I've read on my Google search of histograms that "the histogram is not a light meter"…O.K. Yes, technically that's accurate. So, I'll posit that the histogram is BETTER than an in camera light meter! Your camera's meter is a reflectance meter that is programed to average out the world's whites and blacks to a neutral grey. That's why when your in camera meter says you're properly exposing something white--it turns out grey. Conversely, when it says you're right on when metering something black--that turns out grey as well.
I quit using reflectance meters 25+ years ago and have relied on incident light meters my entire professional career. However, when digital cameras reared their ugly little heads many professional photographers still had exposure problems, even using their incident light meters, because the digital sensors, especially in our earlier cameras, had a pretty narrow dynamic range. Where our film could handle a scene with a 10-stop range, from shadows to highlights, our digital cameras could barely handle 4 or 5 stops! Our current cameras are much better now, but many photographers till have problems with keeping highlights under control.
The answer is to turn your histogram into a spot meter of sorts--we create what we call the FACE-MASK HISTOGRAM to nail the exposure on the most important part of the scene--the Subject's Face.
All you need is a real portrait lens set to at least 200mm--since you need to fill your camera's frame with a full face and not get in so close that you start to block light that could be falling on your subject. I usually pick as my subject, when doing groups, the person with the most pale skin tone--often a blond--to make sure I don't blow-out their highlights. So, you merely fill the frame with your subject's face--make sure NO HOTSPOTS in the background intrude into this image--and take a picture. I should interject that by this time I've already established my baseline exposure with my incident light meter and have imported the custom white balance image into my camera--so, this face-mask histogram is to refine my exposure.
After you take this close-up image you review your camera's histogram. This histogram of a face usually looks like a truncated mountain. If your exposure is accurate this mound should be centered within the histogram's window with none of your data-pieces of that mountain--hitting either the left wall (under exposure) or the right wall (over exposure).
In this first example the histogram, with my exposure at 1/250 sec. @ f6.3, is headed for UNDER exposure:
An example of where I don't want my histogram to be, here's one I purposely OVER exposed:
"Til next week….
Author: Jerry W Venz, PPA Certified Master Craftsman
Training site: http://www.LightAtTheEdge.com