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  1. #1
    kram's Avatar
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    35mm focal length, max aperature and distance from film plane

    I have read that a 50mm f2 lens does not have to be a retrofocus design on a SLR as there is plenty of room between the film plane and rear element of the lens for the mirror box. With a 50mm f1.4 it does need to be a retro focus design (just, depends on camera make there is some variation in this distance) and with f1.2 definitely because the rear element is closer to the film plane than the mirror box will allow.

    With a rangefinder - no mirror box so a 50mm f1.4, f1.2 etc does not have to be of a retrofocus design, therefore can be made simpler and smaller (and more easy to higher optical corrections).

    However, with the light meter in modern 35mm RFs, there needs to be room to take the light measurement, for example, the Zeiss ZM needs 15mm between the film plane and rear element.

    My question can you work out when a lens on a 35mm rf needs to be a retrofocus design because of its focal length and/or its aperature e.g. 25mm f1.4, 28mm f1.4, 21mm f2, 12mm f3.5...?

    Also, the size of the rear element of the lens, for say a leica m mount camera, increases in size with aperature. When would this cause problems in the m mount? 25 f1.4? Any equations out there to calculate this stuff?

  2. #2

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    Quote Originally Posted by kram View Post
    I have read that a 50mm f2 lens does not have to be a retrofocus design on a SLR as there is plenty of room between the film plane and rear element of the lens for the mirror box. With a 50mm f1.4 it does need to be a retro focus design (just, depends on camera make there is some variation in this distance) and with f1.2 definitely because the rear element is closer to the film plane than the mirror box will allow.


    My question can you work out when a lens on a 35mm rf needs to be a retrofocus design because of its focal length and/or its aperature e.g. 25mm f1.4, 28mm f1.4, 21mm f2, 12mm f3.5...?
    The first part makes no sense. Just think about what the F/stop means. A 50mm lens is 50mm. You don't change it to change the F/stop.


    If the lens is shorter then the flange distance for the second part.

  3. #3
    Christopher Walrath's Avatar
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    Talkin' max aperture here. And more often than not focal length dictates interference with the shutter mirror than does aperture size among comparable focal length lenses I would think. Could be wrong though.
    Thank you.
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  4. #4
    bowzart's Avatar
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    I think there must be an error in what you read. It would be a rare 50mm lens that would need to be a retrofocus, or 35mm either, on a 35mm slr.

    I think you could easily tell. A normal, non retro lens can be used like a magnifier looking through it both ways. Looking through my 18.5mm Angenieux Retrofocus, I can get a good image only looking one way. The other way won't form an image at all.

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    You asked

    "Also, the size of the rear element of the lens, for say a leica m mount camera, increases in size with aperature."

    In general, the diameter of the front element of a lens increases as the lens's speed increases. The rear element's size and shape is determined by the lens designer's goals and requirements.

    "When would this cause problems in the m mount? 25 f1.4?" A too large rear element diameter could cause a mechanical interference problem between the lens and camera.

    I have a 21mm f4 Color Skopar lens (non-retrofocus) and a 25mm f4 Color Skopar lens (non-retrofocus). Both of these lenses work great on my M mount Leicas and Zeiss Ikon.

    My 35mm f1.2 CV Nokton is my fastest M mount lens, it is an Aspheric non-retrofocus wide angle design. The front-most lens element diameter is about 52mm. The rear-most lens element diameter is about 29mm.

    "Any equations out there to calculate this stuff?" Yes, plenty of them, an undergrad physics course at your local college is a good place to start.
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  6. #6
    ath
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    Well, if you look at the non-symmetrical 50mm lenses, the 4 element tessar is available for SLRs, the 6 element sonnar not. The sonnar needs the additional elements for correcting the errors coming with the bigger aperture and they interfere with the mirror.
    While there is no direct link aperture - mirror clearance there is an indirect one.

    As for the symmetrical lenses (gauss lenses): they need additional elements as well. A 50mm/1,4 has typically 7 elememts, a 50mm/2 has 5 or 6. The lens block gets physically longer and has to be moved slightly more outside for mirror clearance. This is not really retrofocus but goes in the same direction. There is a reason, why fast normal lenses for 35mm were 55 or 58mm in former times.

    There is no formula for this, its trade-off during lens design.
    Regards,
    Andreas

  7. #7
    Ole
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    Quote Originally Posted by bowzart View Post
    I think there must be an error in what you read. It would be a rare 50mm lens that would need to be a retrofocus, or 35mm either, on a 35mm slr.
    On the contrary, the first Retrofocus lens made (the Angenieux Retrofocus) was a 35mm lens for 35mm SLR's. A 35mm lens for 35mm SLR must be retrofocus if it's to clear the mirror.

    Some (fast) 50mm lenses are, most (not as fast) are not.
    -- Ole Tjugen, Luddite Elitist
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    Ole, in the 1930s TTH made retrofocus lenses for Technicolor cameras. They didn't use the word retrofocus, but that's what the lenses were.

    Many of the early "normal" (in the sense of standard issue) lenses for 35 mm SLRs were around 58 mm. This not because 58 mm is normal, but because many of the makers hadn't yet learned to design good 50 mm lenses that were retrofocus enough to clear the mirror.

  9. #9
    Ole
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    Dan, even an old single meniscus lense is slightly retrofocus - but the word "retrofocus" for "reversed telephoto design", as well as the use of that design to allow wider lenses to be used on SLRs, was introduced by Angenieux.
    -- Ole Tjugen, Luddite Elitist
    Norway

  10. #10

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    Ole, you're quibbling. Using shortish lenses with three strip Technicolor cameras would have been impossible without "reversed telephoto" lenses. And that's what TTH made for them.

    Angenieux didn't invent the design type, although he were the first to push it for the 35 mm SLR market. I'm not sure why TTH chose not to offer lenses for 35 mm SLRs.

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