Digital BW, The Print

Paul D. DeRocco wrote:

>>From: Ernst Dinkla
>>
>>Quotes:
>>
>>Can anyone tell me if there is a standard consensus about what Lab
>>Luminosity value a Kodak 18% grey card should yield? I know there is a
>>formula for calculating L* from density. I saw it once but couldn\ufffdt
>>understand it. But seeing it showed me that L* could be directly
>>translated into density. While testing my camera, which I suspect is
>>slipping off the mark, I discovered that the best exposure of a Kodak
>>Q14 greyscale target is not the same exposure that produces L*54 in a
>>2.2 workspace or L*61 in a 1.8 workspace after conversion through Camera
>>Raw. Now I am wondering if the Kodak 18% grey card is applicable to
>>digital capture. Is there another reflective grey value that is used to
>>calibrate digital cameras and if so, is it standard? Or is something
>>else going on?
>>    
>>
>
>The gamma has no effect on this relation. 18% gray is L=50 (more precisely
>49.5).
>
>  
>
>>Bruce Lindbloom calculated that gamma 2.46 pegs 18% at L50
>>    
>>
>
>A gamma of 2.475 means that a midscale value translates into 18% gray. A
>gamma of 2.44 means that a midscale value equals L=50. So if you accept that
>18% or L=50 represents the eye's idea of medium gray, a gamma of somewhere
>around 2.45 would be about optimum.
>
>However, 2.2 is close enough for rock'n'roll. In addition, sRGB has a linear
>segment at the low end, very much like the linear segment in the Lab curve.
>
>--
>
>Ciao,               Paul D. DeRocco
>Paul                mailto:pderocco@...
>  
>
Paul,

I should have skipped the question that led to that thread. It wasn't 
the 2.46 Gamma or the digital camera behaviour (often overexposure 
though) I was interested in but the selection of gammas made in view of 
the photographer's expectation of the output.  That is quite an 
arbitrary choice and will vary much among users. This subject has been 
covered in other discussions like Mike Chaney's replies on our request 
for the support of the Lab profiles in Qimage. He couldn't accept the 
fact that a QTR Lab space could be assigned to a scan of whatever 
origin. Mainly because his view is that all image data now has an 
embedded space and needs to be converted to another space if that space 
is preferred. That the actual editing in PS for a B&W scan also includes 
gamma and contrast steps + more subjective manipulation than in color 
and by that bending the original data anyway is something he accepted in 
the end. In color this would be a much more dangerous approach. The 
messages in the scanner lists about the import of RAW B&W scans in PS 
are plenty though. That Roy gave us the Lab space to work in is the real 
step forward, the gamma issue is nice for long threads.

Ernst