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  1. #11
    David H. Bebbington's Avatar
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    Quote Originally Posted by TheFlyingCamera View Post
    Bellows compensation is pretty straightforward. For whatever lens you're using, no compensation is required when the lens is focused at infinity. At life size magnification (1:1), the bellows is extended 2x the focal length, and requires 2 stops. At 1/2 life size, the bellows is 1.5x the focal length at infinity, and requires 1 stop. From there you can calculate your intermediate compensation factors. Below 1/4 life size (a half-stop) unless you're shooting chromes, I wouldn't worry about exposure compensation, because you can't set less than a half-stop on the shutter speed anyway. With the Mamiya C2xx/3xx lenses, I don't know if the fstops are marked in less than 1/2 stop increments, but they're small enough it would be hard to precisely set a quarter-stop increment.
    The only practical aspect to note is that exposure compensation is in fact dependent on the number of multiples of back focus distance the lens is extended, not focal length as such. From what I remember of my Mamiya TLRs, all the lenses up to 80 mm were retrofocus designs (back focus longer than focal length), the 105 was of conventional design, and all the longer lenses (135, 180, 250) were telephotos (back focus shorter than focal length).

    Regards,

    David

  2. #12

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    David, are you sure its back focus and not focal length? Really, really sure?

    If you are, please direct me to a reference.

    Cheers,

    Dan

  3. #13
    David H. Bebbington's Avatar
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    Quote Originally Posted by Dan Fromm View Post
    David, are you sure its back focus and not focal length? Really, really sure?

    If you are, please direct me to a reference.

    Cheers,

    Dan
    Haven't got a reference, but the principle worked for me when I had a Mamiya TLR!

    Regards,

    David

  4. #14
    David H. Bebbington's Avatar
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    Dan, I just made an experiment with my "office ornament" Speed Graphic and a 240 mm Tele-Arton:

    Camera extension at infinity focus 6.5", camera extension at nearly life-size (1:1) 12.5 inches (no more available on camera). Certainly no question of needing twice the focal length (480 mm extension) to get to 1:1.


    Regards,

    David

  5. #15

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    Ummm... Why don't you look at the chart on the side of the bellows? C220 gives bellows exposure factor on a line at the bottom.

    Poisonally, I would buy a rb67 with a 90mm lens and a #2 extender tube. Throw the dang paramender away.

    tim in san jose
    Where ever you are, there you be.

  6. #16
    David H. Bebbington's Avatar
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    Quote Originally Posted by Dan Fromm View Post
    David, are you sure its back focus and not focal length? Really, really sure?

    If you are, please direct me to a reference.

    Cheers,

    Dan
    I was just glancing through my Kodak Professional Photoguide (3rd edition, 1998) and found an entry on close-up exposure (pp. 46 - 47) which confirms that the usual formula does not work with retrofocus or telephoto lenses. Kodak suggests a formula for these lenses:

    Exposure factor = (M/P + 1) ²

    where
    M = Magnification
    P = Pupillary magnification

    P is calculated as rear exit pupil diameter divided by front entrance pupil diameter (so it is specific to a particular lens).

    Will send PDF file of these pages to anyone interested.

    Regards,

    David

  7. #17

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    Oh. Correction for pupillary magnification. Well-known, been around forever, but often ignored.

    When I bought my PB-4 in 1970, a sheet giving magnification vs. exposure factor curves for many, many Nikkors was packed with it. The curves for w/a lenses, all retrofocus, are terrifying.

    IIRC, there's a full discussion, with formulae for lens facing normally and lens reversed in Lefkowitz.

    Cheers,

    Dan

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