Quote Originally Posted by Diapositivo View Post
Let's say I have two lamps. By using the dome, and pointing from both sides of the face to the camera, I get those values:

I lamp: 1/125, f/8;
II lamp: 1/125, f/11;

Those measures tell me both what the average exposure would be (1/125, f/9.5) and the illuminance ratio (2:1).
So why would somebody need the flat thing if the with the dome thing one can kill both birds with one stone?

The dome can be used for ratio inspection. Shouldn't there be some application in which the flat thing is necessary? Or more reliable? Which is the reason for the flat thing to exist?

Actually, IMO there is absolutely no point in figuring or finding an average reading.

The lighting on the main, the most important side of the subject, needs to match the camera setting.

That camera setting places the important parts of the subject's "zones and tones" in all the intended places.

At that point your camera and main light settings, are "etched in stone".

The measurement and adjustment of lighting on the the other side of the subject is done "after-the-fact" to control the ratio.