Quote Originally Posted by jeroldharter View Post
I ... 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.)
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.

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.