Head to head
Leica Summicron-M 50mm f/2 (v4) vs Carl Zeiss Planar T* 50mm f/1.7 (C/Y)
Both turn up on the shortlist of anyone chasing a "definitive" 50, and both have a reputation that survives pixel-peeping and grain alike. The honest split is system and money. The Summicron is a rangefinder lens that lives on a Leica M body and costs like a used car; the Planar is an SLR lens off the Contax/Yashica mount that you can find for a fraction of the price and bolt onto almost any mirrorless camera with a cheap adapter.
How they differ
Handling is the first real divide. The Summicron-M v4 is tiny, all brass and aluminum, with a focus throw and a built-in tab that make zone focusing on an M body second nature. You focus through a rangefinder patch, not through the lens, so close work and exact framing behave differently. The Planar f/1.7 is a bigger SLR optic with a longer throw, and you see depth of field directly through the finder, which some people simply prefer for portraits and close subjects. It is also the faster glass at f/1.7 versus f/2, a marginal stop on paper but real when the light goes.
Rendering is closer than the price gap suggests. The Summicron is famous for a gentle, three-dimensional look with smooth out-of-focus areas and beautifully controlled flare, the kind of signature people pay the Leica tax for. The Planar T* is clinically sharp wide open, with high contrast and that Zeiss microcontrast, and it cleans up to bitingly crisp by f/4. Availability also cuts opposite ways: clean v4 Summicrons are scarce and pricey, while C/Y Planars are plentiful on the used market and adapt trivially to Sony, Fuji, or any mirrorless body.
Choose Leica Summicron-M 50mm f/2 (v4)
Pick the Summicron-M v4 if you already shoot a Leica M, or want to, and value a compact, mechanically perfect lens you focus with a rangefinder. It rewards street and reportage work where small size and silent operation matter, and it gives you that classic Leica rendering straight out of the lens. You are paying for the system and the look, so it only makes sense if you actually want both.
Full Leica Summicron-M 50mm f/2 (v4) guide →Choose Carl Zeiss Planar T* 50mm f/1.7 (C/Y)
Pick the Planar T* 50/1.7 if you want most of the optical magic for a small fraction of the cost. It shines on a Contax SLR body, but its real modern strength is adaptability: put it on a mirrorless camera with a $20 adapter and you get a sharp, contrasty, slightly faster 50 with through-the-lens focusing. Ideal for someone who cares about results over badge, or who wants a do-everything fifty without committing to the M ecosystem.
Full Carl Zeiss Planar T* 50mm f/1.7 (C/Y) guide →The verdict
These are close on image quality and far apart on everything else. If you live in the Leica M world and love the rendering, the Summicron earns its keep. For almost everyone else, the Planar f/1.7 delivers comparable results, a touch more speed, and far easier adapting for a tenth of the money. Taste and system decide it, not optics.