Understanding Exposure in Photography
Tip 3 - Histogram
The Scientific Meter
Learning about the histogram is a little daunting at first. But stick with me here. The histogram is a feature well worth spending a few minutes to grasp. Being able to read the information in a histogram will make you a master of exposure.
The histogram is simply a graph that displays the tones in your photograph. The graph runs from dark to light in 255 steps. The dark areas (shadows) are graphed on the left and lighter areas (highlights) to the right. The far left of the scale is perfect black and the far right is perfect white.
If you over or underexpose your photograph, pixel data will run off the chart to the right or left. This is called clipping. And it is bad. In a good exposure, all of your graphed pixel data will stay on the chart. Information that goes off the scale to the left or right is basically lost data (pixels to the right will be recorded as pure white and those to the left as pure black). So try to keep it all on the chart.
On your camera's LCD screen, the histogram can be checked to ensure your exposure settings were right for the given photograph. When I start to photograph in any lighting situation I'll select my shutter speed and aperture settings and make a test exposure. Then I look at the histogram of the test on my camera's LCD screen. The histogram gives me a perfect indication of where my exposure is. If my highlights or shadows are clipping, I can make a quick adjustment to perfect the exposure. Then, until the light changes, I don't have to think about exposure. I know it's dead-on.
The Typical Histogram
The typical histogram will have a bell curve in the middle (the midtones), as most tones in photographs are average tones. On the left side of the graph, the shadows should drop off without hitting the edge of the graph, as should the highlights on the right. Anything going off the edge of the graph is tone information clipped off the scale, usually caused by an incorrect exposure setting. Again, a proper exposure generally keeps all of the graph visible in the scale, avoiding clipping.
Right and Wrong
Above is an example of the histogram of an overexposed photograph. Notice how the highlights at the right of the graph scale up and off the chart. And the shadows at left trail off too far from the edge (meaning I don't have any black in my photograph).
Below is an example of the histogram of an underexposed photograph. It's the same problem, only reversed. While this photograph doesn't have much in terms of highlights, underexposing means we've lost a lot of shadow detail.
Now look again at the histogram on the flower photograph at the top of this tip. You'll see how the tones in that correctly exposed histogram are smooth and mostly fall within the edges of the graph.
On the Computer
Most digital imaging software will display histograms, and slider controls usually appear below the histogram for adjusting the tones in the image. If you move the shadows slider, everything to the left of it will register as completely black. If you move the highlights slider, everything to the right of it will register as completely white. The midtones slider will affect the overall brightness.
In the days of the darkroom, one of the first lessons to learn was how to get a true black in your print. It wasn't intuitive, and many photographers will remember being sent back into the darkroom to try again until they successfully completed the lesson. With the histogram, you can instantly achieve that true black, as well as a true white. And the graph of the histogram gives you mathematical proof.
Protect Your Highlights
If you have to lean one way or the other in exposure, always protect your highlights. Don't let them fly off the right side of the histogram. You're better off slightly underexposing if you have to. Detail in overexposed highlights is lost and basically unrecoverable. Your shadows are much more flexible and can be lightened to recover lost detail.