Brad, as along term Contax user of both regular and MM lenses. There is a difference between them. Basically, the MM lenses allow you to use the program functions on the newer camera bodies. If you look closely at each mount, the MM version has a little pin that sticks out, while the regular mount does not. The pin actuates a newly positioned switch on the newer bodies for program operation. These older lenses can be 'fudged' to behave like an MM lens though by placing in a piece of plastic or match stick in the camera I've heard of several creative ways that folks have gotten round it!!!. That is all there is to it. All aperture priority, manual, will work with all lenses but the MM versions open up the program functions. Hope this helps?