Sensitometry test of unrelated film for process control purpose.

[Bill Burk]

Has anyone done this? I've been consistent to always use a roll or sheet of the "same" film I'm processing whenever I run a sensitometry test for process control purposes.

But tonight as I contemplate turning on the water, I don't happen to have a spare roll of TMAX 100. (But I have several rolls of TMAX 100 to process). You see, when I have fresh film in my hands, I can't seem to resist shooting it ALL!

Now the thought occurred to me. I know my Time/CI for TMAX 100.

I also have Time/CI for TMAX 400, which I happen to have on hand.

I literally could expose the TMAX 400 on the sensitometer, include in the batch of TMAX 100... Develop it all at the time I plan to develop the TMAX 100... Then do densitometer readings on the TMAX 400 to verify it has met its expected curve at that development time.

If the TMAX 400 hits its target, then I know I've developed the TMAX 100 right.

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Additional thought: After formulating this plan, I realized my target time for both films for 0.62 contrast index is 13 minutes 30 seconds in D-76 1:1 at 68-degrees F. So literally this pair of films should process fine together. But I see some value in following this plan with other combinations of film. Example, I could pick up a bulk roll of any film, keep it in the darkroom just to make test strips... And that will let me shoot more of the fresh film I buy in 36-exposure rolls. The bulk roll would only get used in short lengths, and I don't have to hunt for cassettes to load the bulk roll into, since I'll be cutting lengths of it and developing...
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Now I understand how I used to get away with using "Chemco" process control strips with Kodak imagesetter film.

 

[fotch]

Holy smokes. I use the KISS theory myself.

 

[Bill Burk]

A sensitometer is not much more than a "test strip maker" for film. And it's not hard to see why a test strip is good for printing. It's also pretty good for developing film.

 

[Bill Burk]

But you're right fotch about one thing. It IS simpler to do sensitometry on the same film stock you are processing because the results relate directly...

 

[Mr Bill]

Bill, actually the great majority of "official" process-control strips are on different emulsions than user film. So it's pretty typical.

Definitely these films do NOT all behave the same in seasoned developers, at least in the color world where my experience mainly comes from. But they're generally fairly close. I recall somewhere around 20(?) years ago that Kodak C-41 strips were on some version of non-pro film, and did not accurately reflect what was happening in pro-lab processing lines. So Kodak began producing a second line of control strips on pro film stock. Anyway, heavily-seasoned developers can act differently on different film stocks.

I would GUESS that for B&W processes, the effect would be relatively minor, but if you're trying to do some sort of precision work, it would probably be worth staying with the same film type, or at the very least, do a one-time comparison with both types.