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Thread: how to calculate exact bellows compensation

  1. #1

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    how to calculate exact bellows compensation

    Hi

    I am doing some 10x8 macro transparencies ... which cost basically £40 a sheet film and process here in England
    thankfully somebody else is paying but still every sheet counts
    I could bracket +/- 1/3rd but that would probably push the budget over the edge and they would revert to digital which nobody wants

    so what I would like to know is how do I calculate exactly

    I have the math but where would I measure extension from
    its from optical centre to film plane but how do i know where the optical centre is and then the film plane
    after quite extreme movements is all over the place
    4 corners of front and rear standards have probably +/- 10cm difference
    obvious answer is to just average but is there a precise way

    The transparencies will be displayed on light boxes so they need to be as near perfect as possible


    best

    robin

  2. #2

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    Re: how to calculate exact bellows compensation

    Not to be a complete idiot, but why calculate when you can measure? See https://www.pacificrimcamera.com/rl/02077/02077.pdf

  3. #3

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    Re: how to calculate exact bellows compensation

    Quote Originally Posted by Dan Fromm View Post
    Not to be a complete idiot, but why calculate when you can measure? See https://www.pacificrimcamera.com/rl/02077/02077.pdf
    What about a camera / format independent Gossen meter with a measuring probe, https://gossen-photo.de/wp-content/u...esssonde_d.pdf to measure the light on the focusing screen (fresnel)?

    They've also got the Profiselect device: https://gossen-photo.de/wp-content/u...lect_d_e_f.pdf

  4. #4

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    Re: how to calculate exact bellows compensation

    Certainly, being able to meter at the film plane would be ideal, but not everyone has the resources to acquire the necessary equipment...

    Figuring the bellows-extension factors by measuring the extension and calculating is a common and accurate method, so why don't we help the OP with what he asked for?

    @OP,

    I'll take a stab at it.

    The basic formula for calculating the bellows extension factor is 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.

    For the most accuracy, you need to know the exact focal length of the lens you are using and the position of the nodal point. If you have a newer lens that is listed in a manufacturer's specification sheet, then you can take the information from that. At any rate, for many lenses (i.e., not telephoto or retrofocus) the nodal point to measure from is where the aperture blades are.

    Still, it's good to check. If you know the exact focal length of the lens, then you can simply measure from the film plane to a point on the lens shutter/barrel/etc. and mark yourself a reference point. If you don't, then it gets more complicated; you need to know the lens design and the location of the nodal point and then focus on infinity and measure from the nodal point back to the film plane.

    Hopefully, you're using a lens that is easy to find info on, has a nodal point at the position of the aperture, etc. so the entire process is pretty straightforward. If not, you'll need to do your homework for the most accuracy. That said, if you have a lens that you don't have all the necessary info for, you could simply extrapolate what the focal length should be and where the nodal point of the lens most likely is and do a test or two.

    It would help to know which lens or lenses you plan on using. Dan Fromm can certainly help you find whatever information there is about just about any lens that has ever been made; he's the resident expert.

    And, if I'm wrong about anything in my recommendations above, I hope he'll step in with corrections.

    Best,

    Doremus

  5. #5

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    Re: how to calculate exact bellows compensation

    Quote Originally Posted by Doremus Scudder View Post

    Figuring the bellows-extension factors by measuring the extension and calculating is a common and accurate method, so why don't we help the OP with what he asked for?
    Click image for larger version. 

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  6. #6

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    Re: how to calculate exact bellows compensation

    Sounds good, Daniel, but the OP stated

    where would I measure extension from its from optical centre to film plane but how do i know where the optical centre is then the film plane after quite extreme movements is all over the place 4 corners of front and rear standards have probably +/- 10cm difference
    We usually calculate the correction for the center of the field and let what happens off-axis happen, but in this case without more information from the OP -- lens' focal length, how far is the center of the gate away from the lens' axis? -- we don't know what effect cos^4 will have. I'm sensitive to this because when I was writing my Horseman Optical Exposure Computer article I looked into cos^4's importance. If the OP is using a short lens, much decenterment and wants to be within 1/3 stop over the frame measuring directly is the way to go.

    OP, I don't know what you mean by optical center. When doing closeup work and calculating exposure corrections, the distance that matters is from the lens' rear node to the film plane, measured on the lens' axis. Not as easy as most would like with much tilt/swing. For most LF lenses, the rear node is very close to the lens' diaphragm. The big exceptions (no pun untended) are telephoto lenses, whose rear nodes are considerably displaced from the diaphragm.

