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Subject: Re: raw 3.1 professional custom profiles for highend digital cameras

Jeff and Bruce,

This is fascinating, but the scales are lifting from my eyes only very slowly. I need help. I must ask for your indulgence while I try to bring this discussion down to a level that I, as a layman, can understand.

If I get what you are both saying, the old incident light meters, such as my LunaPro SBC, are designed to compute exposure based on the tone response curve of film, which curve revolves around what in the Zone System would be Zone V, which would be a "proper" exposure for an 18% (some would say 13%) Gray Card. Thus, the reading provided by the meter assumes the tone compression and distribution exhibited by film. This means that, if Zone V is properly exposed, the other zones will fall into place on the curve exhibited by film. If I further understand you, the film curve reflects a gamma that mimics human vision (something around 1.8 or 2.2?), all of which assumptions, etc., are built in to the manner in which the old meters compute exposure.

Therefore, if one attempts to compute exposure for a linear capture device, that is, the DSLR, using an old incident meter, those values that we call Zone V values will be placed far down on the 0-255 scale (you say around 50). This means that highlights will fall far lower on the scale than is desirable for digital capture. In other words, the highlights in the shot will be underexposed, at least if one goes by the numbers. Am I correct so far, or have I missed something critical?

If I am right so far, I am confused because my experiments do not seem to bear this out. If I meter on JUST a gray card, I get the same exposure using either the LunaPro (with and without the spot attachment) as I do using the DSLR’s meter. (I use a Nikon D70; either on spot or matrix metering mode).

I don’t understand how that would be so, if my understanding of the concepts that I described above is correct. Shouldn’t the LunaPro be giving me a shorter (less light) exposure? Are they the same because they are both centered on an 18% (or 13%) gray, but they will measure tones above and below differently because of their respective gammas? So, in attempting to place all scene tones onto their respective curves (when I meter the whole scene and not just a gray card), they will give different results? Is this why you are saying to meter and expose for the highlights in digital capture, not the mid tones (in addition to the other reasons related to linear gamma capturing 50% of the data in the first stop)?

Moreover, if I meter on a more complex subject, (such as the 24-patch Macbeth Color Checker), the exposures between the devices are different, and the LunaPro suggests an exposure that it LONGER than that recommended by the camera. Again, I would have expected the LunaPro to suggest a shorter exposure. What am I missing?

Regardless of the reasoning, it seems that you both are saying that one cannot really use the incident meter built for film in attempting to determine exposure for digital capture. If there is an explanation for this that I can understand, I would love to read it. If it is too complex, just tell me so, and I will take it on faith.

Is there any way to use the old meter for digital, or is it simply the wrong technology now?


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