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Hi,
See below
Thanks
Yves
----- Original Message -----
Sent: Tuesday, December 18, 2007 11:53
AM
Subject: Re: OT: 16 bit editing myth or
reality?
I
have a few comments on this topic, then I’ll retreat back to my
lurking/loitering status (hehe).
> I forgot, if as I think PS works with
real number behind the scene, then it should be obvious that only
>
when the original as more then 8 bit, will those extra bit will be maintained
all the way through the editing
> process (16 bit mode only) but when
those real numbers are converted back to 8 bit integers it will be
>
possible that a few pixel value will fall 1 level higher or below its 8 bit
counter part.
It seems to me that the original question is
becoming somewhat convoluted. There’s no doubt in my mind that this is
the case, but this has little to do with the advantages/disadvantages of
editing in 16-bit mode. The following web page gives a nice description
of how PS (and I assume other image apps) handle contrast
adjustments:
http://www.earthboundlight.com/phototips/8bit-versus-16bit-difference.html
Here’s
what happens in 8-bit mode (copied from the above article):
> If an
image is a bit underexposed and lacking in contrast, it may only use values
between 0 and 199
> perhaps (200 out of the possible 256 values). We can
help fix things by spreading those values further
> apart so that they
range between 0 and 255 instead of only 0 and 199, but we have no way to
invent
> new values to fill in the resulting gaps. We still only have
200 discrete values; we've just moved them
> around to stretch all the
way to 255. Worse yet, they are no longer evenly distributed. They can't be,
in
> fact, since all we have are integer values and not fractional ones
(we can use, for example, 128, but not
> 128.5). We'll still have 56
unused values just as we did to begin with, so about every fifth possible
value
> will be left unused (56 out of 256 is about one
fifth).
These “unused” values are what often cause posterization in
prints. On the other hand, 16-bit has 65,536 values per channel.
Granted, PS is only 15-bit, but the numbers are still much, much higher
than 256.
Eric Neilson’s coating analogy nailed it. If you’re
making your image adjustments in 16-bit mode, you have a lot more tones to
spread around, and are therefore much less likely to end up with gaps in your
histogram which will produce a posterized print. This is the real
advantage to working in 16-bit mode.
All
this above is true while in hi bit mode but the question is what happen to
these extra bits when you need to print the image knowing the majority of
consumer printers out there are 8 bit printers? The simple answer is, they go
back to the black hole they came from. The true benefit of editing in hi bit
mode is so small, it is for all practical purpose insignificant on actual
prints but I'll give you this it looks much better while in this hi bit
mode.
Bye,
happy holidays
Yves
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