to deliver amazing test results, but then attract a whole lot of complaints from some owners and a whole lot of love from others.

What’s that? You want some examples?

This is not the bookcase mentioned below; that one is under nondisclosure. This is my bookcase. My bookcase has better optical books.

Service center testing

Years ago, we had in our possession a $ 4,000 lens that was simply optically bad. It went to the service center twice with no improvement. Finally, the manufacturer insisted I send ‘my’ camera overseas with it for adjusting. The lens and camera came back six weeks later. The lens was no better, but the camera contained a memory card with 27 pictures on it. Those pictures were of a bookshelf full of books, and each image was slightly different as the technician took test shots while they optically adjusted the lens.

This, my friends, is why we decided to start adjusting lenses ourselves. And yes – after offering to share those bookshelf images – I was eventually sent a replacement lens.

Non-adjustable lenses

Many lenses have no optical adjustments. They’re assembled, and then what you get is what you get. If in-factory QC detects a really bad one, it might be disassembled and the parts reused, in the hope that random reassortment gives a better result next time. Or it may just get thrown away; the cost of disassembling and reassembling may be greater than the saved parts.

A common type of non-adjustable lens called a stacked lens; ‘element – spacer – element – spacer, etc’ with a front and rear retaining ring holding everything together. The usual method of correcting it is to loosen the retaining rings, bang the lens on a table a few times, and tighten it back up. That probably sounds ridiculously crude, but it sometimes works.

Many fully manual lenses (not those made by Zeiss or Leica) are non-adjustable, as are some less expensive manufacturer and third-party lenses.

Minimally-adjustable lenses

A number of prime lenses have only one or two adjustable elements. This is not necessarily a bad thing; adjusting one or two elements is a lot easier than adjusting six, so the technician is more likely to get things right.

One of my favorite lenses, both to shoot with and to adjust, is the venerable Zeiss 21mm F2.8 Distagon / Milvus. The front element of this lens is adjustable for centering and we’ve done hundreds of these adjustments over the years. The fun part is doing this adjustment lets you choose what type of lens you want. You can have razor sharp in the center with soft corners or you can let the center be a little softer and the corners much sharper. It’s a great example of adjustment being a trade-off, even for relatively simple adjustments.

MTF graphs of a Zeiss 21mm F2.8 Distagon, adjusted for best center sharpness (above), and optimal edge sharpness (below).

Consumer-grade zoom lenses (manufacturer or third-party) and prime lenses with apertures smaller than F1.4 tend to be minimally or non-adjustable. A fair number of better zooms and primes are minimally adjustable, too.

Lenses with many adjustable elements

More adjustments means less variation, at least in theory. It also, however, means when something is wrong it’s far more complex and time consuming to get the adjustments right. Time, as they say, is money and complex lenses can be rather hard to adjust.

I think the most we’ve seen is nine adjustable elements. These are usually top-of the line zooms, but we’ve seen six adjustable elements in some top-end primes. That’s something we never saw even five or six years ago.

So, what’s the key takeaway?

Let’s start with my definitions. A bad copy of a lens has one or more elements so out of adjustment that its images are obviously bad at a glance. Such a lens (assuming it is optically adjustable) can usually be made as good as the rest.

Variance, on the other hand, means some lenses aren’t as good as others, usually as a result of a number of small imperfections. A simple optical adjustment isn’t likely to make them as good as average. All lenses have a little variance. Some have more. A few have a lot. How much is too much depends on the photographer who’s shooting with them.

The Canon 70-200mm F2.8 RF has (give or take one, I’m not certain I recall all of them) 8 or 9 different adjustable elements.

Reducing variation costs money. The reality is the manufacturers are doing what works for them (or at least they think they are). There is a place for $ 500 lenses with higher variation and good image quality, just like there’s a market for $ 2,000 lenses with better image quality and less variation.

Roger


Roger Cicala is the founder of Lensrentals.com. He started by writing about the history of photography a decade ago, but now mostly writes about the testing, construction and repair of lenses and cameras. He follows Josh Billings’ philosophy: “It’s better to know nothing than to know what ain’t so.”

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Roger Cicala: the difference between sample variation and a ‘bad copy’ (Part 1)

03 Nov
We fix a lot of lenses, but not all lenses can be fixed.

With the next two posts, I hope to end the seventh most common forum war; the ‘lens variation is a big problem!’ vs ‘I don’t believe it exists!’ argument. Like a lot of forum wars, it comes down to semantics: Variation and bad copies aren’t the same thing (actually they’re not really related at all), but people tend to use the terms interchangeably.

