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  1. #21
    RalphLambrecht's Avatar
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    Quote Originally Posted by TheFlyingCamera View Post
    I don't think you can make a 16x20 with the Printmaker 35, baseboard or otherwise. The bellows won't compress enough. You're better off trying with the Omega. As a general rule, in addition to increasing the exposure time as you go bigger, you'll have to increase the contrast filtration - if it printed well at grade 3 as an 8x10, you'll probably need to use grade 4 (or even higher - maybe a 4 1/2) filtration to get a good 16x20 out of it. If you start printing at variable sizes, I would highly recommend getting something like an Ilford EM-10 enlarging meter. They're quite simple to use - you first get your enlarger set up, and make a good print from a known negative. With the lens aperture and negative-to-paper distance set, put the EM-10's sensor under a (preferably) middle-gray area of the negative on the baseboard. Turn the calibration dial until only the center green light is lit. Note the number so if you have to re-set it later you can. Then you are all set to print the same negative at a different size and/or a different negative at the same size. With the same negative, just re-position the enlarger head at the appropriate height, re-focus, and make sure the probe is in the same area of the negative that you measured. Turn the lens aperture until the single green light is illuminated on the EM-10. With a new negative, put the EM-10's probe in an image area that should have the same tonal value as the area you metered on the original negative. Then adjust the enlarger lens aperture until you have only a single green light. In both cases you don't have to adjust exposure time - just the lens aperture. It's somewhat primitive because it may be better to adjust time than aperture, especially if you are dealing with a very dense negative, and you don't want to print at wide-open. Of course you can calculate the aperture/time changes yourself after you set the exposure based on the meter.

    i 2nd the em10 recommendation
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  2. #22
    keithwms's Avatar
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    Quote Originally Posted by ChristopherCoy View Post
    I am asking because It took me 3 5x7 sheets, and 3 8x10 sheets to get this one 8x10 right. I want to try and have an idea of how the distance may change my exposure time, before I start wasting 16x20 sheets...
    Some calcualtions will get you in the ballpark, but there are other things besides inverse square that matter too, so it's always best to do a test print.

    Just go ahead and project, seek out some portion of the projected image that contains the tones you want, and do a test on a small fragment of paper. There is seldom a need to test the entire image.

    When I get a new box of paper, I often sacrifice one sheet to make some test pieces for this kind of thing. The test pieces get exposed developed, topped, fixed, washed, dried... before I draw any conclusions about exposure of the "real" prints.

    You shouldn't have to waste hardly any paper at all- test strips can tell you almost everything you need.... before you do the real thing.
    "Only dead fish follow the stream"

    [APUG Portfolio] [APUG Blog] [Website]

  3. #23
    Jim Jones's Avatar
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    Quote Originally Posted by Steve Smith View Post
    Double the distance loses two stops as the light is now illuminating four times the original area.

    The clue is in the title: Inverse Square Rule!


    Steve.
    Yes, inded -- until you refocus. Then you should use an enlarging meter like the Ilford EM-10, calculate the exposure using a formula that includes the change in effective f/number due to focus change, or calculate a chart that instantly tells you the proper exposure compensation. Math is one of our allies, not our nemesis.

  4. #24
    Steve Smith's Avatar
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    Quote Originally Posted by Jim Jones View Post
    Yes, inded -- until you refocus.
    Not relevant as we were referring to moving lights, not the focus (well, I was anyway!) and inverse square rule in general. But yes, on an enlarger the re-focussing will affect it a bit. but only because it will also change the coverage so the inverse square rule will still apply.


    Steve.

  5. #25
    TheFlyingCamera's Avatar
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    Quote Originally Posted by Steve Smith View Post
    Double the distance loses two stops as the light is now illuminating four times the original area.

    The clue is in the title: Inverse Square Rule!


    Steve.
    This is why I was not a math major in college. And another reason why I'm mostly a contact printer these days

  6. #26
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    Quote Originally Posted by ChristopherCoy View Post
    I was able to print an 8x10 using a #3 filter at 3 seconds exposure, around f/5.6 I think, and produce something that I like.

    Last night I was thinking about trying a 16x20 of the same print.... I want to try and have an idea of how the distance may change my exposure time, before I start wasting 16x20 sheets...
    Kodak enlarging dial in the Kodak B+W Dataguide says: 8X 3sec f/5.6 becomes 16X 10.5sec f/5.6

  7. #27
    ic-racer's Avatar
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    Quote Originally Posted by Steve Smith View Post
    Not relevant as we were referring to moving lights, not the focus (well, I was anyway!) and inverse square rule in general. But yes, on an enlarger the re-focussing will affect it a bit. but only because it will also change the coverage so the inverse square rule will still apply.


    Steve.
    You can think of re-focusing as thing that changes the effective aperture. As the lens gets racked out the effective aperture changes. Otherwise you could easily calculate the exposure changes based on conservation of energy and the spread of the light. The change in aperture screws up the simple equation.

  8. #28
    RalphLambrecht's Avatar
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    Quote Originally Posted by TheFlyingCamera View Post
    The inverse square rule applies to enlargement the same way it does to studio lighting - double the distance, lose a stop, halve the distance, gain a stop. So going up to a 16x20 from an 8x10 is roughly 4x the distance, or two stops. If your base exposure is 3 seconds at f5.6, then give it 12 seconds as a starting point on your test strip (still do test strips!). you'll probably want to go up at least one grade in filtration, maybe a grade and a half. So you'll have to add another half to whole stop for the filter grade increase too. Try starting at grade 4, 24 seconds, and give your test strips three exposures at 12, 24, 36 and 48 seconds to see where the truth lies.
    sorry but itdoes not
    Exposure Height and Exposure Correction Whenever the enlarger head is raised or lowered, and the negative magnification is changed, print exposure must be corrected. in the attached pdf on page 511 you will find a chart to determine the magnification of your enlargement and another to estimate the exposure compensation required to accommodate a change in enlarger height. Strictly speaking, projected print exposures fail to follow the inverse-square law, but they follow the inverse square of the lens-to-paper distance if the paper reciprocity failure is ignored, in which case, a new theoretical exposure time (t2) is given by:
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  9. #29
    RalphLambrecht's Avatar
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    Quote Originally Posted by Steve Smith View Post
    Not relevant as we were referring to moving lights, not the focus (well, I was anyway!) and inverse square rule in general. But yes, on an enlarger the re-focussing will affect it a bit. but only because it will also change the coverage so the inverse square rule will still apply.


    Steve.
    noit doesn't!!!!
    Exposure Height and Exposure Correction Whenever the enlarger head is raised or lowered, and the negative magnification is changed, print exposure must be corrected. In ‘Tables and Templates’, you will find a chart to determine the magnification of your enlargement and another to estimate the exposure compensation required to accommodate a change in enlarger height. Strictly speaking, projected print exposures fail to follow the inverse-square law, but they follow the inverse square of the lens-to-paper distance if the paper reciprocity failure is ignored, in which case, a new theoretical exposure time (t2) is given by:
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

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