How does digital process white balance?

Most digital cams have the ability to set the WB for sun, tungsten,
etc.

What actually happens in the camera when you set tungsten, for example?
Is the blue channel amplified or reds/greens cut back? Combination of
the two?

In article . com,
"phk" wrote:

Most digital cams have the ability to set the WB for sun, tungsten,
etc.

What actually happens in the camera when you set tungsten, for example?
Is the blue channel amplified or reds/greens cut back? Combination of
the two?

Exactly.

And the reason it's better to do this in the camera than try to fix it
later is that the camera's sensor returns typically 12 bits per colour
(i.e. 4096 levels) and this is reduced to 8 bits per colour (256 levels)
in the JPG that is saved to tthe card. SO there is quite a bit of scope
for adjusting the level of any one channel by up to a factor of 16
wthout causing a decrease in image quality.

Or, you can save the raw data and do it later in Photoshop or your
camera's software. In which case the camera's white balance and
saturation and sharpening and so forth settings are ignored.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------

Thanks for the explanation. Would anything be gained by using a color
filter?

No...not really. You are as likely to make it worse as better. If storage is
the issue then shoot JPG and find out which situations where your camera can
be trusted....unless its tricky lighting then use RAW and fix it later.

--
Thanks,
Gene Palmiter
(visit my photo gallery at http://palmiter.dotphoto.com)
freebridge design group

"phk" wrote in message
oups.com...
Thanks for the explanation. Would anything be gained by using a color
filter?

"phk" wrote in message
oups.com...
Thanks for the explanation. Would anything be gained by using
a color filter?

Yes, assuming it is matched to the sensel (with Bayer CFA)
sensitivity. You'd ideally optimize (maximize without introducing
clipping) channel exposure, or at least prevent the need to boost a
channel's signal in post-processing. Boosting in 'post' increases
noise and quantization errors. There may be a need to create a special
camera profile for accurate color.

Whether the effort is worth it, depends on the specifics of the
situation.
When cameras start to adjust the analog gain based on white balance,
the differences will become smaller.

In general, I'd recommend using color conversion filters (e.g. 80A or
80B) when e.g. shooting in tungsten light, because it reduces the
noise in the blue channel, although it will increase overall exposure
time with some 2 stops.

Bart

Bart van der Wolf wrote:

In general, I'd recommend using color conversion filters (e.g. 80A or
80B) when e.g. shooting in tungsten light, because it reduces the noise
in the blue channel, although it will increase overall exposure time
with some 2 stops.

....and if you increase your ISO to compensate for these 2 stops
you'll increase noise in ALL the channels.

BugBear

"bugbear" wrote in message
...
Bart van der Wolf wrote:

In general, I'd recommend using color conversion filters (e.g. 80A
or 80B) when e.g. shooting in tungsten light, because it reduces
the noise in the blue channel, although it will increase overall
exposure time with some 2 stops.

...and if you increase your ISO to compensate for these 2 stops
you'll increase noise in ALL the channels.

No need to increase ISO, just expose longer...

If camera shake becomes an issue, then by all means choose between
turning up analog gain (ISO) or under-exposing the blue and green
channels. Turning up ISO should give better results than
postprocessing to compensate underexposure.

Bart