From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 07/20/02-05:51:30 PM Z
At 07:02 PM 07/20/2002 -0300, you wrote:
>DEAR RICHARD,
> Again, the question of resolution (discerning fine detail) and acutance
>(perception of sharpness resulting from exaggerated edges) are two different
>things...and edge effects, whether produced by pyro or any other developer,
>do enhance acutance.
> Another reason that tanning developers are not used in some applications
>is that the tanning creates phase shifts (on the microimage and
>microdensitometric scales) due to the different thicknesses of the emulsion
>and this can distort results.
> CHEERS!
> BOB
Yes, there is both an actual mechanical displacement of the gelatin and a
difference, although slight, in the index of refraction due to tanning.
In general, the conditions which lead to increased edge effects lower
resolution. Whether this is of concequence depends on the application. Its
probably of no great importance in pictorial photography. Acutance is the
contrast at a a sharp transition of density. The eye perceives high
contrast at edges as sharpness. A high resolution image with low edge
contrast will be seen as less sharp than one of lower resolution but higher
edge contrast, as I am sure you know already.
Non-tanning developers produce edge effects because of the local action
of developer reaction products. The size of the edge effects vary depending
on the type of developing agent, the amount of sulfite or other
anti-oxidant, and the diffusion rate in the emulsion. Lack of agitation
tends to increase the concentraton of reaction products since they are not
removed as they diffuse to the surface but even well agitated developers
produce some.
Edge effects are desirable in some pictorial applications but death to
other photographic applicatons such as photographic sound recording.
---- Richard Knoppow Los Angeles, CA, USA dickburk@ix.netcom.com
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