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  1. #1

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    Time calculation when switching print size once again

    After going thru the other thread it made me thinking once again. What I often do is to make 4x5 prints from 35mm using the 80mm lens and when I switch to 8x10 I use the 50mm lens. Assuming that the f/stop marking on the lens is accurate and the magnification for the 4x5 prints is 4x and 8x10 prints is 8x. Also assuming that the same aperture is to be used (I actually use 1 stop larger when go to 8x10). I always use a color analyzer to determine printing time which is simple. However, I wonder how do one calculate? I think I would find the answer in the next few days but I wonder what you're all thinking?

  2. #2

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    Well, I think the exposure time is proportional to the squared of the magnification. In this case it's simply 4x the original printing time. So I guess the neg to paper distance won't work if you change lens as in this case they are 400mm with the 80mm lens at 4x and 450mm with the 50mm lens at 8x.

  3. #3
    Loose Gravel's Avatar
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    Chan's got it exactly right. Exposure is proportional to the square of magnification and directly proportional to the area of the print. When you did the lens change, you screwed up the negative to paper distance calculations unless you want to get complicated and there's no reason for that.
    Watch for Loose Gravel

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    Max Power's Avatar
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    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
    Max Power, he's the man who's name you'd love to touch! But you mustn't touch! His name sounds good in your ear, but when you say it, you mustn't fear! 'Cause his name can be said by anyone!

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    Time Calculation When Changing Column Height
    Where Enlarger Column Height was = 16 in.; then moved up to = 20 in., the following formula would be used:
    Excel: = ((10^2)/(8^2))*20 = 31.25 seconds
    Cursive: = (102/82) x 20 = 31.25 seconds
    [FONT=Times New Roman]MAC[/FONT]

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    hortense's Avatar
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    Time Calculation When Changing Column Height
    Where Enlarger Column Height was = 16 in.; then moved up to = 20 in., the following formula would be use:
    Excel: =((10^2)/(8^2))*20 = 31.25 seconds
    Cursive: (102/82) x 20 = 31.25 seconds
    [FONT=Times New Roman]MAC[/FONT]

  7. #7
    Jim Jones's Avatar
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    Quote Originally Posted by Chan Tran
    Well, I think the exposure time is proportional to the squared of the magnification. In this case it's simply 4x the original printing time. So I guess the neg to paper distance won't work if you change lens as in this case they are 400mm with the 80mm lens at 4x and 450mm with the 50mm lens at 8x.
    Exposure is proportional to the square of the magnification if the lens isn't refocused. I do use the squared magnification rule for moderate changes in big enlargements. It doesn't work well for smaller magnifications.

  8. #8

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    Or, just take a light level reading when through printing the small print. Duplicate "that" light-level when printing the larger print and use the same time; bypassing the calculations. Granted there are some contingencies but they can be overcome with the common solution of "Double and half".

  9. #9

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    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.) If both magnifcations are large, the "+1" terms become unimportant, and the rule reduces the to simple rule that many people cite. The simple rule will break down if one of the prints is small compared to the negative.

    As the previous threads discussed, another accurate rule is to use the image distance (lens to paper) squared. Using the column height will be approximate. Again, this is for refocusing with the same lens. This rule will not work if you switch focal lengths.

  10. #10

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    In the previous thread I have come up with a way to figure out the lens to paper distance from the column height. Also we found way to figure out the magnification from the lens to paper column. So if your formula is correct (which I think so) we can calculate the new printing time if we know the following:
    1. the original printing time
    2. the column height of the original print
    3. the column height of the new print
    4. the focal length of the lens used to make the original print
    5. the focal length of the lens used to make the new print

    I want to use the column height because you can read it off the marking on the enlarger column. Measuring the lens to paper distance is a more difficult task if one wants to do it accurately.

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