Probably one of the least understood and under-used aspects of a DSLR camera is its metering ability; but it’s not as complicated as it sounds. Grasping how to use it can ensure that your photographs come out exactly as you intended. If you read my column on Aperture and Shutter Speeds in the April issue, you’ll remember that if you switch to Aperture Priority shooting mode (A or Av), once you’ve chosen the aperture, the camera selects the appropriate shutter speed to ensure that the picture is correctly exposed – and similarly in Shutter Priority (S or Tv). So why do your photos sometimes turn out too dark or too light?
The Power of Sight
The first thing to remember is that no matter how much money you spend, your camera will never be able to capture the tonal range that your eyes can. Our brains are so quick to interpret the messages from our eyes that if you are looking at a tree in a sunset, you can clearly see not only the detail in the dark bark, but also the deep red background. This range is impossible for photographers to capture in a single shot, so they must choose whether to expose for the bark – which will mean that the background is overexposed – or expose for the sunset, giving them those rich crimson hues, but in so doing transforming the tree into an underexposed silhouette.
How Bright is Right?
The metering system is how the camera decides how bright your photograph should be. Usually you would set this to Evaluative (also known as Matrix or Pattern,) which automatically compares each segment of the scene in your viewfinder with a database of images, then gives extra emphasis to your focal subject, and calculates the correct exposure. This will work accurately for the vast majority of the time, but occasionally the camera is fooled by a tricky lighting situation.
If your subject is significantly darker or lighter than your background, then it is more likely that your camera will judge the scene incorrectly and your picture will come out darker or lighter than you intended. For example, a leopard in the shade of a tree may come out too dark, because the camera has chosen instead to ensure that the sky behind is correctly exposed.
The Quick Fix
The quickest way of correcting this is to ‘dial-in’ exposure compensation (usually denoted by the +/ – button on your camera) and shoot again. This is shown on your light meter: to under-expose (make the picture darker,) move the bar into negative territory, to over-expose (make it lighter,) move it to the positive side. However, it may take a few experimental shots to judge how many stops of compensation you need, by which time your leopard may have disappeared.
Spot the Difference
The solution is to switch to Spot metering, then point the centre of the viewfinder at a neutral mid-tone in your viewfinder (for example, green grass,) and press the Exposure Lock button (* or AE-L). This tells the camera how to expose, and locks those settings for the next shot; although on some models you will have to keep the exposure lock button pressed. If your subject still comes out too dark, or there isn’t really a mid-tone in the viewfinder, the next best option is to Lock the Exposure on the subject itself. Note that this will probably mean that the background comes out overexposed; remember that the camera cannot capture the tonal range you can see. However, this can sometimes lead to a more artistic image, particularly if converted into black and white.
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