Histogram just sounds like a big, confusing word. But it isn't.
A histogram is just a graph of all the different tones in your image. You can find a histogram on the back of your camera or in your developing/processing program.
The darker/black tones are graphed on the left. The whiter/brighter tones are graphed on the right. And all the middle tones (not too dark and not too bright) are in the middle.
The more tones there are of a certain tone, the higher the graph gets in that area. See. Simple.
A Perfect Histogram?
There is no such thing as a perfect histogram. The range of tones in the photo you take will dictate what the histogram looks like. If your photo has lots of white in it (a snow photo), then most of the tones will be toward the right side. If your photo has lots of blacks in it (a night shot), then most of the tones will be toward the left side.
You can view histograms on the back of your camera and in many post-processing programs.
Photographer’s often talk about “clipping” the highlights or shadows. They might also say that the whites are “blown out.” Simply put, this means is that there is no photo information in those areas—they are pure white or black.
Histogram’s with clipped whites or clipped blacks will look like the diagram below:
3 Things to Know About Using Lightroom's Histogram
1) Make Adjustments Directly on the Histogram!
Yep. That's right. No need to adjust your exposure down in the basic panel. You can actually just click and drag on the histogram. The adjustments will show in the sliders within the basic panel.
As you hover your cursor over the histogram, the affected area will be highlighted and the name of the slider you'll be adjusting will appear below the histogram. When you click and drag, the amount you are adjusting that slider will appear on the right below the histogram.
2) See "Clipped" Whites and Blacks
To see any "clipped" values in your photo, hover over the small triangles in the upper right or upper left of the histogram. You can also toggle on/off seeing the clipped areas by pressing "J".
The triangle on the upper right shows clipped whites in red and the triangle on the left shows clipped blacks in blue.
3) View the RBG Color Values
Hovering over areas of your photo will display the RGB color values directly below the histogram. You can see this change as you move the cursor.
This can be used to see "clipped" values as well (0% is black and 100% is white.)
AND, you might also note that when you aren't hovering over your photo, you can see all the photo's metadata information.