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Thread: Are all plasmats convertible?

  1. #1

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    Question Are all plasmats convertible?

    Hey,

    So I'm assuming most of us know how the early Symmar lenses from Schneider Kreuznach are convertible, ie: the 300mm f/5.6 becomes a 500mm f/12 when you remove the front element. I was reading up on the symmar lenses for a friend of mine who is thinking of purchasing one, and I read that the Symmar S is also convertible. This got me thinking, there isn't much difference in the various plasmat lenses, so is my fujinon W 300mm I use convertible as well? I took the front element off my camera, and lo and behold, I was able to focus on a distant tree with a bellows length of approximately 560mm.

    When using my fujinon without the front element, I noticed that the sharpness was noticeably less in the corners with the lens wide open, but all I had to do to fix that was stop down a few stops.

    So, is it just coincidence that both the Symmar S and Fujinon W can act as a longer focal length lens without the front element, or is this the case for all plasmat lenses?

    If any of you have other plasmats in your possession, maybe take them out and see what results you can get without the front element, lets see what we discover!

    Or, maybe better yet, can anyone who knows about lens design add some insight on this apparent property of plasmat lenses?

    Thanks,
    Ethan
    Last edited by Ethan; 15-Jun-2021 at 17:21. Reason: typoes

  2. #2
    Mark Sawyer's Avatar
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    Re: Are all plasmats convertible?

    All Plasmats are convertibles, as are all Dagors, from which the Plasmat descended. (Hint: "Dagor" is short for "Double Anastigmat, GOeRtz", so both cells are independently corrected for astigmatism.) And yes, as you noted, the aperture must be closed down to be acceptably sharp, usually around f/45, as the single cells aren't as well corrected as the two used together. And as a general rule, you're better off using a complete lens rather than a converted half-lens.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  3. #3

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    Re: Are all plasmats convertible?

    Quote Originally Posted by Mark Sawyer View Post
    All Plasmats are convertibles, as are all Dagors, from which the Plasmat descended. (Hint: "Dagor" is short for "Double Anastigmat, GOeRtz", so both cells are independently corrected for astigmatism.) And yes, as you noted, the aperture must be closed down to be acceptably sharp, as the single cells aren't as well corrected as the two used together.
    Thanks for that info! Was the convertible symmar better corrected for use as a single group, or is Schneider just the only company to advertise this more universal feature?

  4. #4
    Mark Sawyer's Avatar
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    Re: Are all plasmats convertible?

    Other companies advertised their Plasmats as convertibles, like the Ilex/Caltar Acuton, which featured two aperture scales on the shutter.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  5. #5

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    Re: Are all plasmats convertible?

    To slightly rephrase what Mark has said about correction, the manufacturer could choose to correct for best results with the indvidual cells or best results with both cells together. As I understand it, the Symmar was a compromise half way between since it was made to be convertible, whereas the Symmar-S was corrected for use as a complete unit only. For best results using a single cell it must be placed behind the aperture.

  6. #6

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    Re: Are all plasmats convertible?

    I did not have very satisfying results when trying to use just the rear cell of a Symmar-S.

  7. #7
    Drew Wiley
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    Re: Are all plasmats convertible?

    There's a problem with this kind of pigeonhole lens terminology. "Plasmat" implies both an original design by that name as well as an entire class of modern lenses, very few of which are "convertible" without a significant penalty in optical performance. Same could be said about "Dagors", which in fact as a design predate plasmats. According to Kingslake, the first plasmat from 1903 was essentially an airspaced dagor, symmetrically 2+1 / 1 +2, rather than the 3 / 3 of the original dagor design itself, starting in 1892. Later plasmats, generically-termed such, became more complex, while the definition of a dagor always remained three fully cemented elements opposite each other. Most "general purpose" view camera lenses with generous coverage angles are classified as plasmats, although some fully airspaced 4-element designs have also been marketed for general purpose usage. Another design with a long ongoing history is the 1+1 / cemented-2 tessar from 1902. I certainly wouldn't use any of my own lenses of any of these designs convertible-option, one side only; but they're all modern, even the Dagor. I actually have tested for that.

  8. #8

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    Re: Are all plasmats convertible?

    I once had a Schneider brochure, introducing the Symmar-S lens line (c.1972). In it they explained that they had eliminated the convertible feature in order to improve performance of the whole lens. Oren Grad (researcher extraordinaire) found a copy and posted it in a recent thread here on the same subject; that would be worth looking up.
    I'll suggest that yes, all 'plasmat' style lenses are convertible- but the converted lens' performance may not meet your standards.
    The folk wisdom of past decades said that the 'convertible' Symmars were meant for portraiture when converted, suggesting that some decreased sharpness in the corners wasn't important. I'll suggest making some tests (as Drew has done) and finding out for yourself.

  9. #9

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    Re: Are all plasmats convertible?

    I tried a couple of G-Clarons as convertibles here:

    https://www.largeformatphotography.i...=1#post1561918

  10. #10
    Maris Rusis's Avatar
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    Re: Are all plasmats convertible?

    I "convert" my Fujinon-W 300mm f5.6 lens by removing the front group. New focal length is about 600mm. Several precautions:

    Add three stops of exposure compensation to allow for the extra bellows extension and loss of magnification of the entrance pupil.
    Start focussing two stops down from wide open. Most of the focus shift happens in those first two stops.
    Stop down a long way until the corners are reasonably nice.
    Safeguard the exposed and fragile iris and shutter blades.
    Photography:first utterance. Sir John Herschel, 14 March 1839 at the Royal Society. "...Photography or the application of the Chemical rays of light to the purpose of pictorial representation,..".

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