Thursday, December 17, 2009

Histograms Part 1

With this post, I'm in danger of covering a subject that is overly complicated and beyond what photomoms want to know. But I'm going to give it a try anyway, so let me know what you think. Hopefully I can explain it simply enough to be practical and useful.

Histograms are graphs that plot data. When used in digital photography these graphs can provide valuable feedback regarding exposure. This feedback is particularly useful because it is objective. It takes a little effort to understand histograms, but once you do, they are quick and easy.

Important Background Information
The sensor in your camera can only record a certain range of brightness values at a given time. That means that if there is too much contrast in a scene, your camera cannot record it all.

It may help to think about your sensor as the paddle in pong. You can move it back and forth from right to left in order to capture a different range of the brightness spectrum, but the paddle can't cover it all. Thankfully, most scenes don't have a "full spectrum" of brightness in this sense, but often the variation in brightness is greater than our sensors can record. Consider these two examples:

In this image I slid my pong-paddle-sensor toward the brighter side of the spectrum so that I could capture my wife and daughter at the other end of this pipe. In doing this, I sacrificed a lot in the darker part of the spectrum. As a result, much of the pipe appears as solid black with no texture.

In this image I sacrificed some of the brighter part of the spectrum so that my daughter wouldn't be too dark. The result is that the sky is pure white without any detail.

If you understand this principle, you are on your way to understanding histograms and using them to your advantage. Read Part 2 next..