Histograms Explained...

 
Explanation of the Histogram
 

For most, the histogram is that weird-looking graph that pops up when you accidentally hit a button on your camera. Instead of frantically trying to make it go away, take some time to figure out what its telling you. It may just help out your shot.

The histogram is simply a graph that shows you the brightness levels of the pixels in your image. The far left represents pure black, the far right represent pure white, and the middle represents every brightness value in between. As for the height of the peaks in your histogram, that represents how many pixels are in that brightness level. 

The only thing you really need to watch out for is that you don't have a lot of pixels hitting the far right or left side. These pixels will be pure white/black and have no chance of being recovered in post due to clipping. If that's what you are intending to do, I'd recommend not hitting either side too much as you can always adjust your image in post to hit them. This way you'll have a much larger dynamic range to play with and can increase or decrease the exposure to your liking.

Your camera may have a "highlight warning" mode or "zebra pattern" mode where you will see areas flashing that are hitting pure black and pure white. Just know if you are shooting a sunrise/sunset, the sun will almost always hit pure white unless you are shooting very under exposed.

I use the histogram often when shooting long exposure landscape shots to make sure I don't have too many pixels hitting pure black. With the filters I use, it's easy to cause certain areas to become too dark. Seeing the histogram lets me know if I should increase my exposure which I often do by up to +1 stop.

Hope that helps explain the histogram. It's not hard to understand, but can be hard to utilize.