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Thread: How to calculate bellows extension from the bellows factor?

  1. #1
    Cor's Avatar
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    How to calculate bellows extension from the bellows factor?

    I sometimes like to play "arm chair" LF using my ancient Palm Pilot. I run pCAM (now a neat iPhone app it seems) and Bob Wheelers extensive Vademecum.

    I am thinking about using my Wolly Raptar 480mm for some (sharp) portraits on 8*10.

    So pCAM gives me the distance from the lens to the sitter (a shoulder/head portrait) through it's nice schematic scene preview function, and the DOF function of the Vademecum gives me the DOF limits and the bellows factor (the correction in stops or time to compensate the bellows extension)

    Now my question is how I can get from a Bellows Factor of 3.2 to the extension or total length of the bellows for said shot?

    yes I know I can mount my camera with that lens and so on, but I still would like to know how to calculate the bellows draw in a given situation, also when I am considering buying a new lens, for which I do not have enough bellows to begin with.

    thanks in advance,

    Best,

    Cor

  2. #2

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    Re: How to calculate bellows extension from the bellows factor?

    At infinity the focal distance is the focal length of the lens. Assuming the nodal point is on the same plane as the lensboard, the bellows extension is 1 * f. When the object is 2 * the focal length form the lens, the focal length is 2 * f (1:1 image size). Between these 2 distances, there is a linear curve so calculating bellows extension is basically an algebra equation. Without knowing the distance of infinity for the particular lens you can not make accurate calculations, but you should be able to get fairly close.

  3. #3
    Land-Scapegrace Heroique's Avatar
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    Re: How to calculate bellows extension from the bellows factor?

    Quote Originally Posted by Cor View Post
    Now my question is how I can get from a Bellows Factor of 3.2 to the extension or total length of the bellows for said shot?
    I’m no good at doing LF w/ palm pilots, iPhone apps, pCAM, or Vademecum, but maybe I can help you with some very, very, very simple guidance:

    Why not focus on your subject, note the length of your bellows, and add an appropriate amount “bellows correction” to your metered exposure?

    A useful rule of thumb – don’t worry too much about “correction” unless the distance to your subject is 10x your focal length, or closer. With your 480mm lens, this threshold would be 10 x 480mm = 4800mm (that is, 4.8 meters, or closer).

    If this is the case, the next rule of thumb is to add ˝ stop for every 25% increase in your bellows extension (beyond infinity).

    -----
    If you really want to start with a bellows factor, and use it to calculate the extension amount, just reverse these rules of thumb. So, if you somehow already know that your bellows factor is 1.5x (or about ˝ stop), then a workable bellows extension would be 25% beyond infinity, or 600mm in your case (480mm + 25%).

  4. #4

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    Re: How to calculate bellows extension from the bellows factor?

    take your focal length in inches (18.8") then measure the distance (in inches) from the film plane to your lensboard with the camera focused on your close subject. Pretend your focal length is an f stop. If it's 32 inches (f32), that's a difference of roughly (f16.5 to f32) 1 1/2 stops. This rule applies to any focal length.

  5. #5
    8x10, 4x5, ..., Tessina
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    Lightbulb Re: How to calculate bellows extension from the bellows factor?

    Quote Originally Posted by memorris View Post
    At infinity the focal distance is the focal length of the lens. Assuming the nodal point is on the same plane as the lensboard...
    Not exactly.

    The distance from the film plane to the front of the lensboard when focused at infinity is the flange focal length (FFL) of the lens, which is on the lens data sheet.

    It's generally close to the focal length only for lenses of moderate FL.
    On wide angle lens, the FFL is usually longer than the FL, to prevent slamming the lens into the ground glass, e.g. 73.4mm for the Fujinon SWD 65mm/5.6.
    On long FL lenses the FFL is generally less than the focal length (e.g. 261mm on the Nikkor-T 360mm/8), so you don't need as much bellows draw.

    In all cases, the bellows extension beyond the FFL required for a 1:1 shot equals the lens focal length.

    - Leigh

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    Re: How to calculate bellows extension from the bellows factor?

    It is measured from the nodal point and that is different from the plane of the lensboard, or flange but as a quick and simple way to do the calculation and get in the ballpark, it works.

    Admittedly I am no lens expert but was taught that the number is when the distance required to focus is 2 X f that yields 1:1.

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    8x10, 4x5, ..., Tessina
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    Lightbulb Re: How to calculate bellows extension from the bellows factor?

    Quote Originally Posted by memorris View Post
    ...the number is when the distance required to focus is 2 X f that yields 1:1.
    That's correct from an optical design standpoint. However, the amount of bellows extension may be substantially different.

    In the case I cited above for the Nikkor-T 360/5.6, its flange focal length is 100mm shorter than its optical focal length, a difference of almost 4 inches. That's a lot.

    You're correct that lens focal length is measured from the rear (second) nodal point to the film plane along the optical axis.

    However, most data sheets don't give the location of that nodal point or of the second principle plane, on which it resides. They do give the FFL.

    - Leigh

  8. #8
    Cor's Avatar
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    Re: How to calculate bellows extension from the bellows factor?

    Quote Originally Posted by Heroique View Post
    If this is the case, the next rule of thumb is to add ˝ stop for every 25% increase in your bellows extension (beyond infinity).

    -----
    If you really want to start with a bellows factor, and use it to calculate the extension amount, just reverse these rules of thumb. So, if you somehow already know that your bellows factor is 1.5x (or about ˝ stop), then a workable bellows extension would be 25% beyond infinity, or 600mm in your case (480mm + 25%).
    Thanks for the feedback of my odd question. Perhaps I phrased it clumsy, so I'll try again, this time starting with the hardware:

    I have this 480mm lens I would like to try on my Toyo 8*10 for a head/shoulder portrait. My max bellows draw on the Toyo is about 650 mm.

    If I plunge this figures in the Palm I arrive at a bellows factor of 3.2.

    So how much bellows will I need ?

    If I follow above rule of the thumb: say bellows factor of 3; 1 1/2 stop; 75% beyond infinity: 480 + (.75*480)=840mm

    So not enough bellows, but I knew that, I am considering constructing a top hat lens board (is that the correct description?) and I know more or less know what extra length I neeed, which is considerable..not sure if that will work..

    best,

    Cor

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    Re: How to calculate bellows extension from the bellows factor?

    Bellows extension exposure factors are calculated using the following formula:
    Ext˛ / Fl˛ = Exposure Factor

    Where
    Ext = Bellows extension measured from film plane to lens nodal point (middle of the lens for most designs) and
    Fl = The focal length of the lens.

    Simply rearranging the equation to solve for Fl results in:

    Ext˛ = Fl˛ * Exposure Factor

    So, you would multiply 480˛ by 3.2 and then take the square root of the product.

    I get 858.65mm

    That should be the answer to your question.

    Best,

    Doremus Scudder

  10. #10
    Cor's Avatar
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    Re: How to calculate bellows extension from the bellows factor?

    That's were I was looking for, thanks again, Doremus!

    Best,

    Cor
    Quote Originally Posted by Doremus Scudder View Post
    Bellows extension exposure factors are calculated using the following formula:
    Ext˛ / Fl˛ = Exposure Factor

    Where
    Ext = Bellows extension measured from film plane to lens nodal point (middle of the lens for most designs) and
    Fl = The focal length of the lens.

    Simply rearranging the equation to solve for Fl results in:

    Ext˛ = Fl˛ * Exposure Factor

    So, you would multiply 480˛ by 3.2 and then take the square root of the product.

    I get 858.65mm

    That should be the answer to your question.

    Best,

    Doremus Scudder

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