Re: 16 bits = 15 bits in Photoshop?
Re: 16 bits = 15 bits in Photoshop?
- Subject: Re: 16 bits = 15 bits in Photoshop?
- From: Ray Maxwell <email@hidden>
- Date: Thu, 14 Apr 2005 11:13:14 -0700
I have been following this discussion with great interest.
However, lets get real.
Those who have read books by Dan Margulis or his Colortheory forum know
that he believes that 8 bits is all you need to reproduce smooth,
non-posterized images. While I don't agree with him and use 16 bit per
color all of the time, I do think that 16 bits is severe over kill for a
2.2 gamma space that is approximately linear with respect to
perception. This statement assumes that the final reproduction is going
to be on reflective media.
Lets work though this.
Let us start in Lab.
1. We know that the typical person can just perceive a difference in
color when we move 1 unit in L*.
2. We know that in typical reflective media there is no common paper
that can produce a white that is brighter that L* = 100.
3. We know that in typical reflective media there is no ink that can
produce a black that is darker than L* = 0.
Now to be safe let us chose to quantitize the dynamic range of this
media into 256 points. This is more than enough to produce smooth tonal
reproduction. This is more than 2 1/2 times the steps that the typical
person can perceive.
Please note that I am not talking about RAW digitization of the signals
out of a CCD. Here the data is linear with respect to the number of
photons that fall on the array and not human perception. Here it is
important to be digitizing to 12 or 14 bits before the data is converted
into a color space like Adobe RGB98. It is also important to work in 16
bit if you are making large adjustments to your data that involve
multiplying the data values. If you do not have enough bits you will
get visible posterization.
In summary, if we have a dynamic range of 100 L* and we digitize to 256
levels or more, we should get smooth reproduction. Each time we add one
bit we double the number of steps we have digitized. 15 bits is 128
times smoother than 8 bit data. It contains 128 times as many steps. I
think this is more than adequate for anything I plan to do.
I use 16 bits for manipulation of data from my Leaf Valeo 22 back. It
digitizes to 14 bits and has a dynamic range of 11 stops. The prints
have very smooth tonal graduations.
I can't imagine needing 32 bits and know that there is no A/D on earth
that can create digital data to that resolution without most of the
lower bits being noise.
Ray
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