Histograms

I have been promising to cover histograms, and so I will start talking about them with this post. I have been talking a bit about metering, so histograms tie right in. You set the exposure before you take a picture using the light meter. You check the exposure after you take a picture using the histogram. Most modern digital cameras can display the histogram, but what does that pattern mean and how do you use it?

Every tone in your image for each color channel is assigned a number between and including 0 to 255. 0 is no color and 255 is all color. Pure red is 255,0,0 in RGB, which means all red, no green and no blue. The histogram can be shown as representing all colors together to show full tonality or you can view the histogram individually for each color channel. I will be dealing here with the the all color histogram.

The histogram shows you how much of your image is at each tone. If all of the pixels are at 250,250,250, or near white, the histogram would show a single line at 250, like this (as shown from Adobe Photoshop):

Histogram for 250,250,250

If all of the pixels are instead at 5,5,5, or near black, the histogram would show a single line at 5, like this:

Histogram for 5,5,5

Understand so far? Good. Now let us introduce a little variation. Here is a gradient from black to white, the same direction of the tones in the histogram, and its histogram:

Gradient and its Histogram

I left a little pure white and pure black in the image. You can see that all of the tones are represented.

Now to move on to interpreting the histogram as an aid in checking exposure. First, there is no single histogram to shoot for. There is no "best" histogram. Each scene is different and you the photographer has to decide on what the histogram "should" look like for the image you are shooting. If the scene is composed of a green field with dark shadows and white clouds, expect to see most all tones represented in the histogram. Here is an example:

Full Tone Image and its Histogram

However, if the shot is dark or contains silhouettes, the histogram will be shifted toward black, like this:

Dark Image and its Histogram

See how the silhouette is represented as a tall line at or near 0 in the histogram? The sky is represented near the middle tones.

If you are trying to capture detail in the brightest areas, make sure the histogram is not clipped at 255. If you want to capture detail in the darkest areas, make sure the histogram is not clipped at 0. Sometimes you can capture detail in the highlights and shadows, but only if the tonal range is handled by you camera. If not, you have to make a decision as to which tones you want to capture and use the histogram to make sure that they are not clipped. Unless of course you want to try your hand at either High Dynamic Range (HDR) or merging shots of different exposures. Both of these are topics for later posts.

Here are a few more sites that are each a good read on histograms:

• The Luminous Landscape
  
• Ken Rockwell
  
• Digital Photography Review
  

I have said this before, but it bears repeating. Get out there and practice. The more you do this and see your own histograms, the better you will be at using them as yet another tool to better your photography.