# Thread: how to do the math for speed film test

1. ## how to do the math for speed film test

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.

2. ## Re: how to do the math for speed film test

You seem to have found a speed of 32.

I think we'll need to know more about your method because this doesn't seem right.

3. ## Re: how to do the math for speed film test

Yeah, it seems odd to me too. Perhaps I need to do the test again and make sure I have done everything exactly correctly.

4. ## Re: how to do the math for speed film test

There's a lot of "slop" in this kind of test - mostly at the printing stage where it is very hard to judge the shortest time to max black, and then visually judge the next lightest tone (you're better off printing to a middle grey tone and then looking for a tone slightly lighter). That being said, some quick things to check for as I read your post:

1. When you do the camera part (filling the ground glass view with an unfocused image of the grey card), make sure the camera is focused to infinity for the lens used

2. Assuming you started at f/45 and then took an additional 5 shots in 1/3 stop increments, I think you only get down to about f/25. Here's a f-stop sequence in 1/3 stops: 45, 40, 36, 32, 29, 25

5. ## Re: how to do the math for speed film test

The instructions I followed were on Barry Thornton's web site. I agree that there are some possibilities for sloppiness on my part. There is another test included in the Way Beyond Monochrome book that I think I will try. In that book there are three methods he recommends, a super easy one, one a bit more complicated and a final, really thorough test with a density meter, which I don't have. Perhaps the middle txt would be sufficient?

6. ## Re: how to do the math for speed film test

Put the negatives over you meter. Look for the negative that drops the meter reading by 1/3 stop over base. That is your 0.1 log d zone one.

7. ## Re: how to do the math for speed film test

Originally Posted by ic-racer
Put the negatives over you meter. Look for the negative that drops the meter reading by 1/3 stop over base. That is your 0.1 log d zone one.
The math to find ISO is not difficult if you know the intensity of your sensitometer and your step wedge densities. (Speed = (0.8 / Hm))
However, when doing your exposure index, no math is needed. Just look back at your notes to see the ISO to which your meter was set when it exposed the 0.1 log d negative.

8. ## Re: how to do the math for speed film test

Oh, you might not have developed for enough time to get the full speed of the film.

I've got lots of ideas how you can tell if you developed long enough.

9. ## Re: how to do the math for speed film test

When you photograph the gray card, you first must have your camera focused at infinity. Then fill the ground glass with the gray card. If you merely de-focus the camera, you've changed the focal length and the proper exposure.

10. ## Re: how to do the math for speed film test

Originally Posted by Peter Gomena
When you photograph the gray card, you first must have your camera focused at infinity. Then fill the ground glass with the gray card. If you merely de-focus the camera, you've changed the focal length and the proper exposure.

So on a 4x5 view camera that means extending the bellows all the way to it's maximum?

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