There has been considerable discussion regarding the performance of the autofocus FA 35mm 645 lens vs. it's manual focus counterpart, the A35mm 645. After testing additional samples of each (since my initial testing of these lenses), I've observed a number of basic things.
There is more sample to sample variation in performance with the FA version than the A version. Since the FA version exhibits fairly extensive field curvature (especially at longer subject distances), it's critical to AF fine tune each individual FA version on the 645D to achieve the best compromise between center and side resolution. Setting in a bit of back focus in the FA version thru AF fine tuning, does help the FA lens achieve better edge to edge sharpness at infinity as does shooting distance landscapes well stopped down. At closer range the FA is extremely sharp even at f4.5-5.6
Just a observation when testing out a number of samples of both the newer D FA 25mm 645 lens and the DA 25mm 645 lens. They both too exhibit field curvature and therefore shooting distance landscapes requires the lens to be well stopped down to overcome this by increasing the depth of field.
Therefore if one is shooting general flat field subjects, the 35mm A version will achieve better edge to edge performance by virtue of sharper sides and corners when shooting long distance relatively flat field subjects.
If the subject being shots contains elements of both near, far and midrange, the 35mm FA version then actually outresolves the A version. It's a tradeoff. At closer subject distances, I have found the edge goes to the FA. If one is shooting a lot of distant landscapes that contain considerable detail at the sides and edges in the general smaple plane of focus, I would opt for the A version. Alternately if one was shooting a wide variety of subjects at various distances with elements not in the same plane of focus, especially if side elements were at closer range, the FA lens would be my lens of choice. It also exhibits a somewhat less CA in bright sunlight.
I've often wondered if the curvature of the front element used on the FA was responsible for the field curvature it exhibits, especially at longer distances. The A 35mm lens has a considerably flatter front glass.
Dave (D&A)