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Thread: APO Sironar S lenses in 300mm + in today's market

  1. #71

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    Re: APO Sironar S lenses in 300mm + in today's market

    Quote Originally Posted by Leigh View Post
    OK.

    But as soon as you loosen the front element you introduce tilt due to clearance in the threads.
    That throws the front cell off-axis relative to the rear cell.
    ???

    A proper optical test jig holds both cells rigid, and allows the spacing to be changed accurately.
    Then the spacing is measured, compared with the shutter thickness, and shims calculated.

    - Leigh
    It's true that the collimation of the lenses may vary a bit if the cell is not tightened, but I guess this is irrelevant for that test, when I made those tests I moved with the hand the front cell to see the effect of that clearance, I saw no practical effect.

    Anyway we can remove that clearance by making/using paper/cardboard/mylar shims.

  2. #72

    Re: APO Sironar S lenses in 300mm + in today's market

    Quote Originally Posted by Pere Casals View Post
    It's true that the collimation of the lenses may vary a bit if the cell is not tightened, but I guess this is irrelevant for that test, when I made those tests I moved with the hand the front cell to see the effect of that clearance, I saw no practical effect.
    Yes. I played with loosening the front element of a Fujinon (240A) and took photo's at the centre and halfway across the image circle at every full rotation of the front element (fully seated and four rotations in all). At every full rotation I refocused before taking the shots (object of focus was 25 metres away). In all five sets of images captured with a digital back (6μm pixel size) there was no discernable difference in image quality at 100% view on the computer screen.

    That said I didn't check to see if there was any difference when the lens was focused closer or focused at infinity. I also didn't check for field tilt*, where the axis of tilt would be along the horizontal plane. It was a casual test just to satisfy my personal curiosity.

    *EDIT 16th Jul.: Field tilt is the wrong definition to use as it correctly refers to the whole lens not the front lens group which I was discussing. My bad. Is decentring more accurate?
    Last edited by duff photographer; 15-Jul-2018 at 15:50.

  3. #73

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    Re: APO Sironar S lenses in 300mm + in today's market

    Quote Originally Posted by duff photographer View Post
    Yes. I played with loosening the front element of a Fujinon (240A) and took photo's at the centre and halfway across the image circle at every full rotation of the front element (fully seated and four rotations in all). At every full rotation I refocused before taking the shots (object of focus was 25 metres away). In all five sets of images captured with a digital back (6μm pixel size) there was no discernable difference in image quality at 100% view on the computer screen.

    That said I didn't check to see if there was any difference when the lens was focused closer or focused at infinity. I also didn't check for field tilt, where the axis of tilt would be along the horizontal plane. It was a casual test just to satisfy my personal curiosity.
    Fujinons usually don't include shims, until I know. My guess is that the Fujinon designs make preformance not that dependant on cells spacing.

    I the other hand we have have designs (like some from rodenstock) that require an accurate shimming (done in the the front cell).

    Let me speculate a bit... My guess is that in some designs the cells spacing impacts in how the aberrations are corrected, so intentionally the design uses the lens spacing as a degree of freedom in the design to optimize the lens.

  4. #74

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    Re: APO Sironar S lenses in 300mm + in today's market

    Papi, several centuries ago I noticed that Polaroid Copal shutters have somewhat longer tube lengths than jes' plain Copal shutters. Since I wanted to put some 210 mm cells of a plasmat type lens in a Polaroid Copal #1 I asked my friend Eric Beltrando (visit his site dioptrique.info) for his opinion. Eric has written a ray tracing program and used it to provide calculated curves, spot images etc. for lens prescriptions in his database.

    He told me to go ahead and use the shutter. His calculations indicated that in general plasmats' image quality isn't very sensitive to small errors in cell spacing and that 0.2 mm is 0.1% of 210 mm, not enough to worry about.

    If no Fujinon LF lens has shimmed cells and some Schneider and Rodenstock lenses do, there are two possible explanations. Fujinon had better quality control, or Schneider and Rodenstock were more nearly perfectionists. In all designs cell spacing affects corrections.

  5. #75

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    Re: APO Sironar S lenses in 300mm + in today's market

    Quote Originally Posted by Dan Fromm View Post
    In all designs cell spacing affects corrections.
    OK, but in some designs it perhaps has a way greater impact.

    For example a Symmar Convertible has its cells individually corrected, being a single cell usable, with the limitations a simple triplet design has.

    But let's imagine that a design undercorrects Chromatic Aberration in the front cell while overcorrecting it in the rear cell. Then if we vary the cells spacing we can adjust the correction to the optimal point, as the amount of deviation added from front cell would depend on the lens spacing.

    Then we can imagine that spherical aberration correction has a well lower dependence on the lens spacing, so if from the production crop we pair cells that compensate well the spherical aberration in particular then we would be able to minimize the chromatic aberration by shimming, obtaining a consistent quality production.

    Just thinking in an hypothetical way to use shimming that's not related to overcoming manufacturing inconsistency, in fact the great amount of shimming that some Pro designs bear are not easy to explain from production inconsistency (IMHO).

    A possibility is that (in some designs) cells spacing is an open paramenter for the manufacturing, that would allow a greater number of front/rear cells excellent combinations found.

    Just speculating...

    _________________

    Oh, yes... I've been exploring dioptrique.info since you pointed it to me time ago. Nice info !

  6. #76

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    Re: APO Sironar S lenses in 300mm + in today's market

    Papi, if we had eggs we could have ham and eggs if we had ham.

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