[Dick England]

Klaus, your mention of AA filter design got me thinking about internal reflections within the AA filter. These will increase, the more the ray's angle departs from perpendicular (i. e. further out in the image circle, especially for non-retrofocus designs). Internally reflected rays will spill over onto adjacent sensels, defocusing the image of tangentially oriented lines. The more prone the AA filter to internal reflection, and the greater the distance between the internally reflecting surfaces, the worse the problem. If this were the case, the blurring should be astigmatic.