Everyone loves the fact that the second you’ve taken a shot with a digital camera, you can instantly view it on the screen at the back. It’s great fun, and also very useful to let you know if you’ve captured the moment you were after or not. The preview image can also be used as a way of checking exposure, though perhaps not in the way you think. What you really need to be using to check your exposure is the histogram.
Many cameras these days will allow you to display a little graph (called a histogram) of an image as well as just a preview of it. You’ll access this in a different way for each camera and manufacturer, so I can’t help you here – I’m afraid it’s a case of RTFM if it’s not immediately obvious.
What this little graph does is show you very accurately where the tones in the image are distributed, from shadows to highlights, usually with shadows on the left and highlights on the right. If there’s a big spike in the graph on the left hand side, and very little in the middle or on the right, this means most of the information in the shot is made up of darker tones, and the image will be dark. If it peaks on the right, and tails off to the middle and left, it’s made up of lots of light tones, and it’ll be very bright. If it forms a perfect “mountain” with a peak in the middle and gently tapering arms to left and right, your exposure is nicely balanced. Think back to when you were taught about bar graphs at school – 5 units in this column means it’s so high, and 10 in this column means it’s so high, and so on. The histogram is just a version of this for your image, the x axis shows how light or dark a tone is, and the y axis shows how much of the image is made up of those tones.
The biggest advantage of the histogram over just looking at the image itself, is that it’s objective, rather than subject to how bright or light the situation you’re in is. Let’s say you’re shooting in a dark, indoor space, and you take a shot and review the exposure by looking at the image on the back of the camera, It looks OK, but then when you download it and start playing around with it on the computer it looks too dark. Because the LCD screen was very bright compared to the room around you, your eyes were fooled into thinking the image was brighter than it actually was. The opposite can of course happen outside on a bright sunny day when it can be hard to see the screen easily and discern how bright or dark the image is. By judging your exposure based on the histogram you can avoid any of these problems.
As always, there’s slightly more to it than what I’ve described so far. Whilst having a big peak on the left (for example) means your image has lots of dark tones, that may not mean your image is under exposed, it may be exactly what you’re looking for. In this image of Peter Crouch, think what it would look like if the histogram was centred in the middle – all those lovely black tones would instead be grey and quite flat – losing much of the impact of the shot.
The same is true the other way round – this studio portrait of a Poker Pro relies for it’s impact on the clean, white, empty space he’s surrounded by, and if that white was grey, it just wouldn’t have the same effect.
As a very general rule, if your image needs to end up looking dark, there’s no harm in the histogram being pushed to the left, and to the right if you want it to look very light and bright.
For more average shots, you won’t go far wrong if you can get your histogram to peak in the middle.