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Thread: 90 inch bellows - what is exposure?

  1. #1

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    90 inch bellows - what is exposure?

    I am setting up a shot I have tried before…unsuccessfully. Here are the specs. I am foggy on the bellow extension factor and reciprocity failure. The set up is for a head shot in the outdoors. It is about a 2X enlargement.

    20 x 24 Ilford FP4 film (125 ASA)

    About 90 inch bellows

    About 50 inch subject to lens distance

    24 inch Goerz Artar Red Dot lens apochromat (American Optical) (no shutter) at f11

    Gossen Luna Pro meter reads 17 which indicates a speed of 1/1000 at f11

    What exposure should I use?

    Thanks,

    Francis

  2. #2

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    Re: 90 inch bellows - what is exposure?

    Are you using flash or daylight?

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    Tim Meisburger's Avatar
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    Re: 90 inch bellows - what is exposure?

    Measure the opening in the lens (the aperture) on its widest f stop. Divide 90 inches by that measurement. Assume for a moment that the opening is one inch, then your actual f stop is f90. Use that to calculate exposure and you should be right on. So in your example, if the meter wants 1/1000 at f11, and you actually have f90, then you need to reduce your shutter speed to 125th of a second. Reciprocity is not relevant.

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    Re: 90 inch bellows - what is exposure?

    Daylight. in shaded area

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    Re: 90 inch bellows - what is exposure?

    Since you know the magnification, deciding what to do is easy. Effective f/# = (1 + magnification) * f/# set. At 1:1, effective aperture = 2 * aperture set. Set f/16, expose for f/32. At 2:1, set f/16, expose for f/45. And so on.

    In your case, if the meter says 1/1000 @ f/11, you want a shutter speed three stops slower. 1/500, 1 stop slower; 1/250, 2 stops slower; 1/125, 3 stops slower. And there you are. Tim nailed it.

  6. #6
    David Lobato David Lobato's Avatar
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    Re: 90 inch bellows - what is exposure?

    If the meter is reading an EV value of 17 at ISO 125, it's is too high for a shaded area. Shade would be close to 12 EV, 1/30th at f11 before bellows compensation. Adding 4 stops exposure for the 90 inch bellows is 1/2 second at f11. Someone correct me if I'm wrong on the 4 stops for the 90 inch bellows draw.

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    Re: 90 inch bellows - what is exposure?

    David, thanks for making me revisit the first post.

    Francis, with a 24" lens 90" extension, 50" lens-to-subject distance (where did you measure from?) and 2x don't seem quite right. Would you please check your measurements?

    2x wants 72" extension (rear node, which will be close to the diaphragm, to film distance) and 36" lens to subject distance (front node, which will be close to the diaphragm, to subject). This isn't your setup.

    90" extension with a 24" lens means 2.75 x, lens to subject distance 33".

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    Re: 90 inch bellows - what is exposure?

    FrancisF - I question your meter reading. I have a Gossen Lunasix and have confirmed that your meter reading of EV 17 does yield an exposure of 1/1000 @f11, but that level of luminance is nearly the highest that the meter can read and is far too high for an outdoor subject in shade. I suspect you're instead reading the Gossen SCALE (the one with yellow background) to be 17, thinking that's an EV value for ISO 125 (it is NOT), then turning the dial to an EV 17 setting to arrive at 1/1000 @ f11. If instead you set the dial at the 17 SCALE reading you would actually be at EV 12 (a realistic reading for such a subject). You would then be reading 1/40 @ f11; this result requires correction for bellows extension.

    The formula for correction due to bellows extension with ANY lens focal length is:
    Exposure FACTOR (i.e., a multiplier) = [extension/focal length]^2. The ^2 means the number in brackets is squared.

    Using your info, given Extension=90 and FL=24, the resulting EF=14 actually amounts to 3.8 stops and can be rounded to 4 stops. Since the EF is a FACTOR used to increase your exposure time, the new exposure time becomes 1/40 times 14, or about 1/3 second; so 1/3s at f11. This is approaching the reciprocity region for most films, and since you don't have a shutter to control short exposures anyway you may want to close the aperture down another two stops (giving 1-1/3s) to lengthen the exposure time enough to allow you to use a flat black card/darkslide/hat in front of the lens to time the exposure by hand. Of course, you’ll be limited by how long the subject of your “head shot” can hold still during the longer exposure. Reciprocity would not be severe.

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    Re: 90 inch bellows - what is exposure?

    Jerry I agree 100% with your math (the bellows factor being 14), and with your advice regarding the actual exposure meter reading.

    Rich
    Last edited by Rich14; 12-Aug-2017 at 22:16.

  10. #10
    Failed Bon Vivant Johnny LaRue's Avatar
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    Re: 90 inch bellows - what is exposure?

    Just a noob guess:
    If your bellows is extended by almost 4x the lens' focal length, add 4 stops to the metered reading.
    So f11 at 1/1000s becomes f11 at 1/60s.
    Too easy? Too wrong? I'm missing something, right?
    JLR

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