I figure that the best way to show what happens to a picture taken with a FF lens and a DT lens on an A850/A900 and on a A700 is to try it and see.
The following four pics, #1 to #4 are the as shot untouched results, only reduced in PS5 to comply with the forum guidelines. NO CROPPING HAS BEEN DONE. The A850 was mounted on a tripod and shot without moving the camera except to change the lenses, see pics #1 and #2 . In pics #3 and #4 the A700 was used and as it is smaller than the A850 the tripod was raised a small amount to keep the pics in the center of the frame. I was very careful to avoid changing the distance from the cameras to the subject. So we can assume that the distance from the sensor to subject is the same in all cases, 24" or (610mm).
The Lenses used were a Minolta 50mm f1.7 full frame lens (FF) and a Sony 16-80 Carl Zeiss (DT) at 50mm. See below the pics for my conclusion.
1/4sec @ 50mm (FF) f4.5, ISO 200. 24.38MP in the photo. A850 + Minolta 50mm f1.7
1/6sec @50mm (DT) f4.5, ISO 200. 10.58MP in the photo. A850 + Sony 1680CZ
1/2sec @ 50mm (FF) f4.5, ISO 200. 12.16MP in the photo. A700 + Minolta 50mm f1.7
1/3sec @50mm (DT) f4.5, ISO 200. 12.16MP in the photo. A700 + Sony 1680CZ
It is interesting to note that the A850 + FF gives a pic (#1) at 50mm (the Epson box in the pic is 3.75" wide on my screen, may be different in yours) and the A850 + DT gives a pic (#2) that is only 1.25x as large (box is 5" in the pic), not the 1.5x crop factor as expected. Note also that the MPixels are less than half the FF, this may be the reason. The MPixel size (10.58MP) agrees with the manual.
The A700 + FF in pic #3 shows a 50mm x 1.5 crop factor (the box is 5.625" wide on my screen and 3.75 x 1.5 = 5.625)) as expected. However in pic #4 we get essentially the same picture as in pic #2, a 50mm x 1.25 crop factor. Both have 12.16 MPixels as expected.
In conclusion I suspect that things do not necessarily work out exactly according to the accepted theories.
However the above exercise should give the original poster, jdlaing, a better understanding of what to expect with his A850. Note also that the A850 is faster than the A700?