On Thu, January 4, 2007 12:37 pm, Rick Drew wrote:
> Thanks - I always wondered about that - I had that happen in a really nice
> blue sky and spent a lot of time cleaning it up. I never thought about it
> being the being the bit depth, especially since the original was 8 bits!
There are two factors that Hans mentioned, and both can have major impacts
on image quality.
Bit depth - it's highly dependent on subject matter *and* the extent of
the edits you perform. For properly-exposed material it's quite possible
to get great results working only in 8bits per channel. There's also a
tradeoff here with color space gamut - the wider the gamut your color
space, the more likely you are to need to work in 16 bits per channel. So
someone working only in sRGB may do fine (with somewhat reduced gamut) and
not see much if any banding working in 8bits, but the same image
originating in ProPhoto or another wide-gamut color space may well cause
banding much more easily. Banding and other negative effects of limited
bit depth are easiest seen in areas of subtle gradations as Hans says.
Compression - this is a toughy. if you're outputting to a compressed
format like JPEG, in all likelihood you're outputting 8bits per channel at
the most to begin with. But the real problem is that the nature of lossy
compression will often make it hard to preserve things like subtle
gradations - skies, walls, etc - without them chunking up into visible
artifacts. This is less related to bit depth than compression strategies
though, and there are ways to minimize it. For instance, adding a bit of
noise in a nice sky or other gradation will actually *help* in many cases.
Note that, inherently, minimizing these artifacts typically means
increasing your target file size. There's no such thing as a free lunch!