>Also, I wondered about this method being used for comparison. It seems that
>unless all the factors are the same these results would not be useful to
>compare one lens against another. If I shot them all at the same time on
>the same film then yes, but otherwise no.
---This is strictly true (I used to use the same targets, though all else
---varied), but, since you assume that the lens does well in the center
---at f5.6, and that most (good) lenses perform about the same in the center
---at f5.6, you have a reasonable reference point built into each test that
---is independent of most variables - you reference everything either to
---what should be identical in a similar (geometrical) part of the frame, or
---to the center performance at f5.6.