Response to Thin Negatives from Film Speed Test

If you get good results shooting at EI 200, I'd say that is what you should use--particularly since it matches most of my own tests with Tri-X. You say it was a medium-toned surface, but that's pretty vague. Was it a grey card? I should note that Phil Davis states that the middle grey of standard 18% grey cards is really a middle grey for reprographic work, whereas the middle grey for general photography falls in the 10% to 12% reflectance range. Most light meters are constructed to give correct readings from 12% grey, which is about 1 stop darker grey than 18%. I suspect that your true EI is 200 to 250.

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