It is, but lens mounts are largely proprietary and SLR lenses are big. Some wide angle lenses are also heavily corrected in order to accomodate a swinging mirro

It is, but lens mounts are largely proprietary and SLR lenses are big. Some wide angle lenses are also heavily corrected in order to accomodate a swinging mirror. Even if you can fit them with an adapter that retains most features, you wouldn't want to lug around a heavier lens, when you could just sell it and buy the native equivalent, which in some cases even performs better due to physics. There is no point in using a SLR mount for mirrorless as it's oversized and will negate the cost and size advantages of shorter flange to sensor mirrorless designs.