Although everyone has their own preferences and shooting styles, you can optimize your photographic imaging by understanding what a light meter and histogram can show you in the four scenarios presented here. Virtually every common shooting situation is represented, and each example shows proper exposure as determined by a light meter, as well as an interpretation of the accompanying histogram.
(1) Avoid subject failure and expose for proper high-key and low-key scenes.
The most common problem of built-in camera meters is subject failure. It caused the exposure system to try to "average" the reflected value of the tones of these two scenes, Figures 7-8. The left halves show that the results of accepting the camera meter settings would have been underexposure (trying to make the white plate middle gray) and overexposure (trying to make the black plate middle gray). Proper exposure (right halves) was achieved using a handheld incident meter that measured the light falling on the subjects and determined that both subjects required the identical exposure.
The histograms, while inadequate for determining proper exposure, show the bias in tonal distribution and give a good indication of how high-key and low-key subjects will reproduce.
(2) Control multiple light sources, set ratios, and adjust tonal range.
When working with more than one light, a meter is essential to evaluate and compare each light source to determine both proper exposure and the effect that each light will have on all parts of the scene, Figures 10-15.
The histogram is useful to gain an understanding of how the overall tonal range will reproduce and to alert when extreme contrasts (pixel blocking on right or left sides) could cause a loss of image detail. However, as it is very difficult to interpret the visual effect by observing the respective curves, the histogram is not a suitable guide for positioning and controlling the power of each light source, especially when making small 1/10th f-stop adjustments.
(3) Balance ambient and flash on location.
Even with the best TTL systems, proper rendering of flash and ambient exposure can be unpredictable, especially when subjects are not "average" and backgrounds are bright or non-reflective. A handheld meter ensures accuracy by separately measuring both kinds of lighting.
Histograms do provide an interesting view of the distribution of tones, and indicate differences in the effective contrast of the lighting. However, there is very little useful information about how the ambient and flash relate to each other in terms of brightness (ratio) or optimum, combined exposure.
(4) Avoid under- and overexposure.
Photographers who learned their craft by underexposing slide film to pump up the saturation should pay attention to Figures 19-23, which show only subtle changes in the histogram when compared to the metered f-stop value and resulting image. Underexposure equals image loss. Relying on a handheld meter, instead of the histogram, ensures accurate, repeatable exposures.
Photographers who learned on negative film, routinely overexposed, and let the lab compensate should pay attention to Figures 23-27, which show only subtle changes compared to the metered f-stop value and images. Overexposure equals image loss. Relying on a handheld meter instead of the histogram ensures accurate, repeatable exposures.
These photographic results show how even a half-stop variation in exposure can cause a rapid blocking of tonal information. Histograms show this in a much more subtle way, but it is difficult to impossible to see them on the average DSLR panel. And as there is no detailed horizontal scale (even in most software, Figure 28), it's hard to visualize and gauge that the error in each exposure here equals a half stop. So there's almost no chance you'll be able to use a histogram to fine-tune settings to get within the ideal 1/10th of an f-stop.