Hi, Drew,
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Originally Posted by drew  Here is another example of the advantages of the greater light transmission of COLORRIGHT. |
Interesting examples.
I note that with a neutral background conveniently in place, "from the camera position" WB measurement can be expected to do a good job if we have a relatively-narrow acceptance pattern (as with the ColorRight diffuser). This is in fact a situation (we can call it the "giant gray card" approach) in which there is an explainable advantage to such a narrow pattern.
Certainly next time I have a subject in front of a neutral background, I will keep that in mind and put my trusty Color Parrot 1.2 into use (assuming I feel impelled to use a diffuser at all).
However, actually, in that setting, we should expect to get a fairly good WB measurement with just the naked lens, zoomed to include as much of the background as possible and as little stuff outside it as possible (and maybe put as out-of-focus as possible).
I don't suppose you made a comparison measurement with that technique, did you?
In any case, a "cosine" diffuser (such as the ExpoDisc diffuser), intended for use "at the subject", can't be expected to give a good WB measurement "at the camera position", even in this setting. That's why they are not recommended for such.
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But, even more importantly, it can lead to large differences when used with a flash. The meter reading doesn't really know how to account for the amount of flash that will be added back into the shot. Are you bouncing the flash? How high is the ceiling? How far away is the subject, really?
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I'm having trouble quite following that story, and why it is that a greater through transmission is helpful in this regard. Note that the meter (both for flash and ambient components) reads through the lens (and diffuser, if in place), and so the photometric exposure for metered operation will not be affected by through transmission (any more than it would by difference in the f/ number).
And the relative contributions of reflected incident light and reflected flash light (in the light whose chromaticity is measured by the camera) will not be changed by differences through transmission.
Of course, as you pointed out earlier, the difference in through transmission can effect whether the camera (Nikon) is willing to use the "exposure" for chromaticity measurement at all or not. I assume that in your examples here, the camera didn't complain in either case.
The bottom line is that, in my opinion, this demonstration did not in any way illustrate an advantage of greater transmission, but rather showed that we can make a meaningful WB measurement "from the camera position" if we have a neutral background, but only if we use a diffuser with a narrow acceptance pattern (or, perhaps, no diffuser at all).
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