Re: [APML] Going to the Luminance side?

[Jerry Lodriguss <jml@astropix.com>]

 >The information content in an image is equal to 2 to the power of the
 >utilized bit depth times the number of pixels. Negative color film is
 >limited to 8 bits (8 photographic stops) of dynamic range per channel.



Hi Matt,

Bit depth is not the same thing as dynamic range.

Bit depth describes tonal resolution, how many steps of tone there are 
between black and white. As Chuck has pointed out before here, you can have 
any dynamic range broken up into any bit depth. You can have a particular 
dynamic range broken up into 256 steps, which would be 8 bit depth, or you 
can have that exact same dynamic range broken up into 1024 steps, which 
would be a bit depth of 10 bits. You could even have it broken up into only 
2 bits of bit depth and there would only be four steps between black and white.

Negative color film is not limited to 8 bits "of dynamic range" (really 
meaning 8 bits on tonal depth) per channel, that is just what most output 
devices work at. Of course, CCD has this same limitation for output 
devices, it must be reduced to 8 bits also. You can scan color neg film at 
any bit depth of tonal resolution that a scanner is capable of, for 
instance, the Nikon LS2000 is capable of 12 bits of tonal resolution per 
color channel, and you can import this information, at this bit depth, into 
Photoshop.

8 bits does not equal 8 photographic stops of dynamic range.


 >After reduction, a good color CCD image typically has content at a depth of
 >10 bits (10 photographic stops) of information per channel. The 2 additional
 >bits of information mean that the information density (per pixel) of CCD is
 >4 times higher.



First, 10 bits of bit depth does not mean 10 photographic stops of dynamic 
range.

As far as information content, not even counting the issue of noise, if the 
CCD image is reduced to 8 bits of tonal resolution for output (and it 
must), and the total number of pixels are the same as the film image, then 
the "information content" would be exactly the same in the final image.

Jerry

Photoshop for Astrophotography Book:
http://www.astropix.com/APBOOK/0_PROMO/PROMO.HTM

Astrophotography, Tips and Techniques
for Digital Enhancement in Photoshop:
http://www.astropix.com

--  APML Archives at <http://astro.umsystem.edu/apml/>  ---
             Unsubscribe at <majordomo@seds.org>