Even $ 2,000 lenses must have variation

Note that I said ‘must’. I didn’t say ‘might’ or ‘could’. I certainly didn’t say ‘shouldn’t at this price’. If you expect every copy of a lens to be perfect, then a dose of reality is in order – unreasonable expectations are a down payment on disappointment.

The key point is what amount of variation is acceptable.

Of course, I define ‘unacceptable’ by my standards. My standards are probably similar to 90% of your standards (and they’re higher than most manufacturer’s standards). A few of you will consider my standards either too low or too high. That’s reasonable. You and I might be looking at the same lens, but we’re doing different things with it, and probably doing them on different cameras. Later on, we’ll talk about the difference between ‘acceptable variation’ and a genuinely bad copy that I would consider unacceptable.

Why lenses must vary

Any manufactured part, from a washer on your kitchen faucet to a component in the Hubble telescope has some variation. Generally (up to a certain point – limited by the state of the technology) you can lower the variation of a part if you are willing to pay more. Why? Because entirely new machines or manufacturing processes may be required, and all of that costs money.

But just ordering more units means you can save money, right? Well yes – in very general terms, ordering larger quantities lowers per-unit costs, but in a fairly linear fashion. Doubling your order of something usually reduces the per-unit cost by some percentage, but certainly not by half. There is never a point where if you order a large enough quantity of an item you get it for free.

This is a 15 cm diameter, 1/10 wavelength optical flat, eyeglasses for scale.

As an example, we use optical flats to calibrate our test benches. The flats come in different accuracies: 1/4 , 1/10, or 1/20 wavelength of flatness. All of those are very flat indeed, and those accuracies cost $ 800, $ 2,200, and $ 3,800 respectively. There is no quantity I could buy that would let me get the 1/20 wavelength plates for the 1/4 wavelength price. And I can’t get 1/40 wavelength of flatness at any price. The technology simply isn’t available.

What varies in a lens? Everything. The screws, helicoids, plates, and spacers vary. Every glass melt is very slightly different, giving elements a very slightly different refractive index. Lens grinding introduces variation, as does the coating process. Even the shims that we use to adjust variance, they vary. And shims don’t come in infinite thicknesses, so if your thinnest shim is 0.01mm then +/- 0.01mm is your maximum attainable accuracy.

What can manufacturers do about this?

The first thing is tolerancing the design. Optical programs let the designers punch in various tolerances for parts, showing how a given variation will affect the overall performance of the lens. For the sake of argument, let’s say that one particular glass element is very critical and even a slight variation makes a big difference in how the lens resolves, while variation among other elements matters less. The manufacturer can pay to have that critical element made more accurately. They can also change the design to make the part less critical, but often only by sacrificing performance.

In addition, manufacturers can (notice I said ‘can’, not ‘always do’) place compensating elements in the lens, allowing for slight adjustments in tilt, spacing, and centering. Emphasis is on ‘compensating’, though: These adjustments compensate for the inevitable errors that accumulate in any manufactured device. They are not called ‘adjusted for absolute perfection’ elements.

The two most common types of lens adjustments: shims and eccentric collars.

Not all lenses are equally adjustable. Some modern lenses may have five to eight different adjustable elements. Many have two or three. A fair number have none at all; what you get is what you get. Here’s a thought experiment for you: imagine you’re an optical engineer and you’ve been tasked with making an inexpensive lens. Knowing that adjustable elements are an expensive thing to put in a lens, what would you do?

I want to emphasize that optical adjustments in a modern lens are not there so that the lens can be tweaked to perfection; the adjustments are compensatory. There are trade-offs. Imagine you’re a technician working on a lens. You can correct the tilt on this element, but maybe that messes up the spacing here. Correcting the spacing issue changes centering there. Correcting the centering messes up tilt again. Eventually, in this hypothetical case, after a lot of back-and-forth you would arrive at a combination of trade-offs; you made the tilt a lot better, but not perfect. That’s the best compromise you can get.

Because many people think of distributions as the classic ‘bell curve’ or ‘normal distribution’ let’s get that particular wrongness out of the way. If you evaluate a group of lenses for resolution and graph the results it does NOT come out to be a normal distribution with a nice bell curve.

Frequency graph of two lenses. For those of you tired of reading already, this graph sums up the rest of the article. The black lens is going to have more variation than the green one. Neither the black nor green graphs are at zero over there on the softest end, bad copies happen to either one, but not frequently.

As common sense tells you it should be, lenses have a very skewed distribution. No lens is manufactured better than the perfection of theoretical design. Most come out fairly close to this theoretic perfection, and some a little less close. Some lenses are fairly tightly grouped around the sharpest area like the green curve in the graph above, others more spread out, like the black one. The big takeaway from that is you can’t say things like ‘95% of copies will be within 2 standard deviations of the mean.’

