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  1. #11

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    Measuring the lens to paper distance is a trick, if you use a regular tape measure you have to convert fractions to decimal. I use a metric tape measure, no conversions. Works like a charm. When using 35mm, I have never wanted to make a print smaller than the negative, not even with 2-1/4 either. So if you into roll film, the simpler lens to paper distance is easier and faster, just use a metric measure.

  2. #12

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    The problem is not metric or fractional etc... To accurately measure the distance you must have a squrare or something on the baseboard to make sure you're not measuring at an angle. You also need to know the nodal point of the lens.

  3. #13

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    If the column distance gives you the distance between the negative and the paper, then you have the have the quantity d_o + d_i, when d_o is the object distance (negative to lens) and d_i is the image distance (lens to paper). But perhaps the column distance measures some arbitary point on the head. (This also neglects the separation of the principal planes of the lens, but that shoud only be a couple of mm.)

    The focusing equation is 1/d_o + 1/d_i = 1/f, where f is the focal length. Using the equations for magnifcation (e.g., d_i = f(m+1)), and some algebra, one can obtain the equation:

    d_o + d_i =f/m (m+1)^2 = f (m + 2 + 1/m)

    So from the column height, hopefully a measurement of d_o + d_i, you could solve the above equation for m for both print sizes, then use the (m+1)^2 equation to calculate a new exposure time. Obviously the above equation is not so easy to solve.

    Possible approaches: 1) measure the lens position and not bother with the above equation, 2) program a scientific calculator or computer to solve the equation, 3) measure the film and prints to obtain m, 4) use a light meter, or 5) test strips.

    To measure the lens distance accurately, you could just mark the optical center line on the baseboard and extend a tape measure from the lens to that point. I would't worry about the nodal points of the lens -- measuring to the aperture or middle of the lens should be sufficiently accurate.

  4. #14

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    My testing has shown if you measure from the same reference points, it doesn't make that much of a difference. If you use the markings on the column, and you change the easel, and its thicker, or thiner, you have a problem. If you just consistently measure from the paper position to the same spot on the lens I find the exposure is close enough for me.

  5. #15
    Jerevan's Avatar
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    Quote Originally Posted by Max Power
    There's actually a PDF floating around in cyberspace which is dead simple to use. I will try to find it and post the link back here.

    MTF

    Kent
    Is it this one? -> http://www.bonavolta.ch/hobby/files/ratiomod.pdf
    “Do your work, then step back. The only path to serenity.” - Lao Tzu

  6. #16
    RalphLambrecht's Avatar
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    Chan Tran

    I thought we went through this already. The inverse-square law is based on the lens-to-paper distance, not the magification.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  7. #17
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    Quote Originally Posted by MichaelBriggs
    As pointed out by Jim above, and by others on previous threads (e.g., http://www.apug.org/forums/forum41/21586-any-calculation-exposure-enlargement-changes.html and http://www.apug.org/forums/forum43/29380-time-calculations-when-switching-print-sizes.html), exposure time does NOT go as the print magnification squared. The correct rule for changing the exposure time with the same lens is time goes as (M2 + 1)^ 2 / (M1 + 1)^2. This rule takes care of refocusing the lens. (I'm not sure if it is applicable to Chan's question about switching focal lengths -- I'll have to think some.) ...snip
    It does. Your equation works with the magnification factor, consequently, you can switch the focal length. It won't matter.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  8. #18

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    Quote Originally Posted by RalphLambrecht
    The inverse-square law is based on the
    lens-to-paper distance...
    Did you forget the lens to negative distance? Any
    computation which does not factor in the change in
    lens to negative distance will not be correct. Have
    any of those interested in this subject found or
    derived A formula integrating the two? Dan

  9. #19
    RalphLambrecht's Avatar
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    You don't need the lens-to-negative distance to calculate the change in exposure. Using the lens-to-paper distance alone will yield accurate results.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  10. #20

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    Quote Originally Posted by RalphLambrecht
    You don't need the lens-to-negative distance to
    calculate the change in exposure.
    You can not ignore the change in lens to negative
    distance when it is that distance which determines
    the speed of the lens. You do recall the formula by
    which the speed of a lens is determined? Dan

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