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Thread: optics question

  1. #1

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    optics question

    Could a lens be designed such that the rear element always produces an image circle of 200mm, but the front elements could be swapped such that the angle of coverage goes from wide to tele? Or could a spacer tube for the front element or rear element change the focal point such that it would render a similar image circle?
    I.e. a kit with effectively 65mm, 90mm, 135, 180, 250mm front elements with decreasing angle of views with same max aperature.

    This would require one back element and one shutter.
    The mountain waters of North Georgia call out to me, I visit and leave only tripod holes behind. The Appalachian Trail is my treadmill and gym.
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  2. #2
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    Re: optics question

    Some of the Nikon telephotos were built something along these lines - the front cell remained constant and the rears were changed. There were two families, 360/500/720 for 4x5/5x7 and 600/800/1200 for 8x10. While Nikon's literature claimed a constant image circle for each family, I believe the IC actually increased as the focal length got longer.

    What you propose is probably possible, but you might not want to pay for it... I hope Jason (Nodda Duma ) chimes in.

  3. #3
    Jim Jones's Avatar
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    Re: optics question

    Some old 35mm Kodak Retina models had interchangeable front cells to change the focal length. Casket lens sets have been around much longer. The image circle might vary with different combinations of lenses, but as long as it was adequate there would be no problem.

  4. #4

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    Re: optics question

    There have been lots of convertible lenses made over the decades for formats from subminiature cameras to large format. Most just switch between two focal lengths, but some have much more flexibility. Several 16mm cameras had wide-angle and tele-photo converters that fit on the front of the normal lens. The Canon Demi C half-frame had an interchangeable front element, as well. I have never heard of a set-up with quite the range you are talking about, but there are a ton of wide-angle and tele-converters -- of numerous strengths -- designed for the front of a normal lenses of any format. Quality? That's another issue. I know some are really quite good -- like the converters for the Minolta Dimage A series -- but some are not worth a dime. Impact on the image circle? I'm sure you will hear from others with more knowledge in this area.

  5. #5

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    Re: optics question

    Quote Originally Posted by Jim Jones View Post
    Some old 35mm Kodak Retina models had interchangeable front cells to change the focal length. Casket lens sets have been around much longer. The image circle might vary with different combinations of lenses, but as long as it was adequate there would be no problem.
    Jim, lots of 35 cameras used interchangeable front elements to change focal length. Besides the Retina the Contaflex Hmong TLR versions), Voightlander Bessamatic, Diax and Braun Paxettes come to mind.

  6. #6

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    Re: optics question

    I assume in theory the lens makers could create the equivalent of a small format 28-100mm zoom lens, but for 4x5 it would likely be quite large and max fstop would likely drop off as you zoom in as it does in smaller formats. In a simple lens system the middle lens would travel from front to rear while outer lenses are fixed.
    The mountain waters of North Georgia call out to me, I visit and leave only tripod holes behind. The Appalachian Trail is my treadmill and gym.
    http://www.esearing.com

  7. #7

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    Re: optics question

    Eric Searing asked about convertible lenses, got good and correct answers. But he added a condition that I don't think anyone addressed:

    I.e. a kit with effectively 65mm, 90mm, 135, 180, 250mm front elements with decreasing angle of views with same max aperature.
    As Bob Salomon in particular pointed out, there have been many 35 mm cameras with interchangeable front elements. None, as far as I know, in which all combinations had the same maximum aperture.

    Jim Jones mentioned casket sets. In the second half of the 19th century, many makers made Rapid Rectilinear casket sets for LF cameras, none with the same aperture for all focal lengths. Casket sets, with single cells well corrected anastigmats, continued to be made for LF cameras after anastigmats came in, were mostly gone by around 1930. Again, each combination had a different maximum aperture.

    The last casket sets offered were Berthiot's Ser. IVc, which contained 3 or 4 individual lenses. Last offered around 1951. These are somewhat an exception to the different apertures for each combination rule, single cells were typically f/12, combinations f/6.7 - f/8. There was only one Ser IVc set for 4x5 (cataloged for 9x12), #1, with six focal lengths, 88, 100, 135, 142, 230 and 305 mm. Not exactly what Eric wants.

    As far as I know there was only one family of wide angle casket sets, based on cells from lenses Berthiot later sold as f/14 Ser VIa Perigraphes. Practically speaking, non-existent.

    Short answer, Eric, get a mule.

  8. #8
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    Re: optics question

    Many 'convertible' lenses have been made for large format. However, if one desires the highest image quality, non-convertible lenses are better. Not everyone wants the sharpest lens, so any lens can be made a convertible. Try using any of your lenses with just the front or rear element. You can try them mounted on the front or rear, you have 4 combinations to try.

  9. #9

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    Re: optics question

    ic, close but no cigar. Not all LF lenses are separable. Separable lenses are, I think, mainly more-or-less symmetrical. Tessars certainly aren't separable. And single cells typically have focal lengths very approximately twice as long as the complete lens.

    Changing a single cell's orientation won't change its focal length. And not all shutters are symmetrical. Compur/Copal/Prontor #1s, for example. Cells that fit them can't be swapped front-to-rear.

  10. #10
    Nodda Duma's Avatar
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    optics question

    Yes, it could be done. However, it wouldn't be any cheaper than a set of fixed focal length lenses on separate lens boards to swap out as needed. Especially in today's market.

    A large format continuous zoom would be fun to design. It wouldn't be as big as one would think.
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