f stops and printing

Discussion in 'Enlarging' started by Gary Holliday, Aug 29, 2007.

  1. Gary Holliday

    Gary Holliday Member

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    I was talking to a printer a few years ago who stated that when she printed nudes for example; the model's back could have a hard or soft edge depending on which f stop she used on the enlarger lens.

    I didn't think that f stops could alter an image.

    Is there any truth in this?

    Thanks
     
  2. w35773

    w35773 Member

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    Sure, similar to a camera lens, if you open it up all the way, you get some sloppiness. This depends a lot on the lens and the light source, too.
     
  3. Gary Holliday

    Gary Holliday Member

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    Define sloppiness? unsharpness?? But surely a negative is flat so there is no depth of field.
     
  4. smieglitz

    smieglitz Subscriber

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    Lenses tend to be sharpest about two f/stops closed down from wide open (e.g., f/5.6 for a lens with maximum aperture f/2.8). So, the entire print would be sharper at that sweet spot but depth of field would not be an affected factor in the image. It is set when exposed in the camera. A similar effect, depth of focus, does exist in an enlargement situation, but increasing it by using smaller apertures again only affects the overall print sharpness for example by bringing the edges of the print to greater apparent sharpness when one has actually focused on the print center. It is the apparent sharpness range of the print image, but has nothing to do with the subject depth of field.

    At some enlargement f/stop, this sharpness all falls apart due to diffraction. You can observe that effect by looking through a grain magnifier as you progressively stop the lens down after initially focusing the grain in. It will sharpen up to a specific aperture and then turn to mush beyond that.

    Joe
     
  5. Ole

    Ole Moderator Staff Member Moderator

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    Unless the enlarging lens sufferes from an unusual amount of field curvature (=soft corners), it's bullsh*t. :smile:
     
  6. w35773

    w35773 Member

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    Yes, I was thinking more of my situation where my lens and enlarger were made during the flintstone age and the abberations in my lens are somewhat soothed by stopping down some.
     
  7. Ole

    Ole Moderator Staff Member Moderator

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    While my enlarger is not exactly "flintstone age", one of my lenses is - flint and crown glass; an Aplanat. Not even that shows enough field curvature to give a visible difference!
     
  8. w35773

    w35773 Member

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    Is it possible to get a focus shift when changing aperatures on a enlarger lens? Maybe that is my problem?
     
  9. Dave Miller

    Dave Miller Member

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    If you do, then it's time to chuck the lens.:sad:
     
  10. Jim Jones

    Jim Jones Subscriber

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    I struggled with such a lens for too long. Dodging and burning had to be done very close to the paper, too. Despite that, the prints were reasonable sharp. Like Dave says, chuck the lens. Good ones are affordable now.
     
  11. Saganich

    Saganich Subscriber

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    I also read that sharpness changes for papers more sensitive to blue light, ie multicontrast papers, and tests should be made. On print dead on via magnifier, and two slightly off in either direction to see if there is shift.

    Chris
     
  12. Dave Miller

    Dave Miller Member

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    Just goes to show - shouldn't believe every thing you read.:smile:
     
  13. Fred Aspen

    Fred Aspen Member

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    Thornton and Ctein spoke of 'smear' when some enlarging lenses are used wide open which I am sure would contribute to some apparent softness that might be used to one's advantage.

    Fred
     
  14. Ole

    Ole Moderator Staff Member Moderator

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    Chromatic aberration? That shouldn't be a problem with any undamaged enlarger lens made after 1900 (with the possible exception of the Voigtländer W.Z., which was designed to be soft).