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  1. #1

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    Adjusting Exposure Times With Print Size

    I recall reading a while ago a formula for adjusting print exposure time as the enlargement size changes. But, I cannot recall where I read this. I assume that with an f-stop timer it's relatively simple to make these adjustments.

    My reason for asking is that I have several negatives I'd like to print on 5x7", 8x10" and 11x14" paper to test my theory that different images look best at one size; at least, they look better than when larger or smaller. It would be nice to quickly adjust the exposure time (or lens aperture) and make a series of three prints while the negative is still in the enlarger.

    Rich

  2. #2
    Sirius Glass's Avatar
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    Twice the linear dimension on one side means four times the exposure [2 stops more] because you are dealing with area and not the length of one side. You would be better to open the lens two stops rather than increase the time because the longer exposure might take you into the realm of reciprocity failure.

    Steve
    Warning!! Handling a Hasselblad can be harmful to your financial well being!

    Nothing beats a great piece of glass!

    I leave the digital work for the urologists and proctologists.

  3. #3

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    Square your lens to paper distance for the first print size.
    Square your lens to paper distance for the new enlarged print size.
    Divide the second result by the first result.
    Take the first good exposure time and multiply it by this result to arrive at your new exposure time.

    Example:

    10"x10"=100
    15"x15"=225
    225/100= 2.25
    If the first exposure time was 11 seconds then multiply it by 2.25 for the enlargement. ie; 24.75 sec.

    My interpretation from "You and Your Prints", by William Hawken ISBN 0-8174-2114-9

    I have used this method many times with good results.
    Last edited by DannL; 11-08-2007 at 12:33 AM. Click to view previous post history.
    "Lo único de lo que el mundo no se cansará nunca es de exageración." Salvador Dalí

  4. #4

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    It's the same inverse square law used in flash calculations: to double the lens to paper distance you need four times the time. In practice, though, I find that it's easier to re-meter or do a new test strip when you enlarge more (or less).

    David.

  5. #5
    clogz's Avatar
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    You might also find that when enlarging you need a slightly harder grade of paper.

    Hans
    Digital is best taken with a grain of silver.

  6. #6
    matti's Avatar
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    Or you can look at this diagram.
    by Ralph W. Lambrecht. From his book Way Beyond Monochrome, I think. It should be easy to use if you have a scale on your enlarger.
    I mostly use a table in Tim Rudmans "Master printer"-book. I often use it to first make test prints and moving up for the finals.

    /matti
    Last edited by matti; 11-08-2007 at 04:28 AM. Click to view previous post history. Reason: missed the url...

  7. #7

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    Quote Originally Posted by Sirius Glass View Post
    Twice the linear dimension on one side means four
    times the exposure [2 stops more] because you are
    dealing with area and not the length of one side. Steve
    The working focal length of the lens must be taken into
    account. As the lens is focused for larger prints it's focal
    length is lessened and the lens approaches it's rated speed.
    The lens becomes faster. The reverse holds true when
    increasing the working focal length.

    An Ilford EM-10 enlarging meter works well and is just
    the thing for what the OP has in mind. Works also as
    a ballpark densitometer if calibrated. Under $30. Dan

  8. #8

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    Hello
    Another variation of the inverse square law, similar to DannL's is the attached formula. It's easier than my scribble looks. The new length (L2) is divided by the old length. (L1) This figure is then squared and this is the factor to multiply your old exposure (T1) to find out the new one.(T2) In this example, a 5x4 print at 15 second exposure is calculated to 60 at 10x8. That's an easy one as if the paper is four times as big, it will obviously need four times as much light. Bit more usefull if it's something like 17.6 seconds. The formula has certainley helped when I suddenly realise I've only got a few sheets of 20x24 paper left to make one print as you can get a small print correct, then go straight for a big one, though as clogz points out a slight increase in contrast may be neccesary.
    Cheers
    Mike
    Attached Thumbnails Attached Thumbnails a-formula.jpg  

  9. #9

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    Thank you all for validating what seemed reasonable to me. Also, the pointers to Ralph's and Tim's books reminded me where I read about this.

    I do have Nick Linden's exposure meter so I can run some tests to calibrate the adjustment to my equipment, lenses, and paper.

    Much appreciated,

    Rich

  10. #10

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    There's also a small programme available at http://home.centurytel.net.dwilder57/timeAdjust.html where you put in the old and new dimensions and it works out the exposure time for you.

    pentaxuser

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