The Camera's Meter Compared to The Histogram
Point your camera at a subject, check the meter, and press the shutter release button. The camera clicks. Later, you review your shots on the camera's LCD display and take a look at the histogram. Both the meter and histogram show you versions of how your images are exposed and both are quite useful. But while they do have a lot in common, they work quite differently. This week we'll take a look at how they compare.
The meter registers whatever you point it at. It only knows what it sees right now and has no memory or what you pointed it at even a moment ago. It also doesn't see anything outside its field of view, an area that may not even cover the entire frame depending on how you have it set. As a spot meter, it may only see a one degree circle in the center of the frame, totally oblivious to whether anything beyond that is over or underexposed. Even on matrix or "evaluative" metering, the whole frame may factor in to the exposure calculation, but not every part equally. It does all this in an attempt to help you emphasize what you think matters most in a shot while not totally neglecting everything else.
Nikon meters generally read right to
left while Canon reads left to right
Whatever goes into the meter calculation ends up being reduced to just a single value. Just one number to represent the entire scene. A "medium" or neutral exposure will register at the zero mark on the meter, with underexposure showing a negative number and overexposure a positive. But you always want a "medium" exposure. White isn't "medium" and neither is black so what the "correct" meter reading is depends on what you want the result to look like. It's up to you to decide how you want to render what you point the meter at. Just set the exposure and shoot. But be aware that what the meter isn't taking into account will still be in the frame. You might want to meter other areas too before settling on an exposure.
By contrast, a histogram graphically represents the exposure of an entire image, weighting each part equally. You don't "point" a histogram to any particular part of a frame — it takes it all in. But rather than reducing all this down to a single number a histogram attempts to show this in a way that helps you make sense of how the entire shot is exposed. The histogram counts how many pixels there are which each degree of brightness, from black to white, and then presents its findings graphically. Dark pixels are on the left and lighter ones progressively towards the right across the graph. At each point, the height of the graph shows you how much of the image has that degree of brightness. Whereas a spot meter is extremely directional, you can't tell from a histogram where in the image each contributing pixel is located. A bright area in the upper left will contribute exactly the same amount to the resulting histogram as it would were it in the lower right.
Histograms always have black
on the left and white pixels on the right
The results of a histogram are actually built from the image itself. You can go back to see the histogram at any point in the future you want to, but using it to influence how a shot is taken isn't so easy. On most cameras at least, you can't see the histogram for an image until after you've taken the shot. By contrast, a meter reading is freely available before you take a picture, but not so much afterwards. Once the camera is moved or its settings changed, the meter won't read the same since it won't see the same thing or at least not in the same way. Once the lighting changes, forget it. Histograms though don't change as they are baked into each image — indeed they are each image, in graphic form. To use a histogram to set the exposure for a shot requires you to take a test shot and then make the needed adjustments based on the resulting histogram. This can work well for static subjects but no so well for more dynamic shots. There's nothing better than a histogram though to confirm you nailed the exposure once the shutter has been fired. The image itself can be misleading when viewed on the camera LCD as it depends too much on ambient lighting and other factors, but the histogram never lies.
Sometimes I hear of photographers who started as film shooters who discount the importance of histograms even now that they shoot digital. I've also encountered photographers who grew up on digital who feel that the meter is irrelevant now that they can get more complete information from a histogram. But both have their uses — they are, in fact, quite complementary. Mastering both can make you a better photographer, especially in tricky lighting situations are you often encounter near sunrise and sunset.
And there are some great photos to be had at such times if you are prepared to take them.