[pure-silver] Re: what are resolution, accutance, and sharpness?

  • From: Claudio Bonavolta <claudio@xxxxxxxxxxxx>
  • To: pure-silver@xxxxxxxxxxxxx
  • Date: Thu, 13 Sep 2007 15:22:39 +0200

----- Message d'origine -----
De: Shannon Stoney <shannonstoney@xxxxxxxxxxxxxxx>
Date: Thu, 13 Sep 2007 06:52:09 -0500
Sujet: [pure-silver] what are resolution, accutance, and sharpness?
À: pure-silver@xxxxxxxxxxxxx

>After I wrote my earlier message, I spent some time exploring the  
>Ilford site and starting dreaming about negatives processed in  
>Perceptol.  Maybe FP4+ in perceptol...  Or even making Delta 3200 look  
>less grainy by using Perceptol.
>
>When I was talking to my partner about the information on the Ilford  
>site, we suddenly realized that we had no idea what resolution,  
>accutance, and sharpness are, as opposed to fine grain.  Can you  
>explain this, and what lp/mm means?
>
>--shannon

Roughly explained, acutance and sharpness are subjective notions indicating how 
many details from a subject are reproduced by a film or lens.
Resolution is a physical measure of this number of details often expressed in 
lp/mm (pairs of lines per millimeter).
There are various ways to measure resolution, some examples:
http://www.normankoren.com/Tutorials/MTF5.html
Actual systems to measure lens resolutions are usually based on MTF curves:
http://www.normankoren.com/Tutorials/MTF.html

Resolution measures apply to both films and lenses.

On the subjective side, contrast affects greatly the impression of acutance and 
sharpness: you can easily find a low contrast print with lots of details that 
seems, at first sight, less sharp than a more contrasty one although it has 
less details.
This phenomenon is used by some developers that improve locally the contrast 
between dark and light areas by an edge effect.
A well-known developer of this type is Rodinal.
Often such high-acutance developers increase grain which may be a concern in 
35mm.
On the other side, some very fine-grain developers give a pretty soft image 
with low local contrast which seems less sharp.
Some others, say D-76, X-tol and the likes try to make a satisfactory balance 
between fine grain and acutance.

Claudio Bonavolta
http://www.bonavolta.ch
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