Hmmm, very interesting piece of information.
Tomorrow I'm doing a film speed test of a new film I intend to use. But this posting has given me something else to do as well
I've just figured out a way to insert small pieces of MGIV to the back of my Nikon F3.
I'll do a few tests on a grey scale and colour chart as well as a pictorial print.
I don't assume a speed of 25 ISO will be great, but it shouldn't take much time to get a working ISO using your information as a start.
Nothing like a bit fiddling here and there to keep the grey matter ticking over, eh?
Many thanks, Mick.