You asked

What are the alternatives to the leica summarit m 90mm f 2 4?

I don't have a list of other lenses for the Leica M mount, so I can't name direct alternatives to the Summarit-M 90mm f/2.4 yet. What I can tell you is that it's a manual-focus prime for full-frame Leica M bodies, and it fits a long list of them, from the M9 through the M11.

If you're choosing a 90mm-ish portrait lens, the Portraits guide explains why focal length matters. It suggests an 85mm f/1.8 as the classic full-frame portrait lens, and a 135mm or 70-200mm f/2.8 for tighter headshots. Check that any of these is actually made for Leica M before you buy, since I haven't confirmed that here.

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