Try the enlarging correction ruler at the Darkroom Automation support files web site.
You measure the size of the original image and the size of the new image. The difference is the exposure correction in stops.
Original image measures 5.2
New image measures 3.7
Difference is 1.5 stops: open the lens aperture by 1 1/2 stops or multiply the time by 2.8
A table of stops<->seconds and a stops dial for GraLab and Time-O-Lite timers are available on the DA web site support section.
Or you can use an enlarging meter. The already mentioned Ilford EM-10 requires you adjust the lens aperture to null the meter at the new magnification. The Darkroom Automation Precision Enlarging Meter lets you adjust the time, the aperture or both to correct for the new print size. An f-Stop Timer is handy for correcting exposure in stops.
I use the EM-10 and never change the time, mostly because I print cibachromes and with a time change comes reciprocity failure..... and a color shift with the change in time.
The inverse square law applies to systems without optics.
Originally Posted by DWThomas
Once a lens is introduced the speed of the lens must be taken
into account. Just as the extension of a camera lens will effect
the amount of exposure so will it with an enlarger. As with a
camera the more extended the lens the slower is the lens.
An enlarging lens extended for 1:1 ratio enlargements
has 1/4 the speed; is two full stops slower. Dan
The two systems are equivalent for practical purposes. Either the effective f-stop is two stops slower (the f-number is twice as big) because the lens-film distance has doubled -or- the amount of light is 1/4 because the area covered is 4x area / 2x linear larger.
Originally Posted by dancqu
Don't use them together ...
OK it looks like I may need to do a little science project as a sanity check and to see if I didn't goof up on my earlier inverse square calculation. Thanks for all the guidance. The "enlarging correction ruler" looks like it might be just the thing I am looking for, and I was not aware of the Ilford meter which seems to be very affordable.
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I do this all the time. I use 8x10 paper to make a good print. Split contrast printing sequence etc.
Then, I often make an 11x14 or sometimes a 16x20. I have a Jobo Colorline 5000 analyzer. I use the Integrated mode with a diffuser over the light meter cell and I hand hold the diffuser over the enlarger lens and take an integrated density reading over the center of the print. I write down the density on my greaseboard. Then I raise the enlarger, compose the print with the same proportions as the 8x10, place the meter in the center of the print, and then open the aperture of the lens until I have the same density reading on the analyzer. That way, I use the same exposure times and sequences programmed into the RH Designs Stop clock timer. (the Darkroom Automation timer and meter would work similarly I think and be less expensive.)
It is remarkably accurate but not perfect. Sometimes the larger print degrades a little and needs a bit more contrast e.g. 1/6 stop more magenta exposure when going to 16x20 but I think the process saves me some time and paper.
The Darkroom Automation meter indicates the number of stops of exposure change required. You take a reference reading, which the meter remembers, go to the new magnification and the meter will read the difference in light intensity. If, as an example, it says the light is 2.20 stops less then you can add 2.2 stops to the time or you can open the lens aperture 2 and an ooch stops so the meter reads 0.0 or you can split the difference - open up the lens a stop or two and add the residual difference shown on the meter to the time. In all cases all the dodges, burns and split-filter exposures stay the same as they are all in stops +/- the base exposure.
Originally Posted by jeroldharter
It is normal for contrast to decrease slightly as prints get bigger. There seem two principle reasons:
1) Stray light: The intensity of stray light leaking from the enlarger and the intensity of light reflected from the paper to the surroundings and back to the paper is constant and doesn’t change with print size. The increased exposure time gives this stray light more time to act and causes highlight fogging - decreasing the exposure to compensate for the fog causes the shadow density to fall - in either case the contrast is reduced. Of course, if you open the lens to compensate then the stray light/image light ratio stays the same, but you then end up with 2).
2) Increased flare: If the lens is opened up a stop or two to keep the exposure time constant then the lens flare will increase.
The standard progression of print sizes are one stop apart. If you go from 8x10 to 11x14, keeping the crop on the long dimension, then the exposure change is exactly one stop. Ditto 4x5 to 5x7 and 5x7 to 8x10. In each case the linear ratio is 1:1.4, or 1:square root of 2. Obviously 4x5 -> 8x10, 8x10 -> 16x20, and 5x7 -> 11x14 are all two stop changes, and 4x5 -> 11x14 and 5x7 -> 16x20 are three stops.
I don't have the Ilford EM-10, but I've got an old Beseler PM??? color analyzer. The white channel on it works the same as the EM-10. But the trick is finding one that still works properly. Taking your chances on Fleabay might cost more than the new EM-10 that will come with a warranty.
Following the lens to paper measurement ratio method (for those of us without metering devices), wouldn't it be more accurate to measure the difference in the lamp to paper difference, which is really the material issue, and would be a different ratio, given the re-focusing necessary?
Wow, there are some really complex ways of dealing with this out there!
Originally Posted by johnnywalker
The method above is probably the easiest way, just remember you need to measure the illuminated area on the baseboard not just the area cropped by the easel blades. The ratio of the two areas gives the factor to change the exposure time by (assuming you leave the lens aperture the same). As others have stated you will likely need to increase contrast (1/2 to 1 grade typically) if it is a large increase in enlargement ratio.