The Histogram is your Friend
To determine if you have proper exposure on your digital images check your histogram on the back of your camera after every photo you take. It sounds like a lot of work to do this, but trust me, if your exposure is correct, you will have less “fixing” to do to your images afterward, so really, it’s a time saver.
The graph above illustrates the exposure of a photograph. The left side represents the dark areas, the shadows. The right side represents the bright areas, the highlights. The middle section represents the mid-tones.
After you take a picture you typically look at it on the back of your camera. You can’t really tell from the image playback screen if your photo is exposed properly, just like you can’t tell if the colour is correct. The brightness of those lcd screens varies a lot, and in the bright sun they can be impossible to see. So to really know if your photo is exposed properly, check your histogram. It will show you whether a photo is too dark, too light, or just right.
Interpreting the histogram
Hopefully the histogram of most images will be easy to interpret. If the graph doesn’t go all the way to the right side, it indicates underexposure. If it goes too far to the right, where it’s climbing the wall, then parts of your photo are overexposed, or too bright.
Sometimes you can have spikes on the highlight side of your histogram, but the rest of the histogram looks fine. This may indicate overexposure to the whole image, but it can also mean there is an area of shiny chrome, a glistening beach, or other bright area in your image. Even on an overcast day, the sky can be overexposed but the rest of the image can look great.
Have a look at the photograph below. I have an orange line going from the overexposed area in the sky to the part of the histogram is represents. I’ve also got a line to the mid-tones. As you can see, the exposure looks good, but the sky is blown out (over-exposed).
If I lower the exposure to make the histogram, and the sky look better (using exposure compensation +/-) then the overall exposure in the shot becomes too dark, like in the version below. I did pick up more detail in the background though, but I don’t think it looks as good overall.
Your job is to determine what parts of the photo are most important and keep those exposed properly.
If you shoot jpg files, it is really important that your exposure is correct. If you overexpose your shot and lose highlight detail, you can never get it back. If your image is too dark, you can make it brighter in the editing process, but it won’t give you the best result.
For best results, shoot raw files
If you shoot raw files, you have exposure “insurance” and you can recover highlights, while retaining the mid tones and shadows.
The photograph below shows a quick correction in Adobe Camera Raw, that gives a compromise for this difficult exposure. Adobe Camera Raw is part of Photoshop and Photoshop Elements.