Originally posted by JinDesu Do Pentax FF lenses work on Sony FF cameras? Like how you can use an adapter to get it working on the NEX? Would that solve the question, since SR only requires a focal length to work?
Pentax's register distance is 45.46 mm and the Sony A-Mount is slightly shorter at 44.50 mm. So nobody makes an adapter since it would not focus at infinity.
The Sony E-mount is 46.1mm wide inside diameter which is wider than the Pentax at 45mm. The Alpha mount is much bigger than either the E-mount or the K-mount, but I don't know the exact number. I don't think it is as large as Canon at 56mm.
My guess is that the A-mount lenses projects a larger image circle, and the IBIS of the A900 is not as good as that of the K-5 or the Sony APS-C bodies which suggests Sony does not have as much room to move the sensor and keep it in the image circle. A900 only give you about 1-2 stops while APS-C systems give you 3-4 stops. Maybe the A99 is better.
I still think the reason invested in OIS (HD 90mm F/2.8 for 645D) is because they will use it on a Full Frame K-mount.
Maybe someone can come up with the technical specifications for the image circle size of the K-mount. A FF sensor requires a minimum of 43.3mm and to reduce vignetting you need more than that. Then the question is how much movement does IBIS require to provide 4 stops of stabilization?