Understanding Histograms
Monday, April 25th 2011 @ 5:30 PM (not yet rated)
Virtually every camera on the market today, from a simple point and shoot to a professional DSLR has the ability to display a histogram. What is it and what does it mean?
It is a simple graph that displays where all of the brightness levels contained in the scene are found, from the darkest to the brightest. These values are arrayed across the bottom of the graph from left (darkest) to right (brightest). The vertical axis (the height of points on the graph) shows how much of the image is found at any particular brightness level.
The histogram can tell you if your image is underexposed or overexposed.
When the values are all smashed up against the right side of the histogram, it means your image is over exposed. There is quite a bit of the image that is completely white with no detail at all. There is nothing that is completely black, either.
When the values are smashed up against the left side, your image is underexposed. In this image, there will be segments of the images that are completely black, lacking all detail. You can see that there are no bright highlights in this image, either, because there is nothing in the right third of the histogram.
So what does a perfect histogram look like? Well, that depends on the type of image you are looking at.
Here is the histogram for the image above:
This tells us that there is a little in the image that is pure black and a little more that is pure white. Because of the clouds and sky over at the upper right section of the image, I would expect that to be white and feel that is appropriate.
What should the histogram for the image of the french horn player look like? Here it is:
There are a few blown out highlights but that is ok. We would expect the light relecting off the horn to be completely white. I would NOT want any part of his skin to be pure white, but the horn is fine. Notice that the biggest block of information is at the far left. Again, that is ok because it is a completely black background. We want a nice, rich black.
Another tool that is helpful when processing images is your "info" palette. When the image is open in Photoshop, I will move the curser over the image while looking at the numbers in the "info" palette (Click on "Window" at the top of your screen and make sure that "info" is checked to see this box). As you move your cursor over the image, you will see the numbers next to the R: G: and B: change. All zeros is absolute black. 255 is absolute white.
Check your histograms as you are photographing to get the optimal exposure.