The Math of Variation

Don’t freak out, it’s not hard math and there’s no test. Plus, it has real world implications; it will explain why there’s a difference between ‘expected variation – up to spec’ and ‘unacceptable copy – out of spec’.

There are several ways to look at the math but the Root Sum Square method is the one I find easiest to understand: you square all the errors of whatever type you’re considering, add all the squares together, then take the square root of the total.

The total gives you an idea of how far off from the perfect, theoretical design a given lens is. Let’s use a simple example, a hypothetical lens with ten elements and we’ll just look at the spacing of each element in nm. (If you want to skip the math, the summary is in bold words a couple of paragraphs down.)

If we say each element has a 2 micron variation, then the formula is ?10 X 22 = 6.32. If I make a sloppier lens, say each element varies by 3 microns, then ?10 X 32 = 9.48. Nothing dramatic here, looser control of variation makes higher root sum square.

The important thing happens if everything isn’t smooth and even. Instead of 10 elements worse by 1 micron, let’s make 1 element worse by 10 microns. I’ll do the math in two steps:

? (9 X 22) + (1 X 102) = ? (36 + 100) = ?136 = 11.66

The summary is this: If you vary one element a lot you get a huge increase in root sum square. If you spread that same total variation over several elements, you get only a moderate increase in root sum square. That is basically the difference between a bad copy and higher variation.

If you have just one really bad element the performance of the lens goes all to hell

The math reflects what we see in the real world. If you let all the elements in a lens vary a little bit, some copies are a little softer than others. Pixel peepers might tell, but most people won’t care. But if you have one really bad element (it can be more than one, but one is enough) the performance of the lens goes all to hell and you’re looking at a bad copy that nobody wants.

More real world: if one element is way out of wack, we can usually find it and fix it. If ten elements are a little bit out, not so much. In fact, trying to make it better usually makes it worse. (I know this from a lot of painful experience.)

What does this look like in the lab?

If you want to look at what I do when I set standards, here are the MTF graphs of multiple copies of two different 35mm F1.4 lenses. The dotted lines show the mean of all the samples; these are the numbers I give you when I publish the MTF of a lens. The colored area shows the range of acceptability. If the actual MTF of a lens falls within that range, it meets my standards.

Mean (lines) and range (area) for two 35mm lenses. The mean is pretty similar, but the lens on the right has more variation.

For those of you who noted the number of samples, 15 samples means 60 test runs, since each lens is tested at four rotations. The calculations for variation range include things about how much a lens varies itself (how different is the right upper quadrant from the left lower, etc.) as well as how much lenses vary between themselves and some other stuff that’s beyond the scope of this article.

So, in my lab, once we get these numbers we test all lenses over and over. If it falls in the expected range, it meets our standards. The range is variation; it’s what is basically inevitable for multiple copies of that lens. You can tell me I should only keep the ones that are above average if you want. Think about that for a bit, before you say it in the comments, though.

The math suggests a bad copy, one with something really out of whack, doesn’t fall in the range. That’s correct and usually it’s not even close. When a lens doesn’t make it, it REALLY doesn’t make it.

A copy that obviously doesn’t meet standards. The vast majority of the time, one of these can be adjusted to return to expected range.

We took that copy above, optically adjusted it, and afterwards it was right back in the expected range. So an out-of-spec copy can be fixed and brought back into range; we do that several times every day.

But we can’t optically adjust a lens that’s in the lower 1/3 of the range and put it into the upper 1/3, at least not often. Trust me, we’ve tried. That makes sense; if one thing is way out of line we can put it back. If a dozen things are a tiny bit out of line, well, not so much.

I know what you’re thinking

You’re thinking, ‘Roger, you’re obviously geeking out on this stuff, but does it make one damned bit of difference to me, a real photographer who gives zero shirts about your lab stuff? I want to see something real world’. OK, fine. here you go.

A Nikon 70-200mm F2.8 VR II lens is a really good lens with very low (for a zoom) variation. But if you drop it just right, the 9th element can actually pop out of its molded plastic holder a tiny bit without causing any obvious external damage. It doesn’t happen very often, but when it does, it always pops out about 0.5mm, which, in optical terms, is a huge amount. This is the ‘one bad element’ scenario outlined in our mathematical experiment earlier.

Below are images of the element popped out (left) and popped back in (right) and below each image is the picture taken by the lens in that condition. Any questions?

On top you see the 9th element ‘popped out’ (left) and replaced (right). Below each is the picture of a test chart made with the lens in that condition.

So, what did we learn today?

