Quote:
Originally Posted by Snapshot  I'm seeing my exposure times rise for exposing enlarging paper, Ilford Multigrade in this case. My times have risen at least 25% (e.g. an 8 second exposure now requires a 10 second exposure for the same results). I'm still working on the same batch of 500 sheets that I've started with last year |
Note that a 25% drop isn't much - about 1/4 of a stop of light or 1/2 a zone of paper density - and there are a lot of things that can cause a variation of this amount: the utility is running the voltage lower; the last time prints were made the airconditioner wasn't running; the lens and or condensers are getting foggy; the aperture click stops are sloppy or the aperture ring was slightly off the stop; the developer is different; the developer temperature is different; the developing time is different; the wrong exposure was written in the original notes; if the enlarger is used a lot then bulb could be blackening, but that takes a powerful lot of use - the enlarger would have to be on for several hours a day for a regular ol' PH212, not nearly as much for a halogen lamp but still a lot of use, enough to have used up the 500 sheet box quite some time ago; the halogen lamp sockets could be failing - a pretty common problem.
The most likely culprit, though, is the paper is aging.
I spend a large part of my time testing paper for printing speed and characteristic curves.
I have been doing this testing for 4 years as part of Darkroom Automation's product development efforts.
And my experience is:
Paper will lose about 1/2 a stop of speed and 1/4 to 1/2 a grade of contrast in six months of intermittent usage. After this initial drop the paper stays relatively stable until it begins to develop fog. Paper that is removed from it's wrapping and spends a month or so in a paper safe is often a basket case with unacceptable levels of fog. My paper is kept in an air conditioned environment and never sees temperatures above 75F/24C.
If the paper package is unopened or the paper is tightly re-sealed and not re-opened it is pretty stable, as Simon says claims. Under typical amateur use this isn't the case.
This behavior is consistent and holds true for Ilford, Seagul and the old Kodak and Agfa papers. Forte, Foma and others are too inconsistent to make any statement about stability.
Since a loss of paper speed is well nigh guaranteed, it is the most likely culprit.
If you have a good enlarging meter you can check lamp stability over time.
If it were me I would shrug my shoulders, use 10 seconds, and not worry about it.