I am not saying that anybody is wrong. However, it is impossible to make meaningful comparisons of film speed (or speed in certain developes) based on field tests. Such tests involve not only variables in equipment (meters, lens apertures and shutters) but also in method and type of metering. And this is before we even bring the film to the darkroom for development.

I completely eliminate these variable in testing for EFS by contact printing transmission step wedges with a light source timer by an integrator, which results in an error of less than 1/100 of a second with a 0.5 second exposure. Therefore the only variables in my testing are the film, developer (composition and temperature) and method of agitation. Of these the most important is method of agitation and I think many people would be surprised if they realized how even small differences in method of agitation can result in a significant change in contrast, and of EFS since there is a relationship between contrast and effective film speed..

Naturally at some point you have to take the EFS values (what should be consistent from person to person if sound testing procedures are used) that results from testing and validate them in the field to give you a working exposure index, or EI. But EI is personal and can be expected to vary significantly because of the field variables mentioned.

Sandy King