I just got a second hand 16mm f/2.8 AIS lens. The lens has a rear mounted filter installed. I took some exploratory pictures with. At f/2.8, the center sharpness was great, but the peripheral sharpness is disappointing to say the least. It is so unsharp that it looks quite soft and fuzzy even on the camera's rear LCD display. At f/11, the edge sharpens up quite a lot, but only compared to f/2.8. It is still quite soft and showing lots of radial chromatic aberration is looked at closely. Is this normal for this lens?