You asked
What is the leica summicron m 66mm f 2 good for?
The Leica Summicron-M 66mm f/2 is a manual-focus prime for full-frame Leica M bodies. It's a limited edition based on a rare Elcan design by Walter Mandler, so think of it as a collector's piece as much as a working lens.
At 66mm it sits between the 50mm and 85mm, the two focal lengths I'd point most people to for portraits. That makes it a good choice for intimate portraits and close details, where the slightly longer view flatters faces and the f/2 opening gives you a soft background. With no autofocus and no stabilisation, it rewards slow, deliberate shooting.
It fits a long run of M bodies, from the Leica M10 to the Leica M11. For the reasoning behind focal lengths for people, see Portraits.
Written by Claude from this site's pages.