Mastering Exposure

Developing an understanding of light, and its interpretation, is probably the best foundation on which to build the rest of your camera know-how. In this, the first of our ongoing camera skills series on mastering your DSLR, we take a closer look at light and how it works to make an exposure, as well as the key features and controls on your camera that will help you in tricky exposure situations. 

How cameras measure light
The word exposure refers to a controlled amount of light that is exposed to the film or sensor to record an image. The camera’s job is to balance the amount of light entering the lens with the sensitivity of the sensor, to achieve a fair representation of what we saw with our eyes. This may sound easy but our eyes are far more sensitive to the range of light than the sensor 
in a camera is, because we can balance out high-contrast subjects and see detail in shadow and highlight areas to a much greater degree than any camera can.

A well-exposed image contains detail from the shadows right through into the highlights. Too much exposure and the image will appear too light, with delicate highlight details disappearing into a pure white mass. But too little and the image will appear dark, with shadow details turning pure black.

Cameras contain three features to control exposure: the shutter speed, the aperture and the ISO setting. The shutter speed determines the length of time that light is passed through to the sensor. The aperture works like the iris in our eyes, opening to let more light in and closing to restrict its flow. The ISO controls the sensitivity of the sensor; in low light, you can increase the ISO to make the sensor more sensitive to light (which in turn, enables faster shutter speeds and minimises the risk of camera shake).

Managing these controls is a light-meter, which measures the light entering the camera and, in auto mode, chooses the right settings to get what it thinks is a ‘correctly’ exposed shot. In manual mode, these settings are suggested by the camera and the user can choose to apply them or override them.

One of the benefits of digital capture is that you can review your exposure instantly and, if necessary, adjust it and take another shot. But don’t be fooled into thinking that good exposure is no longer important as you can always fix problematic images in Photoshop. While it’s true that you can alleviate slight exposure problems on the PC, these can only work successfully on the tones and detail captured in the first place.

Exposure compensation
This button, used in the auto modes, lets you override the exposure the camera has chosen (perhaps because you think it will have been fooled by the background). Add positive compensation when the subject is darker than the background, and negative compensation when it is brighter. The amount of compensation needed will vary according to the conditions, so you may need to experiment.

Tone compensation
A feature found in the menu on many cameras, it’s useful to boost or reduce contrast. At a wedding, for example, you may struggle to get detail in both the bride’s white dress and a groom’s black suit. Turning the tonal compensation down will compress the tonal range to a level the sensor can cope with. You can reverse this process on an overcast day to increase the tonal range and make the images more dynamic.

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