Scottes wrote in post #8240222
I'm curious about this device. Why Black, grey, and white? A grey card would set white balance
and exposure. And the grey cards are a heckuva lot cheaper.
You're right about white balance, and if you take an exposure meter reading from the entire card, the black and white tones average out to the same gray level as the gray tone. But the claim is that the three spikes the target gives you on a histogram will tell you something about how to be sure the exposure correctly places those tones.
Personally, I consider it a gimmick. It can be made to work, but it doesn't give you the information it claims to give.
To do that, it would have to be a two-dimensional target--a cube with three tones on each face, or at least the faces visible to the camera. Only the face illuminated by the main light would show a true white (the highlighted white) and only the face illuminated by the fill light (the shadows) would show you the true black (the shadowed black).
If you have only one target turned to the main light, the black tone (being a highlighted black) is photographically a dark gray. If you turn the target toward the camera, the white tone (being a shadowed white) is photographically a light gray.
And in fact, none of those tones is the tone of primary interest for digital exposure. The tone of primary interest for digital exposure is "the brightest highlight that must retain detail" (which would be Zone Eight in Zone System parlance).
That tone must be identified in the scene and placed at the top of the histogram to gain the greatest dynamic range and the least noise the sensor is capable of capturing. The three-toned target doesn't even have that tone.
