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Thread: DIY: tape measure for easy bellows extension calculations

  1. #1

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    DIY: tape measure for easy bellows extension calculations

    I was just sitting here at 5am on a Sunday morning thinking about an easy way to calculate bellows extension factors and thought of something.

    Maybe it has been done before or maybe it hasn't.

    I have seen some inexpensive small spring-loaded tape measures for sale in fabric stores, craft stores, etc. Unlike hardware store tape measures these can be only 4 or 5 feet long.

    OK, so what if I bought a tape measure for each of my lenses where I would need to know bellows extension? Let's say for example, my 300mm f/5.6 Rodenstock Sironar-N that I use for portraits. I would certainly be extending that lens out in order to photograph someone 8 or so feet away therefore I would need to know the bellows factor.

    It would seem to me to be very handy if I could simply place the hook of the tape measure on the film plane then pull it toward the lens. Then if I had already marked the bellows factor on the tape using a black Sharpie pen, I could adjust the shutter speed to make up for light loss.

    A second idea which is far lighter weight and cheaper would be to use some cloth gift wrap ribbon then note the shutter speed changes needed on it. I could glue a loop on one end that would attach to the back of my Cambo then when I had roughed in the portrait, I could lift the ribbon up to the front standard and quickly know the light loss.

    How fine should I make the correction marks? One second increments, half second, etc.?

    Does this make any sense to those assembled here today?

    Thanks,
    Terry

    Golly, I've got to get some sleep, I need to be at church at 7:45 to warm up for choir.

  2. #2

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    Re: DIY: tape measure for easy bellows extension calculations

    This sounds like the exposure compensation scale on the RB67. Great idea.
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  3. #3

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    Re: DIY: tape measure for easy bellows extension calculations

    It would be hard to do it in seconds, as the lighting and aperture would have to be consistant.
    I would think 1/2 or 1/3 stop intervals would be fine. You can likely estimate smaller intervals between your marks.
    Bill
    "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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    Re: DIY: tape measure for easy bellows extension calculations

    Quote Originally Posted by cowanw View Post
    It would be hard to do it in seconds, as the lighting and aperture would have to be consistant.
    I would think 1/2 or 1/3 stop intervals would be fine. You can likely estimate smaller intervals between your marks.
    Yes, you are right. But I was not thinking of actual seconds but more about how much to increase the time.

    Let's say for example that my light meter said the exposure should be 1/30th at f/8. But the lens was extended so much that I needed to double the amount of light hitting the film. So the tape measure or ribbon might read "2X" which would tell me to use a shutter speed of 1/8th.

    Or let's say the tape measure or ribbon indicated "2.5X". I could over compensate with the shutter speed by going to 1/4th which would be 3X but then stop down 1/2 of an f/stop to f/8 which would bring me back into line.

    Besides, for most B&W or color negative film, I don't think being a half stop off either way isn't going to be the end of the world. If I were using transparency film, that would be a different story.



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  5. #5
    Jac@stafford.net's Avatar
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    Re: DIY: tape measure for easy bellows extension calculations

    I like the idea of using a metric stick along the bellows. Several approaches are outlined on the LF main page article here.

    A clever, simple device was worked out by Philipp Salzgeber. See: http://www.salzgeber.at/disc/index.html

    And this is cool, too.

  6. #6

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    Re: DIY: tape measure for easy bellows extension calculations

    Quote Originally Posted by Jac@stafford.net View Post
    I like the idea of using a metric stick along the bellows. Several approaches are outlined on the LF main page article here.

    A clever, simple device was worked out by Philipp Salzgeber. See: http://www.salzgeber.at/disc/index.html

    And this is cool, too.
    I have known about Phillip's bellows factor tool for many years. But it's designed for close-up photography, so would not work well for portraits when I use my 300mm Rodenstock Sironar-N. That's what got me thinking about a simple alternate.

    Thank you for the link to the Cooksey-Talbott tool. It is exactly what I had in mind. What bothers me about his website is there is no explanation of the Egyptian hieroglyphs used in his ruler design.

  7. #7
    Youngin Daniel Stone's Avatar
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    Re: DIY: tape measure for easy bellows extension calculations

    You can use those bellows-extension factor rulers for general shooting as well. I used one a few months ago when doing a series of 4x5 shots of a friend and his wife, and it allowed me to dial in bellows extension perfectly. Approximate distance of camera to subject was 6 feet(using 350mm lens on 5x7).

    Just sayin

  8. #8

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    Re: DIY: tape measure for easy bellows extension calculations

    Quote Originally Posted by AtlantaTerry View Post
    I have seen some inexpensive small spring-loaded tape measures for sale in fabric stores, craft stores, etc. Unlike hardware store tape measures these can be only 4 or 5 feet long.
    I use one of the loose rubber-coated fabric rulers found in such fabric stores. They are large enough to not get lost easily but easy to stuff in a pocket and carry about. (A small Velcro square would allow hooking the tape end to the camera if desired though I think such an arrangement might generate more problems than it solves.)

    As far as determining what to do with the measurements it makes when figuring bellows extension, I just mentally do the distance as f-stop trick to figure out the exposure compensation for any lens. That has been discussed in detail on the forum but basically you think of the measured extension as an f-stop to determine compensation. For example, an 8" lens extended out 11" gives a one-stop loss in light (analogous to f/8 vs. f/11). At 16" the compensation would need to be 2-stops and so on. I just estimate the in-between extensions visually.

    If you actually want to calculate the value, the key multiplier to remember is 1.4 (or √2) for any lens. For example, 8" x 1.4 gives 11.2" as the one-stop loss point of extension. Likewise, 150mm lens x 1.4 gives 210mm as the one-stop loss point. For that matter, 4 finger-widths at infinity x 1.4 gives 5.6 finger-widths as the one-stop extension loss point, etc.

    Here's an extension table I made for reference that is also easy to carry:
    Attached Files Attached Files

  9. #9

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    Re: DIY: tape measure for easy bellows extension calculations

    Quote Originally Posted by Joe Smigiel View Post
    Here's an extension table I made for reference that is also easy to carry:
    Thank you.

  10. #10

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    Re: DIY: tape measure for easy bellows extension calculations

    I pretty much use Joe S's approach, but only when I remember to consider the bellows extension factor!
    The only trouble with doin' nothing is you can't tell when you get caught up

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