Camera Exposure

26 Aug

In photography, ‘exposure’ means the amount of light that falls onto the sensor of your digital camera. In modern cameras the exposure is usually set to automatic by default and, most of the time, it can be left there and will produce beautiful pictures. There are times though, when the camera lets us down or we want to produce a particular effect and it would be nice to understand what is going on ‘under the hood’.

Getting a ‘correct’ exposure means recording as much of the relevant information in the scene as possible. In the shot on the left, the important information is the bird. The sky has gone completely white as it was much, much brighter than the bird. If I had let the meter expose for the sky, the bird would have been rendered as a black silhouette.

Even when the exposure is ‘correct’ the problem with all cameras is that they cannot record the entire range of contrast (black to white) that the eye can see. Especially when you take into account that the eye is constantly adjusting to cope with high contrast. On a sunny day if you look into the shadows of a scene then into the bright areas, the iris in your eye will quickly adjust so you can see detail in both.

Faced with the task of recording as much information as possible, the camera will try to average out all the light levels and expose accordingly. As burnt out highlights are normally considered uglier than black shadows, the camera, left to it’s own devices, will normally err on the dark side. Which is no good if you are shooting against a bright background. It’s the subject you want to see, and you don’t really care if the background is white.Auto Exposure Modes

The camera manufacturers have come up with all sorts of ingenious metering systems to try to help, there are now multi mode metering systems, which give you a choice of ‘center weighting’, ‘spot metering’ or ‘multi spot metering’ on many of the better cameras, but none can guarantee to give you what you want every time.

A common use for this technique is when you are taking a close up shot of two people and there is space between their heads, if you’re not careful the camera will focus on the wall or trees behind them. If the background is very dark or very light this can alter the exposure significantly and result in faces that are too dark or too light.

Skin tones are what most meters are set up to consider an ‘average tone’, they are also usually the part of the picture that we most want to get right. If I am photographing a group of people in difficult circumstances, like bright sunlight for instance, I will often move close in to the group and take a light reading from someone’s face or, if we are all standing in the same type of light, I will take a reading from the back of my hand. This is no good, of course, if the subject is in bright sunlight and I am in the shade.
Manual Exposure

Now it’s time to turn that dial away from ‘program’ mode and have a look at the dreaded ‘manual’ mode. There are also a bewildering array of other choices such as ‘aperture priority’, ‘shutter priority’, ‘exposure compensation’ etc., but once you understand the basics you will be able to select the most suitable mode.

The amount of light falling on the film is governed by four things.

  • The amount of light reflected from the scene which, if you are outdoors, you can do very little about.
  • The ‘shutter speed’ which is the amount of time the shutter is open, measured in fractions of a second.
  • The ‘aperture setting’ which is the size of the hole through which the light enters. If you look at the lens of your camera you will see a diaphragm in the middle of the glass which the camera adjusts according to the light. This does exactly the same job as the iris in your eye. Aperture settings are measured in ‘f stops’.
  • The ISO setting. This is the ‘sensitivity’ of the sensor. (see ISO Speed)

The shutter speed and aperture settings have other quite separate effects on the photograph which we will discuss in another article, but for the purposes of exposure, making the picture darker or lighter, they are interchangeable. Make the hole twice as big and open the shutter for half the time and you will expose the sensor the same amount.

Why use manual exposure?

The advantage of manual exposure is that the settings do not keep changing as your scene changes. Let’s suppose that you are taking close up photos of cars passing by. Some of the cars will be black or dark colors and some will be light colors or white. If you are filling the frame with almost nothing but car, the meter will be trying to render each car as mid gray. Although it will probably not succeed, what you will notice is that the background is a different shade in each photo.

I often have to take portraits of people, some are wearing very dark clothes and some are wearing white. If I am not careful with my exposure settings will be affected by the clothes.

Although it is by no means always necessary to use manual exposure, an understanding of how it all works will save a lot of disappointment. Below are some links to other articles covering various aspects of exposure.

Any suggestions, ideas? Feel free to comment on this article!

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