All cameras’ internal light meters are calibrated to read a scene to average out as 18% gray. Around 60 to 70 years ago, when meters were first built into cameras, that value was decided upon by the photo powers that be. In the old days, it usually meant lining up a needle in a circle in the viewfinder by manually setting shutter speeds and aperture. With the advent of autoexposure systems, cameras did that for you, but the meters still read 18% gray. Of course, every scene isn’t that tone of gray, and therein lies the rub to correct exposures. You have to compensate if what you’re photographing is lighter or darker.
In the old days of film, one could carry what was known as a gray card. Usually made of cardboard (though there were some that were plastic) it could easily be carried in your camera bag. To read the light at a scene, all you had to do was to whip it out and, through the camera, take a reading off of the card. While it wasn’t very expensive, my photo instructors taught me an even cheaper way to do it. Instead of the card, all I had to do is take a reading off of my left hand (cameras are right-hand operated, only so the left hand was the only option). Most people’s hands were pretty close to 18% gray. With some testing I found that the back of my hand was about 1-stop darker (depending on how much sun I had gotten) while my palm was about 1-stop lighter. Knowing that, it was easy to compensate and set my exposure.
With today’s digital cameras it’s a much easier affair. Autoexposure systems are much more sophisticated and accurate. Even so, they still can be fooled. But one can easily “chimp” (review) pictures on the camera’s monitor. I do that in conjunction with the histogram function (it’s a representation of the exposure in graph form that can be seen in the monitor). So there’s no longer a need for a gray card, but I found that I still need my left hand, photographically speaking.
There are times where we take our studio lights out into the field for a portrait on location. In the old days, one would use a hand-held flash meter to measure lights’ output and the ratio between two or more lights. Today it’s much easier. I set up my lights and just take a picture of my hand in the place where the subject is going to be. A quick “chimp’ of the monitor tells me if the lights’ placement is correct or if I have to move them. A check of the histogram gives me a more accurate picture of the exposure.
When new methods and technologies emerge, sometimes the old ones fall by the wayside. But there are times when the old ways can be adapted to work with the new.