One of the things that turned me off to developing film at home was the short shelf life on C-41 chemistry. I’d read that you can only process up to 12 rolls with one kit, and that it must be used within a month of mixing. As someone who learns through trial and error, I decided to see what would happen if I kept using the same chemicals past their “expiration”.
For this very unscientific experiment, I shot two rolls of Fuji CN100, my go-to test film as it’s the only one I can find with 12 exposures vs the usual 24 or 36.
I took these pictures on the same day using my Minolta X370 and X700, using the same lenses, aperture settings, and focus points.
I developed one roll using 6-month old C-41 chemistry which I had used to develop between ~40 rolls of various Fuji, Lomo, and Kodak film. I developed the other roll in fresh C-41 chemistry the day it was mixed. Both were developed according to manufacturer instructions.
The photos on the left were developed using old chemistry, while the new is on the right. The negatives were scanned using a Plustek 8200i at 2400dpi, using the same color settings.
I had read that using old chemistry would cause color cast, but I never noticed it before doing this test because there was nothing to compare my negatives to, and any undesirable color characteristics are easily corrected using software.
It’s obvious the old chemistry produced much more saturated colors, particularly reds and greens. But that’s about it as far as I can tell. The old chemistry still fully developed the negatives. The only differences that I can see are in color, and as noted above this can mostly be corrected in post. For an amateur enthusiast like myself, this is something I can live with for most pictures.
With that said, most of the film I use is consumer-grade Kodak and Fuji, but as I use professional films (i.e., expensive) more often, I will probably be changing out my chemistry more frequently to be safe.