Going to take some long exposures in a few weeks but I've never done this with film before. So I went to the Kodak website and had a look at the data sheets for 400TX, TMX, TMY and the Portras. The first confusing moment was when I digest the TriX sheet which says
indicated 1s -> add 1 stop -> 2s (fine)
indicated 10s -> add 2 stops -> 50s (? isn't it 10s *2 *2 = 40s?)
indicated 100s -> add 3 stops -> 1200s (??? 100s * 2 * 2 * 2 = 800s)
On the other hand, the TMAX 100 & 400 sheet are comprehensible.
Next I found Gainer's method (http://www.apug.org/forums/forum37/1...sbehavior.html
) which adds to the confusion.
According to Kodak:
400TX @ 10s -> 50s
TMX @ 10s -> 15s
TMY @ 10s -> 15s
400TX @ 100s -> 1200s
TMX @ 100s -> 200s
TMY @ 100s -> 300s
According to Gainer, the corrected exposure is a function of the coefficient tc,1 only. These are
Using this formula (Tcorr = Tmeasured + tc,1*(Tmeasured^1.62)), 400TX @ 100s should be 400s, which seems to be very low. Moreover, TMX has a greater tc,1 than TMY and therefore a shorter corrected exposure. This completely contradicts Kodak's sheets, both at 10s and 100s.
Please tell me, I'm stupid and made a terrible mistake.