Producing a perfect exposure has been the Holy Grail for photographers since the dawn of photography. In times past, taking a properly exposed photo required lottery-winning luck or a degree in mathematics. Ansel Adams developed his zone system to enhance his chances of properly exposing his photos. More recently, hand held light meters and in-camera metering systems made it easier to determine the right camera settings for a given scene. Even still, there was always an element of guesswork and no way of knowing the exposure was correct until the film was developed. Photographers would often burn through tons of film bracketing their shots in hopes that at least one would be exposed correctly. With digital photography there is finally a much improved exposure tool; the histogram.
Today’s digital SLR cameras and even many mid to high-level fixed lens digital cameras have a histogram tool built into their functions. A histogram is nothing more than a basic bar graph. The histogram in digital cameras graphs the light captured when taking a photo. At first a histogram can be a bit cryptic, but with a little practice it is an easy and foolproof way of knowing if you have created a good exposure right after taking the shot.
The basics to understanding a digital camera histogram really are, well, basic. The colors a digital camera can “see” are assigned numbers from 0 to 255. Zero is black and 255 is pure white. On a histogram, the left side of the graph is 0 or black and the right side is 255 or white. The way to know that you have a good exposure is if all the information on the histogram falls within this range.
If any of the information is bunched at the left side of the graph and appears cut off on the left side the shadows have been underexposed and detail in the shadows has been lost.
If a lot of the information is bunched up and cut off on the very right side of the graph this indicates that the highlights have been overexposed and there will be a loss of detail in the brightest parts of the image.
Once either side of the histogram have been “clipped” that data is missing from the image and can not be resotred. The good news is that it is easy to check the histogram when you take a shot allowing you to make the needed adjustments and take the photo again.
Even if all the image information is contained within the boundaries of the histogram, it is preferable to try to keep the bulk of the graph centered or even a bit to the right, as long as the graph doesn’t get clipped off indicating a loss of detail. The reason for this is that darker areas in a digital image are more susceptible to digital noise, especially when adjustments are made. Less noise is introduced adjusting a light image than adjusting a dark image.
Some people make the mistake of thinking that all their histograms need to be perfect bell curves. This isn’t the case since every scene has different proportions of dark, light and all the color in the spectrum. A shot of dark green trees will have more data on the left side of the graph than a shot of snow. The goal isn’t to have the histogram be a perfect curve as long as all the data remains within the histogram range.
So, how do you find your histogram? Consult your instruction manual. If your camera has a histogram it will appear on the LCD viewing screen when accessed. On some cameras it is found within the system of menus. On my cameras I access the histogram by pressing the “Info.” button on the back of my camera while I am viewing an image.