While digital photography has many benefits over old school film photography, the histogram is easily one of the most valuable tools we now have at our disposal. In fact, it sits near the very top of my list.
I decided to put together a longer post about the histogram, why it is such a powerful tool, and more importantly, how and when you should use it to take control of exposure in photography.
What Is a Histogram?
At its core, the histogram is simply a graphic representation of the tones in your image. Light tones appear on the right side of the graph, dark tones on the left, and mid-range tones fall somewhere in the middle. Your goal as a photographer is not to force every histogram into a perfect shape, but to make sure the tones in your image are placed where they belong.
For example, the histogram accompanying the image above is made up mostly of mid-tone greens, with smaller amounts of dark and light tones. All of those tones are properly placed within the graph. If you photographed a field of medium green grass, you would expect to see a vertical band running through the middle of the histogram. It does not matter if that band touches the top of the graph. That simply means there is a lot of that tone in the image.
Now imagine a completely different scene. A black raven perched on a dark branch against a background of pale beach sand. That histogram would look very different. Instead of a central spike, you would likely see a U-shaped graph with a spike on the far left representing the dark raven and another spike on the far right representing the light sand, with very little information in the middle.
Controlling Tones Using a Histogram
Back when I shot film, I almost always worked in manual exposure mode using a spot meter. I wanted absolute control over my exposure, and I did not trust the camera to make decisions for me. Today, because I have a histogram and the ability to instantly confirm how tones are being recorded, I am comfortable shooting in aperture priority using matrix metering. That comfort comes only from knowing I can check the histogram and make corrections when needed.
If the exposure is not quite right, I simply dial in exposure compensation.
Why Dial In an Exposure Compensation?
Before talking about how and why to use exposure compensation, there is one very important thing to understand. Your camera’s meter is stupid. It has no idea what it is pointing at. It simply assumes everything it sees should be rendered as a mid-tone.
So it makes a guess, and it guesses that everything it is pointed at is a mid tone, medium green (a football field), medium blue (the blue sky at noon), medium red (a stop sign), etc. This works surprisingly well most of the time, but it fails whenever you point the camera at something that is significantly darker or lighter than a mid-tone.
In the example of the Bufflehead duck, shown above, most of the scene is dark. When the camera metered the scene, it tried to turn those dark tones into mid-tones. The result was an overexposed image where the blacks turned gray and the whites were blown out with no detail. This problem is clearly visible in the histogram, where a spike on the far right indicates blown highlights.
To correct this, light must be removed from the exposure. In this case, pulling about two thirds of a stop brought the tones back where they belonged. How you do this depends on your shooting mode. In manual mode, you can stop down the lens or increase the shutter speed. In aperture, shutter priority, or program modes, you dial in negative exposure compensation.
This pulls light from the exposure, which will darken the image and bring your whites back, and give you nice rich properly exposed blacks.
How Much Exposure Compensation Is Needed?
How much compensation you dial in depends on how dark your subject is.
Let’s say I was making a full-frame image of that black wolf face. My camera would try to place that black as a medium grey. So, I would need to dial in an exposure compensation of minus 1 1/3 stops to hold light back from the exposure, keeping the wolf black.
Knowing how much compensation to dial in is something that comes from experience, but thankfully, we now have a histogram to double-check what our cameras are doing. Because after all, they are just dumb black boxes that let a certain amount of light in.
How To Properly Expose for Light Subjects
The same principle applies in reverse for light subjects. And remember, when you point your camera at a subject, it assumes everything it is pointed at is a mid-tone.
So, when photographing something like Arctic wolves in snow, the camera again assumes it is seeing a mid-tone scene and underexposes the image. The snow becomes gray and the colors look muddy. To correct this, you must add light to the exposure. In manual mode, you open the aperture or slow the shutter. In the automatic modes, you dial in positive exposure compensation.
Knowing how much compensation to apply comes with experience, but the histogram gives you immediate feedback. It confirms whether your tones are being recorded correctly and allows you to make informed decisions rather than guessing.
At the end of the day, cameras are just black boxes that let in a certain amount of light. The histogram is your window into what that box is actually doing. Learn to read it, trust it, and use it deliberately, and your exposures will improve dramatically.
Good Luck and Good Light!
Steve & Nicole