Encouraged by a Zone System workshop last weekend where we could have our negs read by a densitometer, I finally ran a film speed test. It was an abbreviated form where you shoot a sequence of frames trying to find the frame (and E.I.) with a 0.1 density (Zone I). Each frame was offset from the indicated meter reading. The sequence was: -2, -2.5, -3, -3.5, -4, and -4.5 stops from the meter reading. The scene was a medium toned surface uniformly lit by two BCA (blue) 250w photofloods. The meter reading was 1/60 at f/2.8 (8 on the Pentax dial).(posted 8401 days ago)Using 35mm Tri-X at ISO 400 developed in D-76 1:1 at 20C for 8 minutes, I got the following results:
-2 stops, 1.40 -2.5 stops, 0.07 -3 stops, 0.03 -3.5 stops, 0.01, effectively blank
This implies that my E.I. for Tri-X is approximately 125. That seems suspiciously low. I used a relatively new Nikon N-80 and a Pentax Digital spotmeter (bought used).
I've repeated the test three times and got similar results where I cross the 0.10 density between -2 and -2.5 stops from the indicated meter reading.
Could you hazard a guess where I went wrong? I'm even more puzzled because rating Tri-X at E.I. 200 yields negs that are very easy to print using my standard print time ("maximum" black, minimum exposure).
I'm at my wits end that I'm going to send my meter to Quality Light Metric just to be sure that it's accurate and linear.