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View Full Version : Here's an odd White Balance Q for U



erichlund
12-20-2005, 05:51 PM
To set "Preset" white balance on the new D200 (and from my old D70, IIRC), you can use either a white or gray card. Of course, the card should be a standard 18% gray or as true a white as possible. But, the process to register white balance isn't differentiated based on what you used. How does the camera know whether you used white or grey? Under certain very odd lighting conditions, I could see a case where the camera would not be able to tell.

Admittedly, the incidence when this might happen is probably as rare as snow in Honolulu, but it would be nice to understand how the camera would determine if it's looking at white or gray, so as to understand when you might want to choose one or the other based on the current conditions.

Cheers,
Eric

tekriter
12-21-2005, 03:55 AM
Actually, I don't think it makes any diference. Both shades, white and gray, are tonally neutral with no color shading.

erichlund
12-21-2005, 06:49 AM
So, white, black, and any of the intervening shades of neutral gray should work is what you are saying. Which would indicate that it does not set levels, but merely balances the levels by providing offsets from a neutral position. That makes sense.

Of course, this would indicate that a channel which is offset would then be limited in the direction of the offset. You can't go beyond 0 or 256. I suppose there's more to it than that, but I sure don't know.

Cheers,
Eric

Balrog
12-21-2005, 10:52 AM
Well, the way I see it, a pure white or a pure grey card should theoretically show up exactly the same in a picture, since the camera will expose them to the same "brightness" level (since, as you said, it doesn't 'know' white from gray).

Thus suppose it's aiming for (128,128,128), if any of the color channels fall away from that value, it'll know how much it needs to balance them. The color of the card shouldn't make any difference.