What is a Histogram
The Histogram is a bar graph that shows what tones are in your photograph. Digital sensors can capture about 5 stops of light.
The histogram shows the number of pixels in the image in each of those zones.
This histogram is a screen print from Camera RAW. When you shoot in Camera Raw and open your CR2 file in Adobe Camera Raw, you will see a histogram that represents the tones in your photograph.
From Left to Right, it reads Dark to Bright. The higher the graph, the more pixels in that tone. From end to end, is the 5 stop tonal range. Notice in this image, I have a considerable amount of pixels in each zone. Theoretically, this is good exposure. (As we know, art and photography are subjective.) This histogram shows there’s a good range of tonality in the image. Notice that the far left (very dark) and far right (very bright) areas don’t have any pixels. That means there’s probably good definition in the shadows and highlights.
Notice, in this histogram, the pixels spike at the edges of the histogram – in both the shadows and highlights. This could be a concern as there won’t be any detail in the max black and max white of my image.
Generally speaking, you want to have some pixels in your max black and max white, but you don’t have to have spikes of pixels. The spikes tell us that there is a significant quantity of pixels without detail in the image.
As you can see, the histogram looks very different in each of my examples. There is no “right” histogram and each one will look different. But, the general rule of thumb is to shoot for a bell shaped histogram (similar to my first example). That is, a histogram with the majority of your pixels in the middle three zones with some pixels in the two end zones. That will ensure that you don’t underexpose shadows and overexpose highlights.
I mentioned that these histograms are screen shots of Camera Raw. Most DSLR have a setting to display your histogram after every photo. Or, you can view the histogram through your menu later.
You may have noticed that a lot of photographers glance at their LCD after taking a photograph, and just assumed they were previewing their image. Some of them are previewing their image but many are looking at their histogram to decide if they need to adjust their exposure and reshoot.
One thing worth noting, is that the histogram will look a little different on your camera than it will in Camera Raw or Lightroom, the two examples that I’ve used here. The reason is that the examples I’ve used are based on Raw data but the histogram displayed on your camera is based on jpeg data. Jpegs are 8-bit. My Canon 5D Mark II Raw files are 14 bit. So, when I preview the histogram on the camera, it gives me more information that previewing the image on the camera but not as much information as viewing the histogram on my laptop.
Additionally, depending on your DSLR, your camera may display separate histograms for each channel of color (RGB) and/or a black and white histogram.