We learned that variation among lenses is not the same thing as ‘good’ and ‘bad’ copies. Some of you who’ve read my stuff for a long time might remember I used to put out a Variation Number on those graphs, but I stopped doing that years ago, because people kept assuming that the higher the variation, the higher their chances were of getting a bad copy, which isn’t true. You see, bad copies are – well, bad. Variation just causes slight differences.

I’m going to do a part II that will go into detail with examples about how much you should expect lenses to vary, what the difference is between variation and a genuinely bad copy, and why some people act like jerks on forums. Well, maybe just the first two.

As a bonus, I will tell you the horrifying story of how manufacturers optically adjust a lens that’s really not optically adjustable. And for a double bonus I will show how variation means that there are actually two versions of the classic Zeiss 21mm F2.8 Distagon.

In other words, if you struggled through this article, hopefully the next one will be enough fun that you think it’s worth it. Delayed gratification and all that…

Roger


Roger Cicala is the founder of Lensrentals.com. He started by writing about the history of photography a decade ago, but now mostly writes about the testing, construction and repair of lenses and cameras. He follows Josh Billings’ philosophy: “It’s better to know nothing than to know what ain’t so.”

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Roger Cicala: why I don’t use an MTF bench to test my own lenses

25 Oct
Roughly clockwise from left: 300mm collimator, laser transmission testing, lens test projector, Trioptics Imagemaster HR optical bench, spectrometry measurement. It might not look like much, but the total cost is similar to really nice house in a small city (or a decent house in big city).

I have a complete testing lab at my disposal: MTF benches, lens test projectors, spectrometers, lasers, an Imatest setup gathering dust in a back room; everything all the cool kids have. A lot of people assume I test the hell out of my own shiny new personal lenses after I buy them. (Yes, I buy my own stuff). I do test them, but not in the lab. I go out and take pictures with them.

It’s not because I’m such a great photographer that my practiced eye can tell more about the lens through photographs than any lab test could. I’m a mediocre photographer. Years ago I tried making a living as a photographer. I sold some prints once, made enough to pay for maybe half a lens, and after another six months without a sale I decided to explore other methods of supporting my extravagant lifestyle.

The lab is faster, gives tons of information, and makes cool graphs. But I still don’t use it to test my personal lenses

It’s not because the lab stuff doesn’t give useful information. The lab gives a LOT of useful information. Most people don’t have time to learn how to interpret it, or learn its value and limitations, but it’s useful information nonetheless. And the lab is fast; I can test a lens about 32 different ways in a couple of hours. My ‘test a lens with photography’ time is a half a day or more. So the lab is faster, gives tons of information, and makes cool graphs. But I still don’t use it to test my personal lenses.

Lab tests give a ton of precise information. Understanding and interpreting it is, I’ll admit, not completely intuitive.

That’s because all lab tests have some major limitations. The biggest one is this: real images are 3-dimensional, they are focused at a variety of distances, and almost always contain foregrounds and backgrounds. Optical tests are two-dimensional slices taken at a fixed focusing distance with no background or foreground. The focusing distance is infinity for an optical bench. It’s a single, close distance for Imatest / DxO / and other computer image analysis methods.

So, the lab tests tell me everything I want to know about the plane of exact best focus at one focusing distance. That’s really useful information, especially if you want to find out if a lens is optically maladjusted, want to know what kind of aberrations it has, or are interested in its maximum resolution. And it gives people numbers – the ammunition of choice in many a Forum War.

Even a three-dimensional standard comparison image, such as the kind that DPReview and other sites use, is basically limited to one focusing distance. That distance is different for different focal lengths but it’s always fairly close up. And, if it’s an indoor target, the depth of those targets is usually only a few feet at most; it’s not going to show you what the out of focus area 30 feet behind the image plane looks like.

What I actually do to test a new lens

Photographs give me far more information than the lab, even if it’s less exact. I don’t recommend brick wall or side-of-building photographs. Those are just 2-dimensional slices like the lab gives, but with more variables and less information. I want photographs of 3-dimensional subjects.

With the right background (I prefer a field or yard of grass) you can quickly compare resolution at a half-dozen focusing distances. Sure, some lenses are about the same at all distances, but many are not. No zoom lens is equally sharp at all focal lengths. My favorite grass field is a hill behind my office that slopes up away from me. I focus on the mower tracks and quickly get images at several focusing distances.

Simple grass slope image taken with a Canon 50mm F1.2 lens at F1.4.

Grass (or pebbles or concrete or all manner of things that make fairly uniform photographs filled with fine detail) are great for figuring out the zone of acceptable sharpness (for you) of a lens.

