[pure-silver] Re: 35 MM T-MAX 100 IN PYROCAT?

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 

  
=============================================================================================================
  To unsubscribe from this list, go to www.freelists.org and logon to your 
account (the same e-mail address and password you set-up when you subscribed,) 
and unsubscribe from there.

Other related posts: