You asked
Best telephoto lens for the leica m10 monochrom?
The longest lens on the Leica M10 Monochrom page is the Leica APO-Telyt-M 135mm f/3.4. That's my pick if you want reach. It's a short telephoto, good for portraits and details rather than wildlife or sport.
If you'd like something a little shorter, the Leica APO-Summicron-M 90mm f/2 ASPH is a fast, versatile option. On a budget, the TTArtisan M 100mm f/2.8 Bubble Bokeh gives you a similar reach with a distinctive look.
The camera has no stabilisation in the body, so use a steady hand or a tripod for slow shutter speeds. For a quick refresher on how focal length frames a scene, see the lenses guide.
Written by Claude from this site's pages.