Histograms are your friend and yet they’re one of the most under-utilized features on high end digital SLRs. Some people don’t know about them, others may not know their camera has that capability.
Most cameras have two types of histograms: Luminosity and color. Today I’m going to focus on luminosity and follow up with color in another article, although color also plays into the overall calculation because the human eye is more perceptive of colors in the green range of the spectrum.
At a simple level a luminosity histogram shows the distribution luminosity values from darkest to lightest. Vertical spikes in the graph show the distribution of brightness values in a particular scene.
After looking at a few histograms you start to get the hang of it. Dark values will cause spikes at the near end, light areas at the far end, mid-tones along the center. After a while you don’t even need to see the photo to have a good idea of what it looks like.
The distribution of luminosity values can tell you a lot about the quality of light and how to move exposure more toward the center of the dynamic range.
If the values are dark, you can add a fill flash or other lighting, essentially moving the darks farther toward the middle of the range. If the values are too far to the light side, you can add a neutral density filter to knock down that light end, scrim over your subject, or just wait for better light. DSLR video shooters used to this more than DSLR still shooters because for video you’re limited in the range of shutter speeds you can select and have to use ND filters to get the f-stop you want.
Another option for scenes with really tricky exposure is cheat the dynamic range with HDR. Taking multiple exposures, each optimized for a particular segment of the scene, and combining those elements in post-processing.
Luminosity is one of those subjects that, while it’s easy to grasp the concept, it can take time to master the subtleties. So don’t try to get it all in one day, just remember to call up that histogram display and watch it change as you change exposure values.
Over time you’ll get quite good at tweaking exposure, based only on the histogram!