testing for film speed (again)

[acrobatic_citron]

A few comments:

1. The exposure index that consistently gives you the best negatives (ie what you have determined is best for printing/scanning through practice) is what matters

2. An ISO 100 film will, by definition, have a Zone System speed of 64. Use that as a rough guideline to compare your results with.

Using an enlarger is not the easiest way to do a Zone System speed test, but it can be done. Do you have access to an incident exposure meter? If not, you can make approximate measurements with a spot meter using a few assumptions and/or applying a few formulas.

This is linked to each system developing to a different contrast ratio, is that correct? With Zone system being generally lower

[Michael R]

No. It’s that the speed point in the ISO criteria falls 3 1/3 stops below the metered exposure rather than 4 stops in the ZS.

This is linked to each system developing to a different contrast ratio, is that correct? With Zone system being generally lower

[acrobatic_citron]

No. It’s that the speed point in the ISO criteria falls 3 1/3 stops below the metered exposure rather than 4 stops in the ZS.

ah great thanks!

[quigon]

Ok I think I have got the answer to the question I asked:

(The question: How do I calculate an exposure for zone X for the fomapan 100 exposed for the box speed (100 ISO) in the above illustrated scenario?)

I think the answer is: I can't.

I can't reproduce the highly controlled conditions that the manufacturers adopt to rate their films (of course).
The only way I can progress in the scenario I outlined in the first post is to catch what we generally think is an honestly rated film (and developer)
and expose the film sandwiched with a step wedge, develop, fix, analyse.
Repeat the process (adjusting the height of the head of the enlarger in order to modify the intensity of the light source) until I get 0.1 log density in sector 19 = zone I.
Only then I will have found the intensity of light necessary in my scenario to test a 100 ISO film.
After that, when I expose a Fomapan 100+step wedge in the same conditions I will discover an EI valid for me.
Hope I've got it right
Thank you everybody.
Luigi

[Michael R]

Comparing with a “standard film” should work fine (an approach was mentioned by ic-racer in post #5).

A simpler variation on your proposed methodology would be to expose both the standard film (say TMX 100 or Delta 100) and the test film at the same time and then develop them to the same contrast. Then just compare them.

You can calculate an exposure for the enlarger. It’s just that you need to make a few assumptions since you are using a reflected exposure meter, so it won’t be perfect.

I looked at the Foma data for the 100 film and it seems to be reasonably close to ISO 100, which would mean a Zone System EI ~64. Round it down to 50 for Rodinal and that’s as good a starting point as any. It’s 2/3 stop faster than your current EI of 32, but since you’ve apparently determined 32 works best for you in practice, why change it? Ultimately whatever EI works best consistently is the one you are trying to find, so it seems like you’re kind of going backward. Normally one would run the kind of test you’re describing to get a starting point and then adjust the EI based on real usage/experience if over time you find your negatives are consistently under or overexposed.

[Doremus Scudder]

While striving for precision is laudable, sometimes it's just not practical. I've done all the ZS tests tens of times for different films and different development regimes, etc., etc., but still find the need to tweak my results to better reflect real-world results.

Having a negative that gives you the prints you want is the goal. Be consistent in your metering and exposure practices and keep good field notes. Then, follow the time-tested advice from Kodak: If your negatives lack shadow detail, give more exposure (and vice-versa, except that overexposing is much less of a problem than underexposing). If you need more contrast to print well on a medium-grade paper (or whatever with scanning), then develop longer; and vice-versa.

After a while, you'll be satisfied with your negatives. Remember, the window of possible combinations of exposure and development that will make a great print is not as small as many think. There's some leeway.

After all my years of testing everything before I made a "real" image, I now just rate a new film 2/3-stop slower than box speed, do a couple of tests for N development and then go from there, tweaking as I need from extrapolations for other development times based on my field notes and maybe making a second negative of a scene when I'm still figuring things out. I rarely toss a negative, even when starting out with a new film, simply because exposure and development were not right. It's usually because I didn't see the scene well enough to make an image worth printing.

Best,

Doremus

[popdoc]

Well said, D!


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[quigon]

to IC-racer
Thank you, your post made me take the right track.
to Michael R, WillWilson and Neal Chaves
thanks for your contribution.
to Doremus Scudder
thanks for helping to keep my feet on the earth. I very seldom make tests, but when I do I run the risk of falling in a labyrinth.
Best Luigi