@Alan Edward Klein Thank you for the tip! I will try it next time. If this solves my banding problem, I owe you one!
I have started retesting Acros II, given that some of you have found it to be close to ISO 100 in your own tests. I want to emphasize the fact that I am not trying to dispute your findings. I am simply reporting on my own.
Since I cannot currently change the light quality of my sensitometer, I decided to use daylight as my light source and do the typical Zone System test for film speed. This particular variant of it is based on Fred Picker's method (e.g., Zone VI Workshop, 1974), but it has been recommended in other ZS-focused publications over the years. The method assumes Zone I density being 0.1 over B+F. I don't mean to suggest that this is "correct," only that it is a commonly used approach among ZS practitioners. There are, of course, alternative methods of estimating film speed.
Here's a summary of the method:
- Place a black card in shade or overcast light (to simulate Zone I).
- Put camera on a tripod.
- Focus on infinity.
- Fill the entire frame with the card.
- Set the spot meter to 1/4 of the manufacturer’s recommended speed (here ISO 25) (determined using a Minolta Spotmeter F).
- Close down 4 stops (with aperture and/or shutter speed), thus placing exposure on Zone I.
- Transfer reading to camera and expose.
- Close down and expose subsequent frames in ½ stop increments (I used a Minolta X-570, in good operating shape and a 50mm f/1.4 lens).
Here are the results:
I processed the film in stock XTOL (to give it a fair chance for full speed, as opposed to XTOL-R, which gets a fraction less speed) for 6:30 minutes at 20C, using rotary agitation (aiming for the CI of around 0.62).
I found the film speed to be around ISO 50, which is almost exactly what I expected based on my XTOL-R speed of ISO 41. Again, these are just my findings. There's nothing definitive about them. Here's a cellphone picture of the negative. The ISO changes by the factor of about 1.4 from one frame to the next:
I will continue testing Acros II, developing in D-76.
Edit: I forgot to mention that I ran this test twice, as I always do.