Bellows factor

Discussion in 'Medium Format & Large Format' started by christopher walrath, Apr 6, 2008.

  1. christopher walrath

    christopher walrath No longer a newbie, moving up!

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    So, I have heard that there are like, oh I don't know, a googillion ways to figure bellows factors. How do you do it?
     
  2. Sw1tchFX

    Sw1tchFX TPF Noob!

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    For bellows comp, each 10-inches (i think, might be a foot) is a stop loss in light.

    for example, if your light meter says the exposure is 2 seconds, f/32 at ISO 100, if your bellows is extended 10" (or a foot) it becomes 4 seconds, f/32 at ISO 100.
     
  3. christopher walrath

    christopher walrath No longer a newbie, moving up!

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    But what if it's a 6 inch or a 12 inch lens?
     
  4. Battou

    Battou No longer a newbie, moving up!

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    How are you metering?


    I TTL and don't have any issues about factoring. I have only had two shots fail due to underesxpoure, that was due to not reading it properly and falling short in Max long exposure time, I wanted a shot that exceeded the ability of the camera on it's own and that was my mistake.
     
  5. christopher walrath

    christopher walrath No longer a newbie, moving up!

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    I'll be honest, I'm not doing it. Look below at my gear. Just wanting to stir up some good information in a forum that is near and dear. SO if you have a way of doing it then by all means . . .


    But I will one day . . .
     
  6. Battou

    Battou No longer a newbie, moving up!

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    I need to pay a little more attention to where I am on the forum, I have gotten a little rusty on forums capible of subforums, then again my bellows can't be all that different can it?
     
  7. Judge Sharpe

    Judge Sharpe TPF Noob!

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    There is a formula- can't think of it off the top of my head some thing like the bellows length squared divided by the apeture of the lens (what the heck is a 6 inch len anyway?) to give you a factor. once you have figured it and writen it on a card for a particular lens/ camera, you can forget how to do it. Small changes in bellows length for focus would not matter.

    The used to be a folk band in Birmingham, Alabama named Three on a String- They played at one of my watering holes from a past life. Any relation?
     
  8. Helen B

    Helen B TPF Noob!

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    Apart from TTL metering and marked bellows, the easiest way I know of is to use a Quick Disk.

    As Christopher says, there are many ways of calculating it. I find the easiest way of remembering the general formula is to think of the effective f-number as being the distance from the rear node to the film plane divided by the pupil diameter. That may sound difficult, but it is very easy once you have the physical concept in your head. It's not correct in all cases, but it is often near enough. Retrofocus and telephoto (true telephoto, not just long focus) lenses mess the formula up because the location of the exit pupil (or the pupil magnification) is a key factor.

    For a one-stop reduction the rear node will be 1.4 times the focal length from the film (ie a bellows extension of 0.4 times the focal length beyond the infinity focus position), for a two-stop reduction the the rear node will be 2 times the focal length from the film (ie a bellows extension of one focal length beyond the infinity focus position). You can use the scale on a light meter as a calculator. It's a lot harder to describe than to do it. You just need to have a clear understanding of why you are doing it

    Best,
    Helen
     
  9. Hertz van Rental

    Hertz van Rental TPF Noob!

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    In simple terms, f-numbers only work as marked until an object approaches to within about 10 focal lengths of the lens.
    Various calculations can be used depending upon what values are known.
    The simplest is based on the calculation for f-stops:
    f-number = N
    focal length of lens = f
    diameter of iris opening (aperture) = d

    N = f/d

    This works when the bellows extension is about the same as the focal length of the lens.
    At greater extensions:

    N1 = v/d

    Where N1 is the effective aperture and v = the bellows extension.
    The practical rule of thumb derived from this:
    (Bellows extension relative to the focal length / approx. increase exposure by)

    1.5 x f = x2.25
    1.75 x f = x3
    2 x f = x4
    2.5 x f = x6
    3 x f = x9
    4 x f = x16
    5 x f = x25

    Then you just use a ruler, measure the bellows and relate it to the focal length of the lens in use.
    The table is only approximate so bracket your exposure.
    TTL metering, or using a meter that works off the ground glass screen on LF, will automatically adjust for bellows.
     
  10. Helen B

    Helen B TPF Noob!

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    Just a minor point: the f-number is the focal length divided by the diameter of the entrance pupil, not by the diameter of the iris opening (unless the two are equal, of course). This is a fairly important concept to understand, particularly when you are trying to comprehend zoom lens specifications.

    Best,
    Helen
     
  11. christopher walrath

    christopher walrath No longer a newbie, moving up!

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  12. Helen B

    Helen B TPF Noob!

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    That looks like a less well thought out version of the Quick Disc. There's a reason for the QD to be circular rather than square.

    Best,
    Helen
     

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