Lens Coverage on homemade polaroid/20's plate camera

Discussion in 'Camera Building, Repairs & Modification' started by himself, Aug 17, 2011.

  1. himself

    himself Member

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    As some of you may have seen I recently mounted a polaroid back on an old plate camera http://www.apug.org/forums/forum147/90537-polaroid-back-20s-plate-camera.html, the contraption works fine, but I've since added a new lens mount so I have tilt/swing movement.

    The mechanism works great, but I've encountered a new problem (one I'm sure most here will have already guessed) - just as I start to get some worthwhile blur there starts to be some vignetting and swung further I lose image area.

    so, the problem seems to be one of coverage. The lens is an old IBSOR DRB 105mm shooting on 7.5cm x 9.5cm instant film. the lens covers fine straight on and covers the rise and shift fine.

    my question is, how do I best solve this problem - do I need a wider lens or just a lens with a bigger diameter?

    thanks for any suggestions
     
  2. Ian C

    Ian C Member

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    You need a lens that projects a larger diameter image circle.

    That means a longer focal length lens of the same projection angle—provided that the camera has a long enough bellows to accommodate it, a lens of a similar focal length to the one you have now but that projects a wider angle to give you a larger image circle, or a combination of both of theses characteristics.
     
  3. himself

    himself Member

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    thanks Ian, but how much are we talking?

    the bellows length should be ok, it's a double extending bellows and because the film plane has been moved back so much with the current lens it barely comes out of the box anyway.
     
  4. Ian C

    Ian C Member

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    I looked at the photos of your camera in the link. The lens appears to be a Rodenstock 105/4.5 Doppel Anastigmat.

    I don’t know the exact dimensions of the image rectangle on the film. If we assume it is the same dimensions as the stated film size of 75mm x 95mm, then the diagonal is 121mm. So apparently the 105mm lens you have produces an image circle at least that large in diameter. You can measure the diagonal of the image on film for an accurate size that the lens must cover.

    From your comments I think that you’re able to pivot the lens board horizontally. With a marginal coverage it wouldn’t take much swing for the image circle to fall off of the film at one side.

    Just guessing, I’d estimate that you’d need a lens of at least 150mm or longer, if it has about the same projection angle, to give you a reasonable amount of movement. For an old camera of this type you won’t have as many options as you would with a larger camera. The shutter and lens must still fit the board and its size is limited by the relatively small lens board.

    Hopefully someone with experience with cameras of this type and size might have a specific recommendation that will fit and give you the coverage you need to swing the lens as you choose.
     
  5. michaelbsc

    michaelbsc Member

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    You won't get too much coverage with that, or most, 105mm lenses.

    Look for a lens designed for use with 4x5. The older Angulons should do well for you. Or if you want a little longer focal length try to find an old Polaroid 110 and rob the lens from it (127mm that covers 4x5 well).

    Or perhaps an old Wollensak 90mm WA lens.

    But Ian is right. The focal length of the lens isn't what you need to concern yourself with first. You're first issue is the image circle the lens produces. If you want to tilt and swing the lens, the image circle has to be large enough to give you the movements or you get the vignetting.
     
  6. himself

    himself Member

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    thanks Ian

    I was afraid some one woud say that, 4x5 lenses are out of my price range and probably wouldn't fit the camera.

    thanks tho'
     
  7. holmburgers

    holmburgers Member

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    By the way, awesome looking camera.

    What about the Kodak rectilinear lenses from the old 2A/3A variety folders I heard that if you unscrew one of the elements they cover 8x10". Could be a drop in replacement, or perhaps you can unscrew one element on this lens? I know just enough to be dangerous, but some food for thought perhaps.
     
  8. himself

    himself Member

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    thanks, it was born out of the need to replace a broken polaroid reporter and being fed up with relying on "the magic eye" but being too poor to go full LF.

    I have the camera at hand and with a quick flick of the wrist discovered that unscrewing one of the filters doesn't help with coverage and makes it impossible to focus on anything closer than about 3m, so not very practical.

    great suggestion tho' and I'll keep an eye out for the lenses you suggested
     
  9. michaelbsc

    michaelbsc Member

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    Some of the older lenses are very compact. An old Wollensak from a dead Graphics would work.
     
  10. himself

    himself Member

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    good good, I'll keep an eye out for them.
     
  11. michaelbsc

    michaelbsc Member

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    I noticed that you said the 105mm barely has the front standard out on the rails. Don't get too short or you'll end up not able to get infinity.

    You might check the LF board for someone trying to move an old lens, too. They're a little more focused on something like this than APUG. Here there is quite a hodge-podge of stuff to wade through.
     
  12. himself

    himself Member

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    I didn't think of that.
    is there a way to calculate the image circle so that I don't end up with something too small? or is it one of those things that varies brand to brand, age to age...
     
  13. michaelbsc

    michaelbsc Member

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    It varies lens design by lens design. Some long length lenses have small image circles, while some have large ones. Depends on what the lens was designed to do. Likewise, some short focal length designs have small circles while other have large ones, again dictated by the design of what the lens was made to do.

