Easy way to caculate bellows extension?

Discussion in 'Large Format Cameras and Accessories' started by bmac, Dec 4, 2002.

  1. bmac

    bmac Member

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    Does anyone know of an easy way to caculate bellows extension for exposure?
     
  2. SteveGangi

    SteveGangi Member

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  3. Prime

    Prime Member

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  4. bmac

    bmac Member

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    You guys are great! Thanks for the help. It is great that this board has turned into such a great resource.
     
  5. jmcd

    jmcd Member

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    I square the lens focal length, then divide this into the bellows draw squared. This yields the exposure factor. With black and white, it is easy to arrive at a close approximation of the exact answer by doing the work in your head, which is easier for me than carring one more thing around. If you just use a lens or three, after awhile all you need to do is measure the bellows, and you already know the answer.

    With transparencies an exact answer is appropriate, so I work it out on paper or use a calculator.
     
  6. fineart

    fineart Member

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    This is the easiest way. Convert the focal length of the lens to an f-stop (210mm would be f8 - 8 inch lens; 150mm would be 5.6 - 6 inch lens). Okay? The progression in bellows extension is now the same as the progression in f-stops. A 6" lens (150mm) lens racked out to 8" is one stop more because f8 is one stop from than 5.6. Racked out to 11", it's two stops, etc.

    Strictly speaking, this isn't scientifically accurate. The example above would be about 1/2 stop overexposed. Inconsequential. I've used this for over 20 years and it always works. You should have seen the expression from the RIT slide rule-type "teacher" when I brought this up in class in 1980. Nitwit.

    Also - same size images always requre 2 stops more exposure. For a same size image, the bellows will always be extended to twice the focal length.

    One more thing pleeze - Life is easier if you use short lenses for close ups.

    Okay? Now go take some beautiful pictures

    Anthony - Fine Art Photo Supply
     
  7. Doremus Scudder

    Doremus Scudder Member

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    Brian,

    Calculations take more time than anyone wants to take in the field. Use one of the methods above and make a chart for each of the lenses you own of the exposure factor and stops to open at several different extensions (intermiediate values are easy to estimate). This you can then carry with you along with a small, lightweight tape measure. Determining the factor is then a simple matter of quickly measuring the total bellows extension and consulting the chart. I carry a small notebook with me anyway for an exposure record and bellows extension factors and reciprocity info, along with other stuff I need are there as well. Some people tape the tables to their cameras. I find the tables faster than the disc and measuring the ground glass methods.

    Hope this helps a bit. ;^D)
     
  8. bmoulton

    bmoulton Member

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    I sewed a tape measure to my dark cloth. With camera ready to shoot, I measure the distance from the film plane to the lens. Example: that distance is 12 inches. Then I consider the focal length of the lens I am using. Example: Focal length 210mm, or 8.4 inches.
    Now think f stops. How far apart are F12 and F8.4? Answer: 1 1/3-1 1/2 stops.
    So I need to add 1.33-1.5 stops.
    It works for me.

    Would it be accurate enough for extreme closeups? Maybe not. Then I would ue the Caulmet exposure dial.
    Bob
     
  9. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    I estimate the magnification ratio or determine it precisely by putting a ruler at the plane of focus and comparing the field of view to the width of the frame, then use a chart to determine exposure factor based on the magnification ratio. It works the same way for every format, independent of the lens. The principle is the same as with the QuickDisk. Somewhere here I posted an MS Word doc file that you can download with a magnification/exposure table in small print so it can be taped to the groundglass frame of your camera.