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  1. #11
    Les McLean's Avatar
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    Another factor to consider is that when you stop down the lens after focussing there could be focus shift on the lens. I have had that experience with a Nikor 80mm lens so I always check the focus at the exposing aperture.
    "Digital circuits are made from analogue parts"
    Fourtune Cookie-Brooklyn May 2006

    Website: www.lesmcleanphotography.com

  2. #12

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    Les mentions focus shift at f/16 and that may very well be true. I rarely if ever stop my enlarging lens down that much, usually using them at two or three stops down from their maximum aperture where they seem to be at their best. If you are finding that your exposure times are too short, you can always cut down the amount of light reaching the negative stage. If you have a filter drawer in the light path, simply adding one or two sheets of neutral density cine gels can increase your exposure times by 1 or more stops. See here for an example: http://www.adorama.com/RO3402.html?s...gel&item_no=18. These gels are cheap and you can cut them to size. With a color head, simply dialing in equal parts of cyan, magenta, and yellow filtration can accomplish the same thing.

  3. #13
    Les McLean's Avatar
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    [QUOTE=fschifano;374591]Les mentions focus shift at f/16 and that may very well be true.

    My post
    Another factor to consider is that when you stop down the lens after focussing there could be focus shift on the lens. I have had that experience with a Nikor 80mm lens so I always check the focus at the exposing aperture.

    Not guilty your honour

    Like you, I rarely if ever stop down to f16 usually it's f8
    "Digital circuits are made from analogue parts"
    Fourtune Cookie-Brooklyn May 2006

    Website: www.lesmcleanphotography.com

  4. #14

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    Quote Originally Posted by Les McLean

    My post
    Another factor to consider is that when you stop down the lens after focussing there could be focus shift on the lens. I have had that experience with a Nikor 80mm lens so I always check the focus at the exposing aperture.

    Not guilty your honour

    Like you, I rarely if ever stop down to f16 usually it's f8
    I've been using Prescysol EF on all my films as of late. I have noticed that my negatives have less density than before, although much easier printing. Along with the small paper size I use and condenser enlarger might be causing the shorter printing times.

    I'll check out the ND filter, that may do the trick, or start printing 16x20's!

    I'll do some more research as I really don't understand why the more you stop down an enlarging lens, image quality seems to suffer. Which confuses me greatly. The greater the DOF the greater the overall sharpness? Not the case when enlarging? I feel very dumb...

    Marco

  5. #15
    Dave Miller's Avatar
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    I have yet to be convinced that stopping down an enlarger lens past its midpoint either causes a focus shift or a degradation in actual picture quality. I think this is another example of theory being touted as fact, without practical tests to first prove the calculated result.
    I have about 10 enlarging lens of various makes in focal lengths from 50mm to 150mm, and any one of them would find a place in the trash can if they exhibited either of these characteristics. I am unable to say, when viewing a finished print, whether it was printed at f8 or f32, and that must be the acid test of this matter.
    When producing 5” x 7” prints I generally find I need an aperture of f22 in order to obtain an exposure time of around 10 seconds.
    Regards Dave.

    An English Eye


  6. #16

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    Dave I certainly agree that diffraction stopping down an elarging lens to a small stop when enlarging a 6x7 negative to 5x7 inches will be damn near invisible. Enlarging a extremely detailed 35mm to 16x20 may possibly be a different story.
    Claire (Ms Anne Thrope is in the darkroom)

  7. #17
    Dave Miller's Avatar
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    Quote Originally Posted by Claire Senft View Post
    Dave I certainly agree that diffraction stopping down an elarging lens to a small stop when enlarging a 6x7 negative to 5x7 inches will be damn near invisible. Enlarging a extremely detailed 35mm to 16x20 may possibly be a different story.
    It may Claire, but what I'm asking is that it is tried; before any adverse pronouncement, or statements of "fact" are made.
    Regards Dave.

    An English Eye


  8. #18
    Les McLean's Avatar
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    [QUOTE=Dave Miller;374648]I have yet to be convinced that stopping down an enlarger lens past its midpoint either causes a focus shift or a degradation in actual picture quality. I think this is another example of theory being touted as fact, without practical tests to first prove the calculated result.


    Dave, whilst I have not carried out scientific tests on the lens in question there is no doubt in my mind that when stopped down by 2 stops the focus did shift. I checked it a number of times just to ensure that I had not moved the enlarger due to the imbibing of falling down water or some such inexplicable reason. When closed down I re-focused and the printed image was absolutely razor sharp.
    "Digital circuits are made from analogue parts"
    Fourtune Cookie-Brooklyn May 2006

    Website: www.lesmcleanphotography.com

  9. #19
    resummerfield's Avatar
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    Concerning resolution, I’ve tested my El Nikkor and Rodenstock enlarging lenses ranging from 50mm through 300mm, with a glass USAF resolution target, at both the center and each corner. This was just something I did informally, to be sure I was getting the maximum resolution my lenses could provide. All lenses increased resolution when stopping down, to reach maximum resolution at about 2 stops from open. As they were stopped down further, resolution started to decrease due to diffraction. The resolution loss on my lenses due to diffraction averaged about 20% from their maximum resolution values. I do not have any APO lenses, which may give the best resolution wide open.

    In theory, and with my lenses, any degrading of the final print image would not become apparent until enlargement exceeded about 10x. In actual practice up to about a 12x enlargement (the largest I’ve used so far), I have not seen any difference in the final print image when using any f-stop ranging from the midpoint (2 stops from open) to the smallest. My eyes, my judgement, etc.

    Concerning focus shift, I always focus again when stopping down, and I never considered testing this.
    —Eric

  10. #20
    Dave Miller's Avatar
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    Les,I'm not doubting your word regarding focus shift when stopping down; what I suggested was that the only place for such a lens is the trash can. It shouldn't happen.
    Eric, as you say, it is that actual result that is important, that's the point I want to get across. It doen't matter how wonderful the equipment is, it's the film, and paper that sets the resolution limit; and that's not very high.
    Regards Dave.

    An English Eye


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