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Thread: Questoion about convertible lens

  1. #1

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    Questoion about convertible lens

    I have bought this Symmar 1:5,6/210, convertible to 1:12/370, in excellent condition.

    Today, I tried it for the first time, with Polaroid (another problem there, but that's for another question), and everything looks fine in 210mm.

    I have understood I have to remove the front part of the lens to use it as a 370, and then I can't focus with my Shen-Hao 4x5 (as I more or less expected). Would a tophat board (or whatever it is called) help here ? I have no idea what extension I need for this lens, I didn't find info anywhere about that.

    BUT when I take off the BACK part of the lens, I get an image on the groundglass that looks quite sharp (that one I CAN focus), as far as that can be judged with a loupe, and has a higher magnification then the lens as a whole ... Is there something wrong working that way ? I couldn't test it yet (because of the Polaroid problem), but maybe somebody else has done this before ?

    Thanks,
    Stefan.

  2. #2

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    Re: Questoion about convertible lens

    When you remove the rear element of the lens, it is best to move the front element to the back of the shutter. You should get very good results this way. I won't bore you with the physics of why this is, but it works.

    Sounds like you simply need a longer bellow when working in the 370mm configuration. That is usually the easiest solution, instead of working with multiple lens boards with your lens.
    When I grow up, I want to be a photographer.

    http://www.walterpcalahan.com/Photography/index.html

  3. #3

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    Re: Questoion about convertible lens

    Quote Originally Posted by Walter Calahan View Post
    When you remove the rear element of the lens, it is best to move the front element to the back of the shutter. You should get very good results this way. I won't bore you with the physics of why this is, but it works.

    Sounds like you simply need a longer bellow when working in the 370mm configuration. That is usually the easiest solution, instead of working with multiple lens boards with your lens.
    How can I mount the front element on the back of the shutter ? The front and back element have a different radius, no ? How can the front element fit in the back of the shutter ? Isn't a top hat lensboard a possible permanent solution for both configurations ?

    Thanks,
    STefan.

  4. #4

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    Re: Questoion about convertible lens

    The way it is supposed to be used is removing the front element and using just the back, on the back. (This is per Schneider.) The problem is that doing that takes a lot of bellows draw, more than you'd think from the focal length. So a top hat lens board might do it if you are close to having enough bellows.

    Using the front on the front and taking the back off cures the bellows draw problem and produced a remarkably good image on the two lenses I've had like yours. The aperture scale will be a little off, but not that bad. Try it before everybody tells you this is a dumb idea. Stop down to f:22 or f:32.

  5. #5

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    Re: Questoion about convertible lens

    Quote Originally Posted by Kevin Crisp View Post
    The way it is supposed to be used is removing the front element and using just the back, on the back. (This is per Schneider.) The problem is that doing that takes a lot of bellows draw, more than you'd think from the focal length. So a top hat lens board might do it if you are close to having enough bellows.

    Using the front on the front and taking the back off cures the bellows draw problem and produced a remarkably good image on the two lenses I've had like yours. The aperture scale will be a little off, but not that bad. Try it before everybody tells you this is a dumb idea. Stop down to f:22 or f:32.
    Ok, weather permitting, I'll have a go at it tomorrow; the image looked quite good on the gg, using only the front.

    Thanks,
    Stefan.

  6. #6

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    Re: Questoion about convertible lens

    WRT extension, you're going to need 370mm of bellows to focus at infinity. If you want to get closer, more bellows.
    If you want 1:1 ~750mm of draw. 1:2 ~500mm, 1:4~440mm and so on.

    General info, not critically accurate.

    Check it with the 210 & measure the bellows at infinity, you should be very close to 210mm/8 1/4" of draw.

  7. #7

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    Re: Questoion about convertible lens

    Remember: with many convertible lenses using only one lens group, you need to refocus after stopping down to the working aperature before taking the picture. It gets a little dim, but you'll get used to it quickly and your sharpness will be better than if you don't refocus...look for a bright spot in your focus area and use that to refocus. Many convertible lenses have a small focus shift when stopped down...I think that this it the reason so many folks have a bad opinion of convertibles.

  8. #8

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    Re: Questoion about convertible lens

    The problems of cell location vs bellows extension are given above.
    Someplace on the web is an old lens catalog [ no really] Rodenstock maybe.
    That one recommended putting the cell in FRONT in order to protect the diaphragm leaves from damage, like a finger poke. I DID find my singlecellProtar notes so not-too-long-ago that I remember- The stop down focus shift for the 24 inch cell nominal 12 combined 18+24 was about 3/8 inch, 8x10, nearly infinity, front.

  9. #9

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    Re: Questoion about convertible lens

    The apertures engraved on the shutter are based on using both elements or just the rear element, and is not relevant if the front element is used alone.
    I believe the elements are identical, so there's no point in moving the front element to the back.
    Wilhelm (Sarasota)

  10. #10
    God loves a tryer Scotty230358's Avatar
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    Re: Questoion about convertible lens

    I don't think you will be able to get a top hat big enough. I had a Shen Hao TZ45BII and it had about 315mm of bellows draw. Using my 180/305 convertible I needed about 380mm of draw. This, IIRC, is caused by the front node of the lens moving forward when the lens is used in its converted form. With 370mm converted you would need in excess of 400mm bellows draw. This would mean a top hat of over 85mm. That's fairly large for a Shen Hao front standard.

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