for the pentax 645 system there are two leaf shutter lenses the SMCP 75mm f/2.8 LS and SMCP 135mm f/4 LS - both are optically identical to their A-Series counterparts, as mentioned in the above posts they possess a built in Leaf shutter -Which has to be cocked each time you use it - which makes LS lenses a bit awkward and slow to use, but with Medium format there is no other way of obtaining 1/500th synch speeds. You do not have to use the leaf shutter mechanism - on the shutter speed dial in the front of the lens there is a circle - set the shutter to that and the leaf shutter will remain open allowing you to use the focal plane shutter. The reason for this function is that Leaf shutters have a substantially shorter longevity than focal plane shutters do - as I recall the average leaf shutter is only rated for 20,000 firings. As far as I know the focal plane shutter in the 645D is rated for 50,000, so having two shutter mechanisms at your disposal is certainly appealing.
Last edited by Digitalis; 01-03-2012 at 01:41 AM.