Repeating this set of images at several apertures lets me see at what aperture maximum center, middle, and edge sharpness occur (those are almost always different). It’s good to know things like there’s maximum center sharpness at F4 and the edges are at maximal sharpness at F6.3 or F8 or that they never get very sharp.

Grass is also great because it gives you a nice sharpness comparison as you leave the area of best focus. I also recommend looking at what you consider the depth of field at each aperture and focusing distance. Depth of field is not an area of maximal sharpness. It is an area of acceptable sharpness; there is greater and lesser sharpness within the depth of field. Your definitions of ‘acceptable sharpness’ in your images may be greater, or less, than the calculated depth of field.

You rarely see dramatic changes in a prime lens’ field curvature at different focusing distances, but you will usually see a dramatic change in a zoom’s field curvature at different focal lengths

More importantly, some lenses fall off of the sharpness cliff as they exit their area of maximal sharpness, others drift so slowly down the gentle sharpness slope that it really does seem as if the entire depth of field is maximally sharp. Also, that sharpness slope often changes at different apertures. Those are all good things to know.

The other thing I do is to take some of my grass images and run them through a Photoshop ‘Find Edges’ filter or equivalent. This will let you visualize the field curvature of your lens and see how it varies at different focal lengths or focusing distances. (Pro tip: you rarely see dramatic change in a prime lens’ field curvature at different focusing distances. You will, however, usually see a dramatic change in a zoom’s field curvature at different focal lengths.) That’s really useful information that few people know about their lenses. The find edges type filters are also a good way to look at depth of field at various apertures or with different lenses.

Same image as above (Canon 50mm F1.2) run through a find edges filter – the field curvature is obvious.
Field curvature of Canon 50mm F1.2 as measured on an optical bench. You get about the same information from the grass photo and find edges filter as you would from the $ 250,000 optical bench.

Grass shots also give you a superb way to see if your lens is softer in one area or if the field is tilted. The grass image above is very slightly tilted, an amount that’s about normal for a good prime lens. A more dramatic field curvature might look as though you’d rotated the dark area 15 or 20 degrees in Photoshop.

About half the people who take building or brick wall images and think their lens is ‘decentered’ actually have a lens with a field tilt; the lens is equally sharp on both sides, but not at the same distance as center focus. It’s actually very hard to detect a field tilt by shooting a chart and evaluating a two-dimensional image.

A large field tilt in a prime lens is unusual while a field tilt at some focal lengths of a zoom is pretty common. (I’ve seen 45 degree field tilts in zooms, but 10 degrees or so is routine.) If you return your zoom lens to the store for exchange, the replacement will probably have a different field tilt at another focal length.

People like to talk about a lens’ bokeh like it’s one thing, but bokeh often varies

If the lens is one for which I consider bokeh important, I use the a Bokelizer. Basically, this is a couple of strings of tiny Christmas lights hung in a three-dimensional pattern. I take some images at various focusing distances and evaluate the foreground and background in-focus highlights, as well as the in-focus lights. People like to talk about a lens’ bokeh like it’s one thing, but bokeh often varies in the foreground vs the background, at different focusing distances, and depending on how far off-center the object is for many lenses.

Why do I look at in-focus lights, since they have nothing to do with out-of-focus highlights? Because comparing pinpoint light sources is a superb way to see if the lens is optically maladjusted. ‘Optically maladjusted’ means a lens that has a decentered, tilted or poorly spaced element. On the forums, people often refer to all of these issues as ‘decentering’ but that’s less than correct.

Illustrations of the various types of optical maladjustments. In reality, a given lens usually has several small errors, rather than one single large one.

Each of those optical maladjustments causes different optical problems and often they’re apparent when looking at pinpoint light sources. Looking at pinpoint light sources also gives you an idea of the coma and other aberrations that the lens displays by design.

This image was created from equipment in the repair department that basically just projects pinhole lights. You can easily see the difference between a good lens (upper half) and one that is slightly decentered (bottom half).

Once I’m done with the stuff above, I go out and take the kinds of pictures that I bought the lens for. But the hour or two needed for the checks above gave me a lot of information about how to best use the lens’ strengths and weaknesses before I set off to shoot. It also shows me if the lens is optically maladjusted, and there’s no sense taking a bunch of photographs if I already know I’m going to return the lens.

Will taking pictures tell me if I got a copy that’s every bit as sharp as the copy Reviewer Guy got? Absolutely not. Does it let me spout numbers in ‘my lens is better than your lens’ Forum Wars? Again, no. But it certainly does tell me if the lens meets my expectations and will do the job I want it to do. Lab tests give me all manner of information, but they can’t tell me whether I’m going to like the images from the lens.

