Ilford Delta 3200 Infrared Step Test
Camera was a Mamiya RB67 with a 127mm lens working at f6.7 through a IR720 filter looking at a sunny scene. The steps were done by moving the dark slide between exposures.
The negative step exposed for 8 minutes seemed to be the minimum for reasonable shadow detail. The screen picture above is a scan of a print of the negative enlarged onto Ilford Classic VC FB photographic paper.
Some calculations follow. Please check for blunders.
Ilford's reciprocity exponent for Delta 3200 is given as 1.33 which implies a metered exposure of 104 seconds is needed in order to arrive at 480 seconds after correction.
A metered exposure of 104 seconds at f6.7 is the same as 593 seconds at f16.
Using the old sunny 16 rule that says exposure time at f16 is the reciprocal of the EI (exposure index) this would imply the EI for Delta 3200 in this test is 1/593 = 0.00169. Mighty slow but not zero.
My light meter does not have provision for EI = 0.00169 so I think I will set EI = 16, measure the light, and then increase the displayed exposure time by a factor of 10000X.
Extensive testing of this film and filter combination could get tedious.