<p>The SL2 only requires 2 cam lenses but as has been stated, many earlier lenses have been upgraded by Leica or DAG to 3 cam. When viewed from the rear, the 2 cams are most prominent as the curved silver cams , top and bottom inside the lens housing. The third cam when present is a small (usually black) stair-step immediately inside the bottom silver cam. I have two lenses which were converted by Leica US.</p>