It doesn’t matter to me at all if I have the sharpest copy of a lens or not. I just want to know if it’s acceptable for the purposes I want to use it for

To be completely honest, if I think the lens isn’t as sharp as I expect, then I may actually take it to the lab and measure it on the bench. I’ve done that maybe twice in the last ten years out of a few dozen lenses I’ve purchased, and both times it turned out that the lens wasn’t up to spec. So, really, I knew the answer without using the bench.

Photographic testing won’t tell you if your lens is among the sharpest copies of that lens, or if it’s in the top half of the variation range or things like that. If you want to know that, then really you need to pay someone to test the lens on a test bench. Why don’t I do that? Because it doesn’t matter to me at all if I have the sharpest copy or not. I just want to know if it’s acceptable to me for my purposes.


Roger Cicala is the founder of Lensrentals.com. He started by writing about the history of photography a decade ago, but now mostly writes about the testing, construction and repair of lenses and cameras. He follows Josh Billings’ philosophy: “It’s better to know nothing than to know what ain’t so.”

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‘Who am I and what am I doing here?’ Introducing Roger Cicala

18 Oct
I like big optics.

A fair number of you probably know me as the Roger who started Lensrentals.com, and some may know I used to be a physician before that. A few know I sold most of my share in Lensrentals.com years ago and since then I’ve hung out as their director of Quality Assurance, Lovely and Talented Spokesmodel, and a major contributor to the blog. Other than QA, I haven’t actually managed anything for years.

When I started Lensrentals I had a lot of conversations with service centers that went like this. Me: “That lens you repaired still sucks”. Person at service center: “No, it’s within specs”. Me: “What are the specs?” Service center: “We can’t tell you”. One day, after I raised hell with a factory service manager, he patted me on the head and said, “testing lenses is complicated; you don’t have the background to understand.”

Any of you who has ever seen a physician after someone says something like ‘you wouldn’t understand; it’s complicated’ knows what happened next. I had no option but to spend a couple of years buying testing equipment, offering internships to really smart optical engineering students, and developing a lens testing center and methodology that was as good as anything in the industry.

Pictured: A lens testing center and methodology that was as good as anything in the industry. This machine doesn’t give us numbers, it’s used to optically adjust lenses in real time.

That probably sounds ridiculous, but the reality is that in 2010, everybody (manufacturers included) was still doing metrology (lens testing) the same way that they’d done it with film cameras in the 60s and 70s. In my previous life I’d done clinical research, and my first hobby was writing medical books for non-medical people; putting complex medical terms in plain words. When I started Lensrentals, I started writing again, blogging about the stuff we were doing.

I ended up doing testing and consulting for several major manufacturers, and a fair number of specialty manufacturers

So a few years later, when a service center told me “it’s within spec” I could send them their specs (because we’d tested enough lenses to know them) and the results from the lens in question and say, “NOPE, it’s not.” If you look back to my blog posts in those days, you’ll see I even posted some examples of what service centers claimed was ‘in spec’ versus what was really happening as well as posting actual MTF (as opposed to computer generated) data. As you might expect, this made me rather unpopular with manufacturers.

We then entered the traditional ‘exchange of threats and legal posturing’ period. I managed to convince most manufacturers that we were just reporting facts (emphasis on most). Eventually they started sending engineers to look at our testing methods. I ended up doing testing and consulting for several major manufacturers, and a fair number of specialty manufacturers. I don’t do that much anymore, since we gave our software and methodology to any that were interested, and most then started doing it themselves.

Test results for a lens that isn’t as sharp as it should be in the center, which actually is unusual. Usually the problems are away from center.

I still have a lab in one of Lensrentals’ buildings, but I just do whatever interests me at the moment. They let me put stuff up on their blog but much of what I write only gets widely seen when DPReview reposts it. I’ve worked behind the scenes with the DPReview staff for years, so when Barney offered me the chance to write directly for DPR we sat down and negotiated. I think the terms are fair; they aren’t going to pay me anything, but they won’t tell me what to write about or to STFU [Editor’s note: we offered to pay Roger but he said ‘I already have enough money’ and I didn’t push the matter in case I misheard].

I expect you might see a disclaimer about ‘the opinions expressed in this article don’t necessarily reflect those of DPReview, anybody who works here, or anybody we even know’ every so often. But otherwise I’ll be writing op-ed pieces here when the mood strikes me and when DPReview has a slow news day.

Roger


The opinions expressed in this article don’t necessarily reflect those of DPReview, its parent company, affiliates, anybody who works here, or anybody we even know.

