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Thread: Bellows correction with tele-photo?

  1. #21
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: Bellows correction with tele-photo?

    Quote Originally Posted by Emmanuel BIGLER View Post
    But before we continue, let's have a look at this diagram that I also have uploaded to the forum's database...
    Thank you Dr. Bigler. I'm still digesting the text of your reply.

    I have two questions regarding the first attachment.

    The distances H -> F = H' -> F' = 353mm suggest a 1:1 reproduction ratio.
    However, given the lens FL = 360mm, shouldn't those distances be about 720mm?

    Also, in the telephoto designs with which I'm familiar, H' is closer to the subject than H.
    For example, in the Zeiss 500mm Tele-ApoTessar, H is 80mm behind the film (away from the subject).

    Does the Tele-Arton use a different design?

    Thank you, Sir.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  2. #22

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    Re: Bellows correction with tele-photo?

    Thank you

    You are welcome.

    I have two questions regarding the first attachment.

    I realize that I made a small mistake (a classical cut-paste error when you forget to modify the pasted text) the exit pupil for the 360 tele arton is located about 89 mm behind the front filter ring.
    Here is the corrected file "tirage-tele-arton-360-EN.pdf"; the exit pupil (in magenta color) is located about 89 mm behind the front filter ring
    I have also attached the same diagram here in the forum's database as a JPEG image.

    The distances H -> F = H' -> F' = 353mm suggest a 1:1 reproduction ratio.
    However, given the lens FL = 360mm, shouldn't those distances be about 720mm?


    In fact F is the objet focal point and F' is the image focal point. Those points are not conjugate of each other.
    I have used the classical F and F' symbols as in all French textbooks on geometrical optics, but I realize that those good old textbooks suffer from an inconsitency. Usually A and A' denote an object and its conjugate image. F and F' are the exception to this rule. What I have drawn for F' comes from Schneider's datasheet; and for F, I have estimated "by hand" the position of this reversed focus F myself, by reversing the lens and aiming at a distant object. In principle the image quality of such an asymmetric lens once reversed should be terrible, but actually in the real world is not terrible enough to prevent the approximate pointing of the reversed focal point F. One thing which is granted of course for all lenses is that F is ahead of H by one focal length and that F' is behind H' by one focal length. here, 353 mm.
    An of course if you reverse a lens, its focal length is the same. This is one of the reasons why I try to avoid using the sentence "a retrofocus is a reversed telephoto" because one could dream that by simply reversing a long telephoto lens, you could get a wide angle retrofocus lens with a much shorter focal length;-)

    Also, in the telephoto designs with which I'm familiar, H' is closer to the subject than H.
    For example, in the Zeiss 500mm Tele-ApoTessar, H is 80mm behind the film (away from the subject).

    Does the Tele-Arton use a different design?


    Certainly yes. To the best of my knowledge there is no general rule regarding how H and H' are located in a telephoto design. In the 360 Tele Arton, H and H' come in this order but those principal points can be "crossed" i.e. H' can be ahead of H, like in the telephoto lens you are referring to. In the 360 tele arton, HH' is relatively small and positive: (137 - 63) = +74 mm. However according to old Schneider's specs, for the 250 tele-Arton, HH' = -32.5 mm, i.e H' is ahead of H unlike the 360 tele-arton.
    Why, I have absolutely no idea! The two lenses share the same name but are obviously a different design: the 360 tele arton is not the same as a 250, everything being scaled by a factor 360/250!

    I have looked at the following Zeiss datasheet for Hasselblad V-system cameras and I have found, I m not sure that we are speaking about the same references:
    500mm f/8 Zeiss tele-tessar C Nr. 1046093 - HH' = (413.5 - 125.5) = + 288 mm
    500mm f/8 Zeiss tele apotessar CF Nr. 104615 - HH' = (373.7 - 370.1) = +3.6 mm

    In both cases, HH' is positive but the actual value is very different between the older C model and the newer CF model.
    Note that those lenses exhibit a pupillar magification factor of about 22 mm (exit ) / 62 mm (entrance) = 0.35 which is much smaller than for the 360 tele-arton and would yield noticeable additional bellows factor corrections w/respect to a symmetrical lens design, if ever used in the close-up range (something that probably nobody ever experienced )
    Attached Thumbnails Attached Thumbnails tirage-tele-arton-360-EN.jpg  

  3. #23
    ic-racer's Avatar
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    Re: Bellows correction with tele-photo?

    'Bellows factor' is a change in effective aperture related to magnification. Projecting an object of known dimensions onto the ground glass is an easy way to quantify the effect. Personally, however, I use a film-plane light meter in these situations.

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