RE: 8 verses 16 bit

["Paul D. DeRocco" <pderocco@xxxxxxxxxxxxx>]

They're not rare as Big Foot, they're quite common: they're images that have
a very wide dynamic range. Consider a digicam picture of a scene that has a
bright sky with some nice clouds in it, and a dark foreground with some
shadow detail, as is common when shooting around sunset. To avoid whiting
out the sky, you _must_ expose for the highlights, since you can't recover
information that's been clipped. This pushes the shadow detail down into
very small numbers that can't be represented unless you use a color space
with a very high gamma, or unless you use more than eight bits per channel.
Since the camera's color space will be optimized for more normal pictures,
with a gamma of around two, you have no choice but to use more than eight
bits. (The camera may be sufficiently noisy that you won't be happy with the
results, but that's a separate issue.)

Once you get the 16-bit-per-channel image into Photoshop, then you can
adjust the Levels to bring up the shadow detail. If all you do is move the
middle Levels control to the left (or grab the middle of the curve in Curves
and drag it upward) to brighten the midtones and amplify the shadow detail,
then you are in effect changing the gamma to a larger value. Having done
this, you can then chop down to eight bits per channel, and do the fine
editing from there.

The same is true of scanning slides with a wide dynamic range. Scan in
16-bit mode, adjust the curves to make optimum use of the dynamic range, and
then chop down to eight bits.

--

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@ix.netcom.com

> From: Preston Earle
>
> The important question is whether there is any benefit to using high-bit
> color. Theoretically there is. Practically, for computer generated
> images and for grayscale images, there is. Practically, for color
> images, the jury is still out. Real-world color images that benefit from
> 16-bit manipulations are as rare as Big Foot--maybe they exist but very
> few people have seen them.

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