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Roger Cicala breaks down the do’s (and don’ts) of cleaning your camera gear and workspace

20 Mar

Roger Cicala, founder and owner of Lensrentals, is best known in these parts for tearing down, repairing and reviewing lenses. But not long ago (relatively speaking), it wasn’t just lenses Roger was mending; he was also a physician. As such, his experience in these two fields makes him uniquely qualified to talk about something we should all be mindful of — how to keep yourself and your camera gear disinfected through proper care and treatment of your equipment and workspace.

In the thorough blog post, Roger breaks down what cleaning supplies you should (and shouldn’t) use and what practices will help to ensure you’re being as safe as you can be during the ongoing COVID-19 pandemic (and beyond). From basic gear cleansing tips to advice for keeping your studio or office as clean as possible, he covers it all.

You can read the full blog post yourself over on the Lensrentals blog. If we’re lucky, Roger might even make an appearance in the comments below for those of you who have any additional questions.


Image credits: photos used with kind permission from Lensrentals.

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Roger Cicala tests new Sony FE 135mm F1.8 GM, confirms ‘insanely good’ MTF results

12 Mar

Roger Cicala is a hard man to impress. His team tests a lot of lenses, but the new Sony FE 135mm F1.8 GM is a cut above the rest. Literally, all the rest. Roger’s verdict? ‘This is the sharpest lens we’ve tested. Period’.

We already knew that the FE 135mm F1.8 GM was good, but the MTF results are quite spectacular. In Roger’s words, ‘curves higher than anything I’d ever seen in a normal-range lens’. Compare the Sony’s performance at F1.8 to the Zeiss Batis 135mm F2.8, below. Even if you’re not familiar with MTF curves (in brief – the center of this graph shows resolution at the center of an image, the extreme right and extreme left represent corner sharpness, and higher lines are better), it’s clear that the Sony outperforms the Zeiss in the center and compares well towards the edges, even wide open.

And this isn’t just a standout outlier sample hand-picked by Sony to give the best results – these graphs are created from data averaged from ten copies of the lens.

The 135mm F1.8 was so sharp, in fact, that just for fun Roger ran tests at 100 lp/mm as well as the usual 50 lp/mm, which – again – showed that Sony’s latest lens should perform brilliantly for several generations of even higher-resolution full-frame cameras to come.

Read Roger’s full article on the Lensrentals blog

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Canon EOS R teardown: Roger Cicala takes a look inside Canon’s first full-frame mirrorless

20 Oct
Lensrentals.com, 2018

It’s been less than two weeks since the Canon EOS R started shipping and already Roger Cicala, founder of Lens Rentals, has taken one apart in incredible detail to see what’s inside.

Per his usual routine for gear teardowns, Cicala makes notes of various features and components found inside the camera along the way.

Lensrentals.com, 2018

The EOS R teardown started with the removal of the adhesive grip tape from around the body of the camera to better see where all of the screws are. From there, the Cicala stripped the EOS R of its various elements piece by piece from the outside in.

While Cicala called it ‘a rather a boring disassembly,” the resulting photos and look inside the camera are anything but. Canon appears to have done a solid job across the board considering the price point and feature set of the camera, but there’s certainly room for improvements.

Lensrentals.com, 2018

The buttons on the camera are thoroughly protected with weather-sealing gaskets, but the body itself is only water-resistant by tightly overlapping two pieces of the seams of the polycarbonate frame. In Cicala’s own words, “that means, I think, that it will be fine in a misty rain for a while, but don’t get it saturated and don’t set it somewhere wet.”

Lensrentals.com, 2018

Cicala also notes that “it’s not very crowded inside [the EOS R],” meaning there’s plenty of room to pack in more features and tech inside if Canon decides to do so. He specifically mentions that much of the extra space he noticed between the circuit board and image sensor is where the in-body stabilization (IBIS) is seen inside the Sony A7R III he took apart. But don’t hold your breath for seeing IBIS in future EOS R cameras. Cicala adds “Canon has been very clear that they think lens stabilization is superior.”

Lensrentals.com, 2018

Overall, Cicala says the EOS R appears to follow most of the design and engineering elements of past Canon DSLR cameras. “It was rather a boring disassembly, really, about what we should expect for Canon doing a Canon 6D Mark II quality mirrorless camera […] It’s neatly laid out and nicely engineered inside.”

Lensrentals.com, 2018

To see more photos and more thorough insights from Cicala, head on over to the full Canon EOS R teardown. Cicala notes that a similar dissection of Nikon’s Z7 is complete and will be written up as soon as he can get around to it.

