I am testing a film for personal film speed. Thanks to one of our forum members I have a method to follow. Since I am really bad at math and find formulas difficult, I wanted to ask how to interpret my results.

The formula I followed said the take a reading of a grey card, filling the entire unfocused ground glass with the blurry grey image. Then I took a spot meter reading with a Pentax V and it gave an exposure of f 11 at 1/60 second. The instructions said to stop down the aperture to 45. That theoretically would be zone one.

Then I took five shots, each at about 1/3 stop greater than the previous. I made a exposure test strip from a developed unexposed negative to get base plus fog. Then after developing and printing under controlled conditions I should find the first negative/print that varied from maximum black. That print would indicate my proper zone 1 for my film.

Is this correct so far?

I have done all that and found that having started with f 45 at 1/60 second, the first print that showed a difference was shot at f 18 at 1/60 second.

So, seeing those figures, how do I determine what the true proper speed for my set up? The film I am using is Arista EDU Ultra 200 box speed.

Thanks so much. A greatly challenged math student.