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Miles: Thanks for the response. The Davis article talks about the film speed point location at 2.4 using the plotter program. I don't have the plotter program and do the data "massaging" myself. I think I understand the basic speed point issue- if one tests a 100 speed film as
Phil suggests( Ilford D-100; D76 @1/1; 70F) and sets the enlarger light source to provide 4.0 EV, exposing at .5 sec. or say .25 sec. will still provide 4.0 EV. However, the.5 sec exposure will provide light for 2X the duration as that for .25 sec. and possibly overexpose the film. In my hands the .5 sec test results in a speed point of about 19 (2.85) and suggests a test at .25 sec exposure will also require about 1/3 stop less EV.
This doesn't address the Q I raised about the actual film speed to be used for incident metering.
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