Re: Bruce and Dan are both right: was: 16 bits = 15 bits in Photoshop?
Re: Bruce and Dan are both right: was: 16 bits = 15 bits in Photoshop?
- Subject: Re: Bruce and Dan are both right: was: 16 bits = 15 bits in Photoshop?
- From: Ray Maxwell <email@hidden>
- Date: Sun, 17 Apr 2005 17:05:21 -0700
This has been a very interesting thread.
However, I think I can show why Dan and Bruce come to different
conclusions on this subject. In a word...it is noise.
If you have an image with very low noise (ie. the noise does not appear
in the least significant bits of the most significant 8 bits) you can
make rather large moves and see the posterization that Bruce refers
to. If you do the same manipulation in 16 bit you will not see the
posterization.
If you have an image that contains noise in the two least significant
bits of the most significant 8 bits this noise will cover up the
posterization when you do moves in either 8 or 16 bit.
So, if you have scanner or digital camera with very low noise follow
Bruce's advice. If you have the typical scanner or camera that does
have noise in the data you can follow Dan's advice and you will not see
any difference.
If you want to see this for your self try the following experiment.
1. Create a vignette in a 16 bit file using two grays that are less
than 20 units apart. Be sure you have dither turned OFF.
2. Save this file.
3. Convert this file to 8 bits.
4. Save this file.
5. Open the 16 bit file.
6. Open the "Levels" dialog and set the black pointer to the low end of
the histogram and the white pointer to the top end of the histogram.
7. Repeat the same adjustment on the 8 bit file.
You will see that the 8 bit file is posterized. The 16 bit file is smooth.
Now repeat this same experiment except that when you make the vignette
make sure you turn the dither ON. This introduces noise into the vignette.
Now you will get smooth vignettes in both 8 or 16 bit files.
I have done this same experiment with images from my digital back. It
has low enough noise that I get banding in the blue sky when I do this.
I will try to get some space on a web site so that I can upload the
images that demonstrate this.
Thanks,
Ray Maxwell
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