Originally posted by Dr_who Agreed, the filter is across the frame not more or less strong depending on how close it is to the center of a image.
The Birefringence material will affect the acceptence angle of the microlens depending how for fromt the pependicular it is this will affect vignetting at least.
To imagine it simply put a conmb infront of your eye now alter the angle of the comb (move on end away from your face) the light level will vary.
This reducing lightlevel will also cause localise difraction
ergo soft borders directly casued by the AA filter.
Interestingly Pentaxs AA simulator won;t suffer the same issue.