    Measuring, given the gear, seems easier and allows better control when the subject isn't evenly illuminated.

  7. #7

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    Re: how to calculate exact bellows compensation

    At 40 pounds a pop I would consider a 4x5 reducer and Horseman meter or exposing B&W and developing to check or some old Polaroid film any of which might cost 1-2 sheets.

  8. #8
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    Re: how to calculate exact bellows compensation

    This is what I use:

    http://www.salzgeber.at/disc/


    Kent in SD
    Qui sedes ad dexteram Patris,
    miserere nobis.

  9. #9

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    Re: how to calculate exact bellows compensation

    Quote Originally Posted by Dan Fromm View Post
    Sounds good, Daniel, but the OP stated

    OP, I don't know what you mean by optical center. When doing closeup work and calculating exposure corrections, the distance that matters is from the lens' rear node to the film plane, measured on the lens' axis. Not as easy as most would like with much tilt/swing. For most LF lenses, the rear node is very close to the lens' diaphragm. The big exceptions (no pun untended) are telephoto lenses, whose rear nodes are considerably displaced from the diaphragm.

    Measuring, given the gear, seems easier and allows better control when the subject isn't evenly illuminated.
    Perhaps the problem is neither to measure the distance of all four corners of the lens board to all four inverted corners of the screen, nor the use of a relatively inexpensive Profiselect attachment.

    IMHO the problem - beside the "mother-of-all-problems" character of a 8x10 macro exhibition diapositive (!) - consists in forming a reasonable average value from all four measurements, given the costs of 40 £ for each try.

    Anyway, I find it disadvantageous to spend 40 £ on developing an 8x10 macros of which I don't know if it is exposed exactly 1/3 EV or if its depth of field is large enough. - Remember that colours will bleach when transparencies are displayed on light boxes.

    I would take the respective picture in 4x5 or even 6x7 (cheaper roll film, more dept of field than a 8x10 macro, bracketing) and then reproduce the best one using the 8x10 camera, without moving the lens or the film, just as a simple reproduction with a constantly increasing bellows factor.

    How about digital test shots? You can attach a digital camera to a large format camera back. How about a digital image with many megapixels, which would be exposed on a transparent carrier?

  10. #10

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    Re: how to calculate exact bellows compensation

    Quote Originally Posted by brighamr View Post

    so what I would like to know is how do I calculate exactly

    As you want exactitude... first you have to know how to get the exact numbers you use in the well known formula.


    1) Know the Real Focal your lens has from datasheet, normally comercial denomination is quite close in practice to the real one, but for example Symmar-S 360mm has a true 350.5mm focal (http://www.mr-alvandi.com/downloads/...mar-s-data.pdf)


    2) Know your Effective bellows draw. Real Belows draw depends on the Flange Focal Distance, so if you measure directly Belows draw you may get some error in the calculation, so to measure your Effective Bellows draw in the macro conditions first focus your camera at infinite and then measure what Relative Increment you need for close focus from the infinite focus position. Then add this Relative Increment to the Real Focal and youl'll have the Effective Bellows Draw bellows you have to use in the formula alongside with the Real Focal. This is exact.

    3) Use belows formulas here: http://kennethleegallery.com/html/tech/bellows.php, but measure extension in the way I explain if wanting an exact compensation: Use Real Focal and caltulate the bellows extension by adding the Real Focal to the Relative Increment (close vs infinite), reiterating it.


    Resources:

    To determine the Real Focal, when unknown:
    https://www.largeformatphotography.i...-unmarked-lens


    Tele and Retro lenses have particular Bellows compensation formulas for very close subjects, this is first class knowledge from a true master in optics, Mr Bigler:

    https://www.largeformatphotography.i...=1#post1480718


    ___________

    Beyond that, if wanting total consistency just attach a DSLR (or SLR) in the view camera back (without the dslr lens) to spot meter exactly, you'll project the image on the sensor. If you use a SLR in the back you may practice with bracketings, flash, etc with the real slide film you are to use.

    If not using flash, use a shutter tester, LF shutters were sold new with a +/- 30% tolerance, so 1/30 could be 1/20 or 1/40, modern shutters were usually accurate than that 30%, but after many years best is checking the real speeds.

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