[pure-silver] Re: 35 MM T-MAX 100 IN PYROCAT?
- From: "Jacques Augustowski" <py1hy@xxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Thu, 29 May 2008 16:24:41 -0300
For me Richard sintesized perfectly de definition of acutance and the notion of
sharpness without going into line pairs/mm and etc...
----- Original Message -----
From: Richard Knoppow
To: pure-silver@xxxxxxxxxxxxx
Sent: Thursday, May 29, 2008 2:29 PM
Subject: [pure-silver] Re: 35 MM T-MAX 100 IN PYROCAT?
----- Original Message -----
From: "BOB KISS" <bobkiss@xxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, May 29, 2008 5:55 AM
Subject: [pure-silver] Re: 35 MM T-MAX 100 IN PYROCAT?
> DEAR JONATHAN,
> Here are some definitions (curious choice of words!) of
> acutance.
> 1) "A measure of the sharpness with which a film can
> reproduce the edge of
> an object" (Dictionary.com)
> 2) The measure of lens performance or of picture quality,
> in terms of the
> sharpness of the transition across the boundary between
> the images of light
> and dark areas. (Photographic Optics; Cox)
> I have much more sophisticated definitions in Neblette and
> Mees &
> James but the above cover it.
> ******For an excellent example please see the following
> link.
> http://www.cambridgeincolour.com/tutorials/sharpness.htm
> Scroll down (about mid page) to the close up images of the
> man's
> sweater. The images clearly show the difference between
> resolution (ability
> to reproduce fine detail) and acutance (perceived
> sharpness, often due to
> edge effects).
> Soooooooooooooo, as mentioned, although the T-max 100 35mm
> negs I
> have processed with other developers held lots of fine
> detail, there was an
> overall lack of sharpness.
> CHEERS!
> BOB
The term acutance was invented by Kodak to describe the
contrast at the edges of sharply focused bright and dark
areas on film. Edge sharpness of the image due to film
characteristics is separate from the edge contrast of the
image from the lens. Acutance is partly a characteristic of
the film and partly its determined by processing. The edge
contrast on a negative may be in excess of the contrast of
the image generating it. Acutance is lowered by scattering
of light in the emulsion (irradiance) and can be increased
by edge/border effects in development and also by optical
effects in film that has been processed in a tanning
developer. The edge or border effects can increase the
contrast at a sharply defined transition because the
reaction products of the developer can diffuse sideways in
the emulsion. So, at the border, reaction products from the
denser areas tend to diffuse to the lower density area thus
reducing the amount of development while fresh developer
from that area diffuses into the dense area tending to
increase development there. The scale of the effect depends
on the rate of diffusion in the emulsion, the thickness of
the emulsion, and the nature of the developer. The reaction
products of some developers have little effect on the rate
of development, in others a relatively large effect. The
reaction products of Metol, for instance, are restrainers,
those of hydroquinone are accelerators.
Since sulfite, or other preservatives which become
sulfite in reaction, are oxygen absorbers they tend to
prevent the generation of reaction products by selective
absorption of oxygen. This is actually one reason for the
presense of sulfites in developers. Developers with much
sulfite tend to produce relatively weak acutance effects.
Also, the concentration of developing agents and
sulfite has an effect: dilute developers tend to produce
stronger edge effects than stronger ones. As an example
developers like D-76 or Microdol-X have very little edge
effect when used full strength but both will generate strong
effects when diluted 1:3. Rodinal generates strong acutance
effects at around 1:100.
Agitation also has an effect. Since the acutance effect
is due to "exaustion" of the developer (really a misnomer)
rapid agitation tends to reduce the effects although it can
not overcome the minimal effect due to the diffusion rate of
the emulsion.
Where a tanning developer has been used there is a
slight variation in index of refraction and in emulsion
thickness dependant on the amount of tanning. This can have
an effect similar to acutance at the edges of dense vs:
light areas.
Keep in mind that acutance is an optical illusion. It
depends on a characteristic of vision which tends to
interpret contrast as sharpness. Actually, strong acutance
effects can _reduce_ resolution. This is a problem in
photographic sound recording and in microfilm. However, the
eye is relatively insensitive to resoltion and will see an
image with relatively poor resolution but strong edge
contast as sharp.
Note also that acutance is fixed in scale so it can be
quite noticable in images made from 35mm film but is of
virtually no importance for 4x5 negatives.
I find too much acutance to be rather unpleasant.
Another note: a sharply defined grain pattern also
gives the illusion of sharpness to otherwise somewhat blurry
images. This is one reason 35mm users sometimes like grain:
it helps cover up poor optics or poor film resolution.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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