There’s nothing magical going on. Whatever light intensity illuminates the negative, the light that passed through the negative now passes through the lens at the stated f/8 resulting in 2-4 second exposures on the paper you’re using.
Most of us find that faster than we’d expect at the approximately 4.7X needed to make a 10” x 10” print of a 6 x 6cm negative at f/8 (assumed cropping from an approximately 10 ¼” x 10 ¼” projection).
Assuming that the lens aperture is correctly closing to the stated f/8, then the likely explanation is: brighter than usual light source, thinner than usual negative, or a combination of these.
You didn’t specify what filters you’re using. Obviously an unfiltered exposure is shorter than a filtered exposure—possibly much shorter.
Regarding #14, the closer the light source to the negative the brighter the projection and the shorter the printing time. Placing the source farther from the negative would result in longer printing time.
Thus, one way to increase the printing time is to open the upper bellows to move the light source farther from the negative. This will work so long as the negative remains evenly illuminated.
Last edited by Ian C; 02-05-2012 at 02:22 PM. Click to view previous post history.