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  1. #1

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    Using multiplier of area to scale print size and adjsting for print density

    I just want to confirm what I am seeing is real.... not some error in my process.

    Paper in question is Ilford MGIV FB Glossy in 8x10 cut and 11x14 cut. These were purchased at the same time from the same vendor. (no way to tell if they came from the same master roll)

    I have a negative which prints exactly the way I want on 8 x 10 paper with 1/4" borders using #1 1/2 filter, f/8, and 21 seconds.

    When I wanted to print this on 11 x 14 with the same border, I calculated the ratio of surface area and used that multiplier for 21 seconds, then came up with computed figure of 42 seconds exposure.

    Comparing the results, the larger version has shadow area is about 1/6EV darker.

    Is it correct that this method will give me fairly accurate figure but fine tuning is still required?
    Develop, stop, fix.... wait.... where's my film?

  2. #2
    keithwms's Avatar
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    Well, normally you just compute a linear magnification number, i.e. 8 inches becomes 11 inches, and compute the exposure difference from that. Did you try that and compare?

    I wouldn't recommend working in terms of areas. That'd take more math.

    Try this:

    http://www.fineart-photography.com/xc.html

    As an example of calculating a new time, let us assume that we have made an 8 x 10 enlargement from a 35mm negative and that the short dimension was not cropped and, thus had a magnification of 8.5. Also, assume the original printing time was 16 seconds. We now want to make a 11 x 14 print.

    If we enter 24 and 11 into the top table, we will get a (New) magnification estimate of 11.6. Now we are ready to calculate a new exposure time. In the bottom table enter 16 as the original time, 8.5 as the original magnification and 11.6 as the new magnification. An extimate of the new time would be 28.4 seconds.
    Last edited by keithwms; 02-28-2011 at 11:17 PM. Click to view previous post history.
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  3. #3
    Dan Henderson's Avatar
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    I learned to use changes in enlarger height to calculate exposure for a new print size, using the following formula:

    (new height+1) squared / (old height+1) squared.

    I found a tape measure with a sticky back side at the local home improvement store that I attached to the column of my enlarger to facilitate height measurement, and keep a small calculator in the darkroom to do the math.


    web site: Dan Henderson, Photographer.com

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    I am not anti-digital. I am pro-film.

  4. #4

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    Years ago Kodak published a darkroom guide that as I recall had a dial that would give exposure changes for enlarging from one size print to another. As for myself, I prefer to make a new test print when increasing print size. Different sizes can give a different feel to an image and depending on the size a different viewing distance.

    http://www.jeffreyglasser.com/

  5. #5

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    I would think that finding an accurate measurement of the two projected areas, and then using their ratio as a multiplier, would give the best approximation of the new exposure. 8x10 and 11x14 have aspect ratios that are darn close, but I would still use the true projected areas outside of the easel arms, not the areas within the easel arms. Beyond that, I think the answer to your question about tweaking is "yes". Probably due to the granular nature of film, the tonality, micro contrast and overall contrast of a print can sometimes change in subtle ways as it is enlarged.

    Regards,

    Dave

  6. #6

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    Thanks everybody. Just to clarify, two prints are very close. I found some subtle differences in deep shadow where on larger print, the details aren't as clear as the smaller. I'd bet the adjustment is only a few seconds worth (out of 43 second exposure).

    I may just dodge that area a bit as everything else looks about right.

    Thanks again.
    Develop, stop, fix.... wait.... where's my film?

  7. #7
    wiltw's Avatar
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    Quote Originally Posted by jeffreyg View Post
    Years ago Kodak published a darkroom guide that as I recall had a dial that would give exposure changes for enlarging from one size print to another.

    Kodak Black-and-White Darkroom Dataguide
    ...
    'School solution' says that 8x10 at 10 sec. becomes 18 seconds for 11x14

    For the OP's numbers, 21 second 8x print becomes 38 sec 11x print

  8. #8
    Nicholas Lindan's Avatar
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    What you are seeing is real: ratioing the areas will give the wrong answer, as you have found out. Sometimes very wrong, sometimes not terribly so.

    There is a long drawn-out convoluted sticky thread at the top of this forum that resolves this topic.

    The correct correction can be calculated as either:
    • The ratio of magnifications (m + 1) ^ 2 / (M + 1) ^ 2
    • The ratio of lens heights h ^ 2 / H ^ 2. When using lens height it is important to use the position of the diaphragm in the lens (there is a further tweak, but it can be ignored).


    A scale for calculating exposure correction using lens height is available on the Darkroom Automation web site support files section, about 2/3 the way down the page:
    http://www.darkroomautomation.com/support/index.htm
    DARKROOM AUTOMATION
    f-Stop Timers - Enlarging Meters
    http://www.darkroomautomation.com/da-main.htm

  9. #9

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    Using difference in enlarger height to figure this out is theoretically accurate in a single step, not area...but this is what test strips or for. IMHO, there is no point in trying to go for broke on the first print after changing the enlargement. You can calculate to you heart's content, but this is analog photography. Something will always be slightly different from your calculations. I would use enlarger height to get close, but still use test strips before devoting a whole piece of paper to the enlarged print.
    2F/2F

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  10. #10

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    I appreciate everybody for comments. Experimentally, I found 8x10 print at 21 seconds can be matched EXACTLY to 11x14 print at 38 seconds. Margin of error is +/- 1 second at 11x14 scale.
    Last edited by tkamiya; 03-01-2011 at 05:45 PM. Click to view previous post history.
    Develop, stop, fix.... wait.... where's my film?

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