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  1. #1
    arigram's Avatar
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    Enlargment Mathematics

    Alright, half my brain doesn't work, I am pretty much braindead when it comes to mathematics. So I am asking you mathematically non-challenged people a simple question:

    To enlarge a 6x6cm negative with a 80mm lens up to 48x48cm (on a 50x60cm -20x24" paper) what should be the distance from the negative plan to the paper?

    I am working on my darkroom and I need to see if I need a new table as I will be rearanging certain elements such as wall hanging the enlarger.
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  2. #2
    gma
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    I suggest that you mount higher and farther from the wall than the required minimum dimensions to allow for cropping.
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  3. #3

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    Z=(m+1)f


    Z = distance from the lens to the baseboard

    m = magnification

    f= focal length

    So Z=(8+1)8.0

    Z=72 cm

    Roughly. The focal length of your lens is likely a little off the nominal 80mm. You need to measure from the right spot. Still should be pretty close.

    The negative to the lens is:

    X=(m+1)/m*f

    X=(8+1)/8*8

    X=.14cm


    I'm assuming 48cm from 6cm is 8x magnification. You can check the formula with smaller enlargements on your enlarger.

  4. #4
    gma
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    The numbers posted by the previous member are very incorrect. For a 10.4 X magnification the distances are theoretically:

    negative to lens is 88 mm ( NOT 0.14 cm ??? )
    lens to easel is 920 mm

    Keep in mind that the lens has thickness. As I mentioned previously please consider cropping and allow extra distance from the wall and extra height. I like to use a counter that can be adjusted lower than I typically use.
    [FONT=Century Gothic][/FONT][SIZE=7][/SIZE][COLOR=DarkOrange][/COLOR] I may be getting older, but I refuse to grow up!

  5. #5
    127
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    For a "thin" lens

    F/f=W/w

    where:

    F=Paper-Lens
    f=neg-lens

    W=paper width
    w=neg width

    For a 10x enlargement W/w = 10 so F/f=10 (approx)

    This agree's with GMA
    90mm x 10 (aprox=) 90 cm.

    Ian

  6. #6
    Joe Lipka's Avatar
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    Let's see. Put the negative in the enlarger. Turn off the room light, turn on the safelight. Raise the enlarger. Focus the image. When the image is the size you want, measure the height from the enlarger to the baseboard.

    So far, you have two answers and they both disagree. Practicality trumps science every time. This is the difference between scientists and engineers...
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  7. #7

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    Yup did the math wrong for the negative to lens distance. It's 9cm.

    Which formula are you using to figure lens to paper distance? Or are you using a different magnification factor?

  8. #8
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    Nick and GMA are using different magnification factors (8x and 10.4x, respectively). I think Nick is judging by the nominal frame size of 6x6 cm, and GMA is assuming a slightly smaller more realistic frame size, plus a little crop space.
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  9. #9

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    Don't forget that the focal length gets shorter as you rack in the bellows for larger print size. Find a rule of thumb. Avoid the calculus.

  10. #10
    gma
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    1/focal length of lens = 1/distance lens to film + 1/ distance lens to easel

    1/80mm = 1/ 88mm + 1/ 918mm

    Magnification = 918/88 = 10.4 X
    [FONT=Century Gothic][/FONT][SIZE=7][/SIZE][COLOR=DarkOrange][/COLOR] I may be getting older, but I refuse to grow up!

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