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Roger Cicala finds innovation sealed inside the Sony 35mm F1.4 ZA

27 Jan

Sony FE 35mm F1.4 ZA teardown

We’ve already looked at Roger Cicala’s teardown of the Canon EF 35mm F1.4L II, where he showed it to be a heavy duty design with extensive adjustability to bring it back to spec after a period of abuse. LensRental’s look inside the Sony FE 35mm F1.4 turned up a lens designed with a radically different approach. It’s easy to over-interpret the differences and start trying to picture the different use-cases they’ve been designed for, but even if you don’t want to extrapolate so far, it’s fascinating to see how unconventional Sony’s approach is.

Taking a more linear route

The most fundamental difference Cicala highlights between the two designs is how the focus elements are moved. The Canon uses a traditional helicoid design – rotating the focus element along a helical track, in much the same way as manual focus lenses would. The Sony design instead uses a piezoelectric drive to push and pull the lens element along a rail, meaning that there’s no rotational movement occurring at all. Instead it can be shuffled back and forth in very fine increments.

Sony calls this design ‘Direct Drive SSM’ (with SSM standing for Supersonic motor), and it’s based on technology used for in-body image stabilization in Sony A-mount cameras, hinting at the speed and precision that such systems can provide. It’s distinctly different from the linear motor technology used in the FE 55mm F1.8, which you can see in operation here.

Direct drive SSM

The Piezoelectric drive mechanism is fascinating, and we saw it embedded in a cut-in-half FE 35mm at CP+ last year. It involves a drive element that can be expanded or contracted by applying an electric current. Expanding it slowly moves the lens out along its mounting rail, but the connection to the rail is designed to slide in response to rapid movement, so rapidly contracting the drive element leaves the lens in the more distant position but with the drive element retracted. Repeating this pattern of slow extension and rapid contraction progressively nudges the lens away from the drive element. Pulling the lens back again involves reversing the process: rapidly expanding the drive element so that it slips through the clamp, then slowing drawing the lens back in, one step at a time.

Why do it this way? Well, it’d certainly be quiet and it allows very fast movement, giving the lens possibly the fastest focusing we’ve ever seen in this class: when paired with the a7R II’s phase-detect AF system, autofocus is quick and precise. A single element able to move quickly back and forth in tiny steps lends itself well both to contrast detection focus in video as well as being able to reverse directions when subjects erratically switch between approaching and receding.

Adding a snap to aperture-by-wire

Another interesting design detail is the switch for engaging and disengaging the stepped, clicking aperture. A weather-sealed switch pushes a small, sprung ball bearing against a series of tiny teeth, to give tactile feedback as you rotate the aperture ring. However, no other mechanical connection is engaged: the aperture is entirely controlled by-wire, with a sensor detecting movement of the aperture ring and relaying it to the aperture motor.

Locked on place

The other major difference between the Canon and the Sony is the philosophy behind lens alignment. Where the Canon had a series of shims and adjustment screws to allow the different elements to be re-aligned, and re-centered, the Sony has most of its elements glued together in one giant module. This whole module then has three shims offering only a small degree of adjustment. LensRentals’ testing of its copies suggests this adjustment isn’t sufficient to give the consistency you might hope for.

Individual replacement parts are not available for after-market repair: the only option is to slot a whole new module in, with limited adjustment to ensure its alignment within the lens barrel. This approach means Sony has a good level of control over the alignment within each module but means the lens is harder and more expensive to service if it goes out of alignment or if the front element gets scratched.

In summary

The construction and adjustment isn’t quite as extensive as in the Canon but, as Cicala highlights: nor is it in most lenses. Instead it appears Sony has designed its lens so that it’s durable and everything is fixed in place, whereas Canon has built its lens to be tough but accepted that as a photojournalist’s workhorse, it’ll need to be beaten back into shape every now and again.

it’s clear, though, that Sony has built this lens to be tough: not only is each element individually positioned, rather than being spaced apart, relative to another, but Sony has included extensive amounts of weather sealing at every step of the design (just look at the size of the rubber gaskets, in the picture above). Cicala concludes: ‘This lens has the most rubber gaskets I’ve ever seen. The weather and dust resistance in the lens itself should be superb.’

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Rezension: Roger Steffens – the family acid

13 Jul

© Roger Steffens - The Family Acid

Es gibt eine ganze Reihe von Auslösern, ein Fotobuch zusammenzustellen und zu realisieren. Seien es nun konkrete Projekte, Serien oder auch Monografien, die eine Rückschau auf das Lebenswerk eines Fotografen liefern. Eine Herausforderung ist dabei immer auch, aus einer großen Menge Bilder eine wirklich starke Auswahl zu treffen und diese in eine stimmige Reihenfolge zu bringen.
kwerfeldein – Fotografie Magazin | Fotocommunity

 
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