To the point…
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Quick and to the point: that’s the reasoning behind the use of linear focus motors, but it’s less true of the latest blog post on the subject, over on LensRentals.com. That’s what we love about the crew’s in-depth teardowns. In their latest post they tear apart a series of linear drive lenses and discuss the various designs they’ve encountered. Some are pretty robust and others, well, take a look for yourself…
Click here to read the LensRentals Blog post about Linear Electromagnetic Motor focusing
The need for new designs
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The ring-type focus motors [pictured above] that were traditionally the default choice for high-end DSLR lenses are not especially well suited to the needs of mirrorless cameras or video shooting. Contrast detection autofocus requires not just being able to move a focus group quickly but also the ability to stop it, then drive it back in the other direction, all with high precision. Video requires silent and carefully-controlled focus drive, to allow smooth refocusing while the camera is recording. These different requirements have prompted the adoption of new types of focus motors.
Linear electromagnetic motors
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Among the more popular alternatives to ring-type drive is the linear motor, which features a permanent magnet and a coil of wire that, when electricity is run through it, slides along a bar parallel with the magnet. In principle these fulfill the things demanded of them: fast, precise and quiet (we’ve been very impressed by how fast some of the linear motor lenses we’ve used can be).
Surprisingly, the internet has very few good diagrams of these designs, but you can sometimes recognize lenses that use this type of motor because the focus element rattles around when the camera is switched off. This is because in many linear motor lenses the focus element is only held in position when power is being provided to the focus coil – the rest of the time, the focus carriage can just slide up and down its rails. This isn’t true of the Sony and Zeiss designs that much of the blog post discusses – these appear to have some sort of brake to stop this disconcerting behavior.
Rattle and, er, break
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Generally we don’t worry too much about this rattling, but perhaps we should. LensRental’s experience with large numbers of hard-worked lenses reveals that not all linear motor designs are the same. Early Sony motors attach the moving coil to the focus element carriage with just a single blob of glue. Oddly enough, this can fail; leaving the coil racing up and down the rail but with the focus element uncoupled. Later designs do a better job of securing the moving coil to the carriage, prompting Roger Cicala to define two categories within lenses of this kind: Type 1 motors and Type 1a designs that are very similar but don’t break so readily.
No right answer
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As well as highlighting a failure mechanism of poor designs, Cicala and Co’s teardowns hint at a fundamental shortcoming of linear motor’s capabilities. Fujifilm’s use of two, three and four linear motors in some lens designs suggests that they struggle to move large, heavy lens elements quickly, taking a brute-force approach.
This is also likely to explain why Sony adopted three different focus drive technologies (linear electromagnetic motor, piezoelectric direct drive and ring-type motors, sometimes in combination) in its recently-announced GM series of lenses: because there isn’t yet a single technology that provides all the necessary characteristics in a way that works for all lens designs.
Results, not technologies
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Like LensRentals, we’ve seen very different results between the best and the worst examples of each lens motor type, which is why we try to concentrate on performance, rather than technology, when we write about lenses. We’ve also been lucky not to experience any of the motor failures (perhaps better described as motor detachments), that LensRentals has seen, but it’s interesting to see the designs of lenses improve as manufacturers become more experienced at using each technology. Or, as in the case of the Sony 70-200mm F2.8 pictured here, a mixture of technologies.
We also hope Cicala makes good on his promise to look at other emerging focus technologies, and the ways in which they’re developing, in the coming weeks.
Click here to read the LensRentals Blog post about Linear Electromagnetic Motor focusing
Articles: Digital Photography Review (dpreview.com)