enlarging lens focus shift

Discussion in 'Enlarging' started by michael_r, Aug 17, 2010.

  1. michael_r

    michael_r Subscriber

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    If I buy a new high quality APO enlarging lens from Rodenstock or Schneider, is there any focus shift at all when stopping down? I doubt it, but just want to get some comments on this. In the literature from either company, they point to the diaphragm preset feature as useful because you can focus wide open and then stop down to your working aperture. But I was always taught to try and focus at the working aperture if possible to avoid even the slightest focus shift. So what do you think? With these top-line lenses is it safe to focus wide open?
     
  2. BetterSense

    BetterSense Member

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    Why don't you do a test?

    I have never noticed focus shift with any lens, enlarging or otherwise.
     
  3. tkamiya

    tkamiya Member

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    I have EL-Nikkor and Scneider as main lens. They are not APO kind. I've heard of focus shift per aperture and with contrast filters, but personally, I have not seen one. I enlarge to equivalant of 11x14 and use a grain focuser. I would think, if there are any, they will be masked by larger DOF stopped down.
     
  4. michael_r

    michael_r Subscriber

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    I will test, but wanted some opinions. I would assume modern enlarging lenses, particularly high quality ones, are fully corrected for focus shift, but in some of the old books you come across it as something to watch for.
     
  5. fotch

    fotch Member

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    Never had any with my old Kodaks.
     
  6. ic-racer

    ic-racer Member

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    Scroll down on this page and move the mouse over the aperture scale and see the focus shift of that Zeiss 1.4 lens. Now compare f4 to f8 -> not much difference.
    I have about 13 enlarging lenses and have not detected any focus shift with a grain magnifier. They are all slower than f1.4. Perhaps a f1.4 enlarging lens would have some shift, but maybe that is a design factor in producing good enlarging lenses. ie enlarging lenses are not that fast.
     
  7. michael_r

    michael_r Subscriber

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    Interesting stuff. Thanks for the link.
     
  8. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    You need quite a bit of under-corrected spherical aberration to get any meaningful focus shift. The old Dagor and Angulon lenses exhibit noticeable focus shift as did some of the old convertible lenses - especially if only one group was used. This led to the apocryphal advice to focus at the working aperture.

    I have never seen any focus shift when stopping down a modern enlarging lens. It is likely any shift is likely to be far less than the problems of negative and paper flatness and to be masked by increased depth-of-field.