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  1. #11
    Nicholas Lindan's Avatar
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    Quote Originally Posted by rshepard View Post
    I recall reading a while ago a formula for adjusting print exposure time as the enlargement size changes ... I assume that with an f-stop timer it's relatively simple to make these adjustments.
    I certainly do hope so.

    As the subject of threads seems to be pre-ordained
    by the stars and planets, from a recent post on the Pure Silver list:

    The easiest solution is to use an enlarging meter and
    f-stop timer.

    Measure the difference in light with the
    meter [the meter has a special function for
    doing just this] and add the meter
    reading to the f-stop timer setting. You
    are done ... accurate to 1/10th of a stop
    or better.

    And all the dodges and burns made by the timer
    automatically track the change.

    I do confess I make f-stop timers and enlarging
    meters http://www.darkroomautomation.com/da-main.htm

    Another solution is to use an enlarging comparator
    (Ilford EM-10 or an Analyte) and open/close the
    lens so the same amount of light falls on the easel.
    However, you are no longer using the lens at optimum
    aperture and high-end lenses have fixed apertures
    so the method is a non-starter.
    DARKROOM AUTOMATION
    f-Stop Timers - Enlarging Meters
    http://www.darkroomautomation.com/da-main.htm

  2. #12

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    Excel formula for scaling prints

    Hi,

    I made an Excel spreadsheet to do the calculation for me. I have cells labeled, (in the following order):

    Original exposure time: Original dimension: New dimension: New exposure time.

    These are all on row 5 of my spreadsheet and in columns A,B,C and D respectively.

    The formula in the D5 cell is the following (if you use other rows or cells, you must compensate accordingly, of course).

    =SUMME((C5/B5)*(C5/B5))*A5 (Note: my software is in German, SUM is probably correct for English versions)

    This is based on the formula: To(Ln/Lo)2=Tn
    Where:
    To is Original exposure time
    Ln is New dimension (Length)
    Lo is Original dimension (Length)
    Tn is New exposure time

    As mentioned above, this is not completely accurate, but will get you in the ballpark. There are always small changes in exposure/contrast to be made when scaling a print up or down anyway, since the difference in size changes our perception of detail, contrast, tonality. It does save time, however, to start here instead of making test strips.

    Hope this helps,

    Doremus Scudder

    www.DoremusScudder.com

  3. #13
    Dave Miller's Avatar
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    Quote Originally Posted by DannL View Post
    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.
    I always use this method because any resizing / cropping of the image does not effect the calculation. By the way, stating the obvious, but it also works in reverse.
    Regards Dave.

    An English Eye


  4. #14
    Bruce Osgood's Avatar
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    My Excel program works thus:

    NEW TIME = ((NEW DISTANCE / OLD DISTANCE)^2) X OLD TIME

    This works for either enlarging or reducing size by distance and is is based on the NASA explanation of the Inverse Square Law.

    http://quest.arc.nasa.gov/pioneer10/education/temp/

    What is lacking in this is the fact that contrast is affected and we do not believe it should be, or at least I don't. But it is and I don't know why.

    When enlarging from 8X10 to 11X14 there is an ever-so slight reduction in contrast. I thought there was a problem with the math but the math is correct.

    I'm not too concerned with this mystery as the results are generally quite pleasing.
    Bruce Osgood
    Chinese proverb
    "Print with #3.5 and burn with #1.5." B.J. Confucius
    http://www.flickr.com/photos/camclicker/

  5. #15
    Nicholas Lindan's Avatar
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    -- oops, wrong thread, this is/was a dupe --

    New thread started...
    DARKROOM AUTOMATION
    f-Stop Timers - Enlarging Meters
    http://www.darkroomautomation.com/da-main.htm

  6. #16
    Monophoto's Avatar
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    Many years ago, Unicolor offered a series of workshops on the use of their products. One of the handouts you received was a card that showed the approximate adjustment in printing time as you moved from one print size to another - 4x5 to 5x7 to 8x10 to 11x14 to 16x20, etc.. While I haven't printed in color for many years, I still keep this card hanging in my darkroom.

    It doesn't guarantee a perfect print, but it gets me into the ballpark so that the first workprint is close.

    I would offer to scan it, but there are those copyright issue. But you can generate a similar card/table from the formulas that others have suggested.
    Louie

  7. #17

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    Dancu writes:-

    "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."

    This may well explain why the inverse square law calculations quoted here have never worked that well for me, I have always had to make adjustments to time but have not been able to incorporate the differences into a new model.

    Thanks Dan,
    Regards,
    John.

  8. #18

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    Quote Originally Posted by John Roberts View Post
    Dancu writes:-

    "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."

    This may well explain why the inverse square law calculations
    quoted here have never worked that well for me, I have always
    had to make adjustments to time but have not been able to
    incorporate the differences into a new model. Thanks Dan,
    Regards, John.
    The exact formula is somewhat more complex. I've never
    made a point of having it at hand and do not use it. Likely
    it is some where on the Web. The exact exposure will be
    some little shy of that indicated by the inverse square
    law on greater magnification and the reverse when
    down sizing. Dan

  9. #19
    Dave Miller's Avatar
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    Quote Originally Posted by Bruce Osgood View Post
    My Excel program works thus:

    NEW TIME = ((NEW DISTANCE / OLD DISTANCE)^2) X OLD TIME

    This works for either enlarging or reducing size by distance and is is based on the NASA explanation of the Inverse Square Law.

    http://quest.arc.nasa.gov/pioneer10/education/temp/

    What is lacking in this is the fact that contrast is affected and we do not believe it should be, or at least I don't. But it is and I don't know why.

    When enlarging from 8X10 to 11X14 there is an ever-so slight reduction in contrast. I thought there was a problem with the math but the math is correct.

    I'm not too concerned with this mystery as the results are generally quite pleasing.
    [FONT=Times New Roman][SIZE=3]You are correct, I believe the effect is an optical illusion[/SIZE][/FONT]
    Regards Dave.

    An English Eye


  10. #20
    brian steinberger's Avatar
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    This evening I re-printed a photograph at a larger size. The previous print was 7x7" on 8x10 Ilford MGIV FB glossy. The exposure was for 9.5 sec. at f/16 at grade 3 1/2.

    Tonight I printed the image up to 10x10" on 11x14 Ilford MGIV FB glossy. I had read this thread previously and instead of doubling the exposure, I just opened the lens up one stop (essentially the same thing). After a few test strips my final exposure was at 9.5 sec. at f/11 at grade 3 1/2... exactly one stop more exposure than the smaller print. Its funny how simple things are sometimes. But then I thought about it, the 7x7" print has 42 inches of area, and the 10x10" has 100 inches of area.. Approx. double.

    I just wanted to add this for anyone, because this was an eye opener for me and will certainly give me a great starting point when re-printing from 8x10 to 11x14 (on MGIV anyway). No calculations or densitometer needed.

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