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  1. #11

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    wow that seems like such a bad design but now I know I'm not completely crazy I'm going to borrow a camera with aperture priority and a similar focal length zoom and run some tests with the two TTL metering systems and the incident meter and a grey card and see what is actually going on at 210mm with this lens. I will post the results here in case anyone is interested.

  2. #12
    RalphLambrecht's Avatar
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    [QUOTE=

    So with this lens zoomed to 210mm the largest effective aperture is F5.6. This is presumably the amount of light which is let in when the lens aperture is set to F4 on the aperture ring.

    However, if I set the lens aperture to F5.6 on the ring and then zoom to 210mm there is clearly less light coming in than when zoomed to 210mm and set to F4. This is clear when looking through the lens and is confirmed by the camera's TTL meter. So would the effective aperture actually be F8 with the lens at 210mm and the aperture ring at 5.6? Do I have to compensate a whole stop throughout the entire range of the lens? [/QUOTE]

    that is my understanding,yes!
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    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
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  3. #13
    MattKing's Avatar
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    The Tamron lens in question was most likely marketed at a time when auto-exposure functions were common in cameras, and directed to photographers who would use them.

    So the variable aperture probably went unnoticed by many, at least until they tried to use flash .

    And if the lens happened to be used on a camera with TTl flash, not even the.
    Matt

    “Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”

    Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2

  4. #14

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    This is the best discussion on the topic I can find.
    http://neilvn.com/tangents/exposure-...erture-lenses/
    "There are a great many things I am in doubt about at the moment, and I should consider myself favoured if you would kindly enlighten me. Signed, Doubtful, off to Canada." (BJP 1914).

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    Bill

  5. #15
    Leigh B's Avatar
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    Quote Originally Posted by nwilkins View Post
    wow that seems like such a bad design
    Not at all. In fact it was a simple and inexpensive solution to a very complex lens-design problem.

    These lenses were designed for use with cameras having through-the-lens (TTL) metering, not with hand-held meters.

    Using the lens on a TTL camera, when you zoom to a longer focal length, it's as though the ambient light decreased.

    From a metering standpoint there's no difference. The TTL meter thinks the lens is still at f/4 and the scene got dimmer.

    - Leigh
    “Wise men talk because they have something to say; fools, because they have to say something.” - Plato

  6. #16

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    This interested me so I put such a 28-200 3.8-5.6 Zeiss Jena zoom on a manual 35mm camera and looked through the open back at shutter speed B. I expected the size of the aperture to stay the same as I zoomed out, resulting in a dropoff of F because of: F= focal length/apparent aperture
    I was surprised to find another issue. The apparent aperture decreased in size as the focal Length lengthened.
    So the F number decreases not only because the focal lenth changes, but also because the apparent aperture changes. A double wammy.
    Serious testing is required to use this lens on manual!
    "There are a great many things I am in doubt about at the moment, and I should consider myself favoured if you would kindly enlighten me. Signed, Doubtful, off to Canada." (BJP 1914).

    Regards
    Bill

  7. #17

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    Quote Originally Posted by cowanw View Post
    This interested me so I put such a 28-200 3.8-5.6 Zeiss Jena zoom on a manual 35mm camera and looked through the open back at shutter speed B. I expected the size of the aperture to stay the same as I zoomed out, resulting in a dropoff of F because of: F= focal length/apparent aperture
    I was surprised to find another issue. The apparent aperture decreased in size as the focal Length lengthened.
    So the F number decreases not only because the focal lenth changes, but also because the apparent aperture changes. A double wammy.
    Serious testing is required to use this lens on manual!
    And folks wonder why I don't use zooms, ever.

  8. #18
    Diapositivo's Avatar
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    For what I understand, when using an external meter with a variable aperture zoom lens you normally worry only when using the lens at its maximum aperture. When using the other positions in your diaphragm ring, the f/number indicated remains the same regardless of the focal length.

    An example: sun high, bright day, front lit subject, you use the "sunny 16" rule, and decide for 1/250 @ f/11 with ISO 125.

    Your variable aperture zoom should give you f/11 at every focal length.

    That's because when you use the lens at a less-than-full-aperture, you cut away the geometric limit given by your front lens. Your front lens is larger than what is needed to give you f/11, at all focal length.

