[Stephen Benskin]

I'm also considering stuff like shutter efficiency, for example. Slower shutter speeds and small apertures are more efficient than faster shutter speeds and larger apertures. It's a simple question of the percentage of the exposure time the shutter covers the aperture. Here's a graph of it from Photographic Materials and Processes.

Shutter Efficiency001.jpg

Tests that use only a single exposure aren't as effected as doing a Zone System type test which consists of multiple exposures using different combinations of shutter speeds.

Also, any optical system produces flare. Even if everything is masked off, flare will still be coming from the thinner steps on the tablet. Not only is there veiling flare, but local flare, where one step on the step tablet will influence the one next to it. Veiling flare will not only reduce what the results of the contrast testing, it will also increase apparent film speed. And since flare is hard (next to impossible) to measure, you can never adjust for it. It's always best to do a flare free test and factor in flare later. This is how the ISO standard for black and white film speed determination does it.

This isn't all make of break kind of stuff. It's just something to consider when testing. Each variable has the potential to influence the test and the influence from multiple variables could be cumulative.