You have probably heard this quite a lot that the picture you have taken is over or under exposed. If you have just started out in digital photography, this is a sort of remark that you could expect to hear a lot. That is not until you have mastered the art and science of proper exposure.
What is over or under exposure and how can that affect your photographs? Overexposure means when your photographs have more highlights. The photo looks washed out. Underexposure is just the opposite of it. The photo has more dark areas or what we call as dark tones. A majority of the picture looks as if it was shot in poor lighting conditions. So how to correct your exposure for avoiding either of these? One good way is to use a live histogram and if you don’t have a live histogram on your DSLR, then using the histogram when reviewing the picture you have taken on the camera’s LCD screen. While pro bodies will always have a live histogram and that makes a lot of sense too, as professionals want their exposure to be right on the money, amateurs should not be left behind either. All DSLRs thus have an option to check the histogram on playback option. Once you have taken a shot, analyze it by pressing the playback button and pulling up the histogram option. If everything is not where it is supposed to be then compensate for the right exposure and retake the shot if you have to.
Histogram is the tool that allows you to understand whether you have more highlights or dark tones or basically how your picture is exposed. The ideal histogram (if there is anything like this at all) is one where the resulting mountains (the vertical spikes that you see on the histogram) are more evenly spread out. That points towards a properly exposed shot which has more of the mid tones, little bit of highlights and of course little bit of dark tones. While shooting outdoors or even indoors what you see from the LCD screen is never a proper judge of the quality of the exposure. What looks great on the LCD screen may actually look washed out or underexposed on the computer. So the histogram is a better evaluative measure of the right exposure.
In a histogram each spike that you see points to a particular color tone. The extreme left side is for black and the extreme right is for white. Anything between makes up the mid tones. If you have a washed out picture, the spikes will tend to be more towards the right and if it is underexposed then more towards the left. A correctly exposed shot will have the spikes more towards the middle and evenly spread. Now there would be situations where you will see that is impossible to achieve. Say when you are shooting black and white portraits. You are bound to get more highlights and dark areas and some areas of gray in the histogram. But this cannot be avoided, as that is what you need. Again if you are using side lighting whether in a studio or an outdoor shoot (depending upon any additional source of light) you are going to see more of the darker tones and gradual decrease of the mid tones to nothing on the light tones. So histogram is ok as long as you don’t intentionally want to shoot a special situation. At such circumstances, simply ignore the spikes and follow your intuition.