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[OM] Scanning B&W photos and the Callier Effect

Subject: [OM] Scanning B&W photos and the Callier Effect
From: Joe Gwinn <joegwinn@xxxxxxxxx>
Date: Sat, 14 Sep 2002 12:48:55 -0400
All the stories of the problems that some kinds of transparency scanners have, 
especially with black and white negatives, brings to mind a debate from the 
1970s and 1980s, the one about the relative advantages and disadvantages of 
condenser versus diffuser lightsources in photographic enlargers.

Condenser enlargers are basically Kohler designs, but done sloppily (an Abbe 
design using crude and thus astigmatic lenses).  This yields very directional 
light coming through the negative, and thus increases contrast (the Callier 
Effect) and is quite sensitive to scratches et al.  

The Callier Effect is caused by the directionality of the light.  In a Kohler 
system, scattered light is lost.   The denser the negative, the greater the 
absorption, and the greater the scattering loss.  In a diffuser system, 
scattered light isn't lost, it's replaced by light from other directions.

Diffuser enlargers may have much the same condenser lenses, but have a piece of 
opal or frosted glass between the last condenser lens and the negative; or use 
an inherently diffuse light source right from the start, and omit the condenser 
lenses.  Because the light in a diffuser enlarger isn't directional, contrast 
isn't enhanced, and scratches are far less noticable.  The cost is that much 
light is lost in the diffusion process, either slowing the printing process as 
exposures must be longer, or requiring a far larger lamp (and thus more 
cooling, etc).

The late Fred Picker of Zone V Labs in Vermont was a very vocal proponent of 
diffuser enlargers, and Zone V made a replacement for the lamphouse and 
condenser assembly of many enlarger models.  This replacement contained only a 
cold light source (a white "neon" tube that zig-zagged to make a sheet of 
glowing white tubing).  By all reports, this replacement source worked very 
well.  (For black and white only; the cold light source was not color balanced. 
 But Fred abhored color anyway.)

So, given the above history, I suspect that we are repeating history, now with 
scanners.  If I have followed the threads correctly, the Polaroid 4000 uses a 
diffuse light source, while many or most others use condenser (Kohler) 
illumination systems.

The difference between color film (negative and transparency alike) and 
traditional black and white film seems to be that because the silver is removed 
from color film during development, there is far less scattering (and thus 
Callier Effect) with color film than with traditional black and white.  
Likewise, color has soft clouds of transparent dye rather than random clouds of 
black particles (the silver grains) so a scanner that's satisfactory for color 
may well fail on black and white film.  (Presumably, chromogenic black and 
white film will scan like color film, as that's exactly what it is.)

Turning the argument around, a scanner that does well on traditional black and 
white film is likely to be excellent on color film (ignoring color balance and 
the like).


Joe Gwinn

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