    In the case of the OP I am quite puzzled about what would happen with the second value, 5.6, supposing such a value exists on such a lens as a separate case from full aperture, but that would appear very strange to me.
    The most normal case is that the variable aperture lens has full aperture indicated as f/4-f/5.6 and the next aperture is straight f/8.
    In this case, f/8 is reliable at all focal length when using an external light meter.
    Fabrizio Ruggeri fine art photography site: http://fabrizio-ruggeri.artistwebsites.com
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  9. #19

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    Quote Originally Posted by Diapositivo View Post
    For what I understand, when using an external meter with a variable aperture zoom lens you normally worry only when using the lens at its maximum aperture. When using the other positions in your diaphragm ring, the f/number indicated remains the same regardless of the focal length.

    An example: sun high, bright day, front lit subject, you use the "sunny 16" rule, and decide for 1/250 @ f/11 with ISO 125.

    Your variable aperture zoom should give you f/11 at every focal length.

    That's because when you use the lens at a less-than-full-aperture, you cut away the geometric limit given by your front lens. Your front lens is larger than what is needed to give you f/11, at all focal length.

    In the case of the OP I am quite puzzled about what would happen with the second value, 5.6, supposing such a value exists on such a lens as a separate case from full aperture, but that would appear very strange to me.
    The most normal case is that the variable aperture lens has full aperture indicated as f/4-f/5.6 and the next aperture is straight f/8.
    In this case, f/8 is reliable at all focal length when using an external light meter.
    What you said is only true with modern lenses and mounted on a camera that control the aperture. The camera interfaces with the lens and know what zoom position you set the lens at and open up the aperture to compensate automatically.

  10. #20
    Diapositivo's Avatar
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    Quote Originally Posted by Chan Tran View Post
    What you said is only true with modern lenses and mounted on a camera that control the aperture. The camera interfaces with the lens and know what zoom position you set the lens at and open up the aperture to compensate automatically.
    I don't fully understand why it should be so.

    The f/number should be, broadly speaking, the result of the focal length divided by the diameter of the front lens.

    For instance, 100/50 = 2. In order to have a f/value of 2, a 100mm lens must have a front lens of 50mm of diametre. A 200mm must have a front lens of 100mm in order to have a f/2 aperture. A 200mm with a front lens with a diameter of 50mm has a f/value = 4.

    This corresponds to what we observe in our lenses. The ordinary 50mm f/2 lens actually has a front lens with a diameter of 25mm.

    We are talking here about the f/value which is just the result of this simple division, and doesn't take into account internal losses of light due to refraction, the T/value or such other measures.

    When you "stop down" a lens, you cut out of the picture the contribution of the outer part of the front lens. You are actually using a smaller part of the entire optical system.

    The thing, as I see it, is entirely "geometric" and has nothing to do with aperture and focal length communication between lens and body.

    The variable aperture zoom has a variable aperture because the same front lens must serve all focal lengths. Given that the diameter of the front lens is invariable, the f/value will vary according to the focal length of the zoom state. At the maximum focal length the entrance pupil will yield less light, a higher f/value.

    When you stop down, you artificially reduce the part of the lens that you use. It was so far my understanding that by cutting away the outer part of the entrance pupil, the part which is used varies with the focal length, but you always have "enough pupil" or "enough light" for that closer f/value.

    Said in other terms, the front lens is not "wide enough" to give f/4 at all focal lengths, but it is "wide enough" to give f/8 at all focal lengths. The actual part of the front lens which is "used" at various focal length position varies (the lens is used "more" or "less" during the zooming action) but at f/8 there is "enough glass" at all focal lengths.

    I do use my external light meter with my 28-85 3.5/4.5 lens, and never considered correcting for the variation at f/8. (It's also true that between 3.5 and 4.5 there is around "only" 2/3 of EV, but I think using slides I would have noticed).

    Thinking about it, considering 50mm (the half excursion) the "right" value (f/4, or f/8, or whatever) I would have an exposure mistake which is "up to 1/3 EV" on the wide zone and "up to 1/3 EV" on the tele zone. That makes it not so easy to spot. Yet, I don't understand why what I wrote above, perhaps not very clearly, shouldn't hold valid.
    Last edited by Diapositivo; 08-25-2012 at 02:09 PM. Click to view previous post history.
    Fabrizio Ruggeri fine art photography site: http://fabrizio-ruggeri.artistwebsites.com
    Stock images at Imagebroker: http://www.imagebroker.com/#/search/ib_fbr

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