    For example, the 105mm you have in your camera has a relatively small image circle because it was designed for use in a portable camera with little to no movements required. Larger coverage wasn't required, and would have been a waste of money that didn't contribute value to the product.

    Whereas many 90mm and 120mm lenses, both longer and shorter than yours, do have image circles large enough to cover 4x5 because that was part of the design objectives, so the glass was made to do it.

    Ergo, in isolation focal length really tells you very little about the image circle size.
     
  14. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    I take it you are looking for a 'Petzval look': sharp center, blurring in the surround, and swirly bokeh.

    http://www.flickr.com/photos/richard314159/galleries/72157624453174055/

    If you are on a budget you might want to play around with homemade lenses. Try using the front objective from a set of binoculars, close-up lenses, magnifying glasses, reading glasses. Sometimes removing a corrective element from a decent lens can result in some interesting effects. Try unscrewing the negative element from a triplet or Tessar - use a long focus lens as the starting point; the focal length will shorten when you remove the negative element. You can also try using 1/2 of your Dopple Anastigmat, when one element is removed the focal length will increase as both elements are positive.
     
  15. himself

    himself Member

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    what about the larger plate cameras that cover 9x12?
     
  16. himself

    himself Member

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    I'm not sure what it's called but selective focus is what I want.

    actually what I originally wanted was a plate camera that could shoot polaroid, but ambition has pushed me beyond that.

    I'll keep in mind what you suggested and if I get hold of any of those things I'll give it a go
     
  17. holmburgers

    holmburgers Member

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    Yeah, removing one of the lens elements on the aforementioned Kodak Bausch & Lomb Rectilinear (OTOMH) would increase the focal length as well, so that wouldn't help much I guess.

    How about a close up filter? Again, just an errant suggestion...
     
  18. michaelbsc

    michaelbsc Member

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    Not only would it "approximatly" double the FL, but it would also substantially increase the image circle. Which is exactly what the OP needs in the short run, a way to test his movements without running out of the circle.

    @himself: Now, keep in mind it isn't going to increase the *AMOUNT OF LIGHT* that's pouring in through the lens. So since the same light will be spread over a circle that's potentially 4 times the area (+/-) you may lose a couple of effective stops.

    Is this making sense? If you can get to them, screw out either the back or the front element of your lens. (I'd take out the back if you can.)

    Then you'll have a lens somewhere in the neighborhood of about 210mm, and your image circle will be a lot bigger. So you can try out your movements, but the image will be dimmer because the available light is spread thinner. Exposure is kind of a guess until you figure out exactly how it's changed.
     
  19. holmburgers

    holmburgers Member

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    Hmm, but if his FL is doubling (we could all hope to be so lucky) then he's gonna run out of bellows it sounds like.
     
  20. himself

    himself Member

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    tried and...

    he's right, ran out of bellows.

    I managed to drag back the FL with a +2 close up diopter to about 130mm (I can still focus to about half a meter too) and the image circle has increased to a point where I can tilt so much that it's pretty much impossible to focus at all.

    the problem now is that it's completely unusable with a giant filter taped to the front... looks real nice too

    my question now is - can I get a 130mm filter for this shutter?
     
  21. michaelbsc

    michaelbsc Member

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    Well, it only works that way on a symmetrical lens. Other designs behave differently, and in ways that I don't understand.


    Oops. Didn't think of that.
     
  22. himself

    himself Member

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    just wondering if anyone knows what sort of coverage I can get out of a Carl Zeiss Jena Tessar 1:6.3, 120mm.

    thanks
     
  23. Dan Fromm

    Dan Fromm Member

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    Depending on whose exaggerated coverage claims you believe, anywhere between 138 mm (60 degrees) and 168 mm (70 degrees). 60 degrees is much more likely.
     
  24. himself

    himself Member

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    thanks for the info, but that's actually bigger than I've been previously told, so maybe I should just ask the guy if I can try it.
     
  25. himself

    himself Member

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    for those of you that still may have a small interest in what's going on here - the lens in question has now been moved to the home-made view camera I have (that can be seen here http://www.apug.org/forums/forum147/18930-pix-your-home-built-cams-here-please-32.html), so instead of starting a brand new thread I thought I'd hijack my own and change course a little.

    So the new camera shoots pretty much any size up to 4x5, but as you can imagine the lens won't come anywhere near that, but because the lens is on a camera with a longer extension than the plate camera some options have opened up:

    the thought of buying a new lens crossed my mind for all of the amount of time it takes to do an ebay search for large format lens, so I decided to go back to some of the suggestions I already had.

    taking one of the rear cells out of the shutter gives me more than enough coverage for any movements I have, but the image quality is poor and there doesn't seem to be much light getting through.

    so what I've now discovered is that if I tape the dipoter from holga-polaroid back to the lens, the focal length increases (not as much as taking a cell out) without any lose of quality, and so, but here's the thing - so does my image circle, now that can't be right can it?

    can turning my 105mm lens into a 150mm with the aid of a small plastic lens actually increase my image circle enough that I can swing and tilt to my hearts content, maybe not on 4x5 but certainly on 3x4 and all at infinity... I can scary believe it.