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  1. #1
    davetravis's Avatar
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    How to Determine the Magnification Factor?

    I have a Beseler 23C XL II, printing 6x7 cm with an 80mm Rodagon.
    Does anyone know what the "factors" are for 11x14, 16x20 & 20x24?
    Thanks.

  2. #2
    David A. Goldfarb's Avatar
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    For photographic purposes, it is conventional to use the linear magnification factor, so the magnification factor would be the size of the print along one dimension divided by the size of the negative along the same dimension. So if 6x7cm is nominally 2-1/4x2-3/4", the magnification factor for an 8x10" print could be computed as 10/2.75=3.6X magnification.
    flickr--http://www.flickr.com/photos/davidagoldfarb/
    Photography (not as up to date as the flickr site)--http://www.davidagoldfarb.com/photo
    Academic (Slavic and Comparative Literature)--http://www.davidagoldfarb.com

  3. #3

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    I do what David suggested, only in millimetres!

  4. #4
    Dave Miller's Avatar
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    Quote Originally Posted by David A. Goldfarb
    For photographic purposes, it is conventional to use the linear magnification factor, so the magnification factor would be the size of the print along one dimension divided by the size of the negative along the same dimension. So if 6x7cm is nominally 2-1/4x2-3/4", the magnification factor for an 8x10" print could be computed as 10/2.75=3.6X magnification.
    Just a thought; shouldn't the "size of print" above be replaced by "size of enlargement". If printing full frame they are of course the same, but not with a selective enlargement.
    Regards Dave.

    An English Eye


  5. #5

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    IMO the easiest method is to ignore image size and use enlarger lens to baseboard distance. This way selective enlargement or not, you are always measuring the same thing. You are never going to get caught out by not noticing that you have masked off the image etc.

    Say you do a 10x8 ish print and the distance is from lens to baseboard is 50cm
    Then you want to do a bigger print and having racked up the column and getting it how you want it the distance is now 70cm

    70 squared (4900) divided by 50 squared (2500)= 1.96

    If your exposure was 10 seconds for the first print it will be 19.6 for the second.

    Simple - just keep a tape measure and calculator next to your enlarger. Takes a few seconds and never fails. Bear in mind that if exposures change a fair bit the relationship is not quite linear because lamps get hotter ie if the factor is 4 so goes from 5 seconds to 20, I find that with my enlarger the second time would be somewhat less than this, maybe 19 seconds. To fix this I tend to make small images at say f11 and the bigger ones at say f8 or f5.6. This way I can use aperture changes to help keep exposure time close. You can of course use a combination of both. Say the factor is 2.6 and you originally used 10 seconds at f11. This would be 26 seconds at f11......or you could use 16 seconds f8.

    Sometimes I confuse myself too.

  6. #6
    Dave Miller's Avatar
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    No confusion Tom, that is the system that I use, with the slight difference that I measure from lens to easel surface to the nearest centimeter.
    Regards Dave.

    An English Eye


  7. #7
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    print image distance / negative image distance = magnification.

    be sure to measure between the exact same two points on both the negative and the print. I will measure a particular distance on the negative before I place it in the carrier, then i can calculate the magnification easily when enlarging by measuring the same two points on the easel.

    example: 8" on the easel / 1" inch on the negative = 8x magnification

  8. #8

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    One more way to skin the cat - measure the size of the neg holder opening (always stays the same). When you get the magnification you want, measure the uncropped image at the easle plane. Should give accurate image enlargement ratio, I think(works for me). If you are after relative exposures, though, I like the idea of measuring relative neg stage to easle plane distances (change to bulb distance is the material issue, I think), I'm gonna try it next time.

  9. #9
    Dave Miller's Avatar
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    Quote Originally Posted by George Collier
    ........If you are after relative exposures, though, I like the idea of measuring relative neg stage to easle plane distances (change to bulb distance is the material issue, I think), I'm gonna try it next time.
    Yes, it’s the Inverse Square Law relating to light spread; hence the squaring referred to in Tom’s post above.
    Regards Dave.

    An English Eye


  10. #10

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    Quote Originally Posted by Dave Miller
    No confusion Tom, that is the system that I use, with the slight difference that I measure from lens to easel surface to the nearest centimeter.
    Same....I meant easel !

    Foolproof and very accurate as the distances from lens to easel are big enough to make measuring them accuarately easy.

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