Chet wrote: > It goes back to something I was wondering about earlier when I noticed > that the SMC 300 is marked to focus to 300 feet before infinity; the M* > and A* dropped to 150 feet before infinity; and the F* has dropped to 60 > (or 80?) feet before infinity.
That would be due to the amount of rotation it takes to go from minimum focus to infinity, combined with changes to the minimum focus distance. The F*300mm goes from infinity to 2m in half a turn (180 degrees). The A*300mm goes from infinity to 4m in 3/4 of a turn (270 degrees). So the A* can have more space for distance markings. The F* will be more difficult to focus accurately. I suspect the turning reduction was done to speed up AF. Otherwise the lens would put more strain on the motor due to a higher gearing ratio to achieve the same AF speed. > So what happens if the subject is at 200 feet (with a distant background)? > Would the SMC 300 display DOF effects, whereas the F*300 would have it all > at infinity????? I am speculating here but my assumption is that it'd be identical (except wide-open where its f/4 vs f/4.5). Cheers, - Dave http://www.digistar.com/~dmann/
