Hi,
I have few thoughts on white balance with DSLRs, and I would like
to discuss them to hear your opinions.
I have two sort-of questions:
1) White balancing is IMO done by increasing the gain of R or B
channel. At least DCRAW and most OEM raw conversion
software works in this way. I don't know, however, if in-camera
jpeg or raw WB is done after the AD conversion, or somehow before
it. If it was done before (on the _analog_ signal), setting
correct WB in camera would be better than just correcting it in raw
conv. software in computer. If it was done on the already AD
converted signal (_digital_, quantitised signal), there would be no
difference between in-camera WB and raw conv. software WB correction.
(of course, in-camera WB is still useful when shooting JPG).
In the first case, correctly setting WB in camera would probably
leave the RAW file less colour noisy than with correcting wrong WB
in raw conv. software.
Anybody knows more? Or the answer? Or a way to test it? This is
pretty important if one wants the cleanest possible files.
2) IMO, every chip & its RGB filter set is calibrated for one type of light,
probably D5000 or D6500 daylight. If the scene is lit by another
light source, the camera (software) must increase the gain of some
channels to get the balance right. This means, when shooting under
tungsten light with ~3200K, setting the WB (either in camera or in
software, see [1]) for 3200K will mean it (camera or software) is
increasing the gain of the BLUE channel to make it look like ~5000K
lit scene. As all the channels have fixed quantum efficiency,
increasing gain means increasing noise as well. Increasing noise in
the blue channel, in our example. Thus, an indentical scene lit by
5000K light in one case, and lit by 3200K light but balanced by the
DSLR to 5000K light in the other case... the second case will have
quite more blue channel noise! Right?
The point I want to make, WHITE BALANCE is not miraculous. For best
results, we should use light temperature conversion filters on the
lens or on the light to bring the light temp back to the one the
camera is calibrated for (probably, daylight ~5000K). Otherwise, we
get lesser image quality in way of increased colour noise in some
channels.
It works the same with colour negative film. You can correct it
during printing, but at the expense of some quality.
So far, my real world experience seems to support these ideas. The
shots I have under tungsten light have more blue channel noise,
than the ones under daylight.
Of course, we can decrease colour noise digitally, but even with
"smart" software, the result is the well-known sport-photography
bleached colour details look. Small colour details like subjects'
eye colour, pink lips, etc... all lose more or less saturation as
the "smart" filtering software mistakes them for noise.
If anybody disagrees, please write me. I would be glad to be
proven wrong on this issue.
Good light,
Frantisek Vlcek
