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Thread: Convertible lens query

  1. #1

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    Convertible lens query

    I’ve a barrel mounted convertible lens that has been identified by Schneider as a 180mm Symmar from the 1960s.
    However, I’m confused over the two sets of aperture numbers on opposite sides of the ‘iris ring’.

    With the front lens cells fitted the aperture set of
    5.6 8 11 16 22 32 45 is used
    Is used whereas with the front lens cell removed the aperture set of
    12 16 22 32 45 is used

    My question is why, when there are seven click stops on the iris ring, do I only get five aperture settings when the lens is in its ‘converted’ mode?
    Regards
    Tony

  2. #2

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    Re: Convertible lens query

    Because.

    The single cell's maximum aperture is f/12. That sets the wide open end of the scale. Schneider thinks you shouldn't use apertures smaller than f/45. That sets the other end of the scale.

    In other words, you have been short changed and should write angry letters to editors.

  3. #3

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    Re: Convertible lens query

    Quote Originally Posted by tonyowen View Post
    I’ve a barrel mounted convertible lens that has been identified by Schneider as a 180mm Symmar from the 1960s.
    However, I’m confused over the two sets of aperture numbers on opposite sides of the ‘iris ring’.

    With the front lens cells fitted the aperture set of
    5.6 8 11 16 22 32 45 is used
    Is used whereas with the front lens cell removed the aperture set of
    12 16 22 32 45 is used

    My question is why, when there are seven click stops on the iris ring, do I only get five aperture settings when the lens is in its ‘converted’ mode?
    Regards
    Tony

    In fact when converted by removing the front cell, f/12 and f/16 deliver some softness in the corners, if you want better sharpness in the corners you should use f/22 or f/32. So in reality (when converted) you may want to use two of the 5 stops.

    At original 180mm you have a very good lens (single coated, anyway), when used converted to the longer focal you have important limitations, most important is focus shift: This is if you close diafragm from f/12 (while focusing) to f/22 (for the shot) you should adjust again bellows extension for best focus on your subject. So you end focussing at f/22 with a very dim image on the ground glass, so you need to use bright points in the scene or use a very light tight cloth. At 180mm configuration there is no appreciable focus shift, only when converted !

    Time ago some people were saying that the converted configuration was very bad because they didn't know the focus shift effect, IMHO.

    Well, the converted configuration is not as good as the original one, and has drawbacks, but still it is very usable, just less practical, but it also saves weight in a backpack !!

    Here you can see a test, speaking in lp/mm comparing the 150 symmar with its 265 conversion, it is not a lab test but it is good enough for a fair comparison: http://www.hevanet.com/cperez/testing.html

    This test also shows that the converted lens does not improve with a yellow filter and that some statements were a mith. This suggests that the slightly lower performance of the conversion is due to more Spheric Aberration rather to more Chromatic Aberration, as bandpass filters tend to lower Chromatic Aberration effect in sharpness. In fact we see near the same performance with the filter.


    Regards

  4. #4

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    Re: Convertible lens query

    Thanks Pere,
    A month later and I’m still somewhat confused over the settings of the lens’ two conditions [180mm & 315mm FL].
    Given that aperture diameter = FL/f# I get the following aperture diameters for both focal length conditions and available stops.
    F# 5.6 8 11 12 16 22 32 45
    180mm 32.14 22.5 16.36 11.25 8.18 5.62 4
    315mm 26.25 19.59 14.32 9.84 7

    The factory barrel with integral iris is common to both lens conditions and the f# is controlled by click stops so how/why can the aperture diameter vary for a specified stop?
    The below table shows the relative aperture diameters assuming that f5.6 [180mm] and f12 [315mm] are the initial and identical click stop setting
    F# 5.6 8 11 16 22 32 45
    180mm 32.14 22.5 16.36 1.25 8.18 5.62 4
    315mm 26.25 19.59 14.32 9.84 7
    F# 12 16 22 32 45

    Regards
    Tony

  5. #5

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    Re: Convertible lens query

    sorry about the misaligned tables I set them up correctly when doing the reply
    Tony

  6. #6

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    Re: Convertible lens query

    Tony, the f/number has a dividend and a divisor. The dividend is the lens' focal length. The divisor is the diameter of its entrance pupil, not the aperture's diameter.

  7. #7

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    Re: Convertible lens query

    I used an example of your lens for many years on the job at Kodak. The marked apertures were accurate (although I seldom used the 315mm focal length.)
    When using both groups, front and back, you have a 180mm lens with a max aperture of f/5.6. When using the rear group alone, you have a 315mm lens with a max aperture of f/12.
    You can prove the accuracy of the f/stop markings by setting up a photo. Make one exposure with the 180mm @ f/22, and one exposure with the 315mm @ f/22. The resulting negatives should have the same densities.
    If you need to understand this more deeply, you'll want a book on photographic optics. I'm sure this is covered in the classic book "Lenses in Photography" by Dr. Rudolf Kingslake, from the 1940s, as well as more modern publications.

  8. #8

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    Re: Convertible lens query

    Yes Tony, as Dan says the physical aperture is not used to calculate the f/ number, as optical paths can flow compressed or expanded by the diafragm section because refractive action of the front/rear cells.

    http://hyperphysics.phy-astr.gsu.edu...oopt/stop.html



    If you want you have the choice to calibrate diafragm by using a ($20) luxmeter, I used this way to calibrate a lens after shutter replacement.

    > This consists in framing a well illuminated smooth wall with constant light level, (I use a light table), set bellows for focus at infinite.

    > Measure the Lux level in the film plane with a lens than has well calibrated f stops.

    > Stop down the diafragm of the lens that has to be calibrated until it is reached the same Lux level, mark the position. It is not an "optician guy" job but at least it calibrates it very well for exposure determination.


    Most of LF lenses have similar transmission performance at the same stop, being single coated not far from multicoated, and uncoated lens has lower transmission. In general a SLR/DSLR prime lens has similar transmission for same f/ stop than a LF lens at the same stop.


    Regards.

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