Head to head
Leica Summicron-M 50mm f/2 (v4) vs Canon FD 50mm f/1.4 S.S.C.
Both land on the shortlist when someone wants a "real" 50mm on a film body and starts reading forums, and that is where the similarity ends. One is a rangefinder lens built for a Leica M; the other is an SLR lens on Canon's old breech-lock FD mount. The single biggest difference is what surrounds the glass: a Leica M body and the Summicron together cost many times what a Canon FD body plus this 50mm will run you. You are choosing two different systems, not just two optics.
How they differ
Speed and handling split first. The Canon opens to f/1.4, a full stop and a bit faster than the f/2 Summicron, so it pulls ahead in low light and gives you that thinner SLR-finder slice of focus when shooting wide open. You also focus through the lens on the Canon, framing exactly what the sensor or film sees, with no parallax. The Leica focuses by rangefinder patch, which is faster and more certain in dim light once you trust it, and the v4 Summicron is tiny and dense in a way the FD never is. The Canon's breech-lock ring is its own ritual, smooth but slower to mount than a bayonet.
Rendering and cost are the other axis. The v4 Summicron has a reputation (some of it earned, some of it forum mythology) for a particular smooth out-of-focus transition, and it holds contrast and micro-detail beautifully stopped down. The FD 50mm f/1.4 S.S.C. with its Super Spectra Coating is a genuinely excellent performer that gets sharp by f/2.8 to f/4 and flares less than its bare-coated predecessors, but wide open it is softer and lower in contrast than the Leica. Availability favors Canon hard: clean FD 50mm f/1.4 copies are everywhere and cheap, while a good v4 Summicron is a deliberate, expensive hunt, and adapting it to other bodies is easy for the FD and a fiddly proposition for the M.
Choose Leica Summicron-M 50mm f/2 (v4)
Pick the Summicron v4 if you already shoot a Leica M, or intend to, and want the optic that matches that body and that way of working. It rewards a photographer who values rangefinder focusing, a pocketable kit, and the last bit of contrast and resolution wide open. If you keep lenses for decades and want one 50mm that just renders cleanly without fuss, and the price does not stop you, this is the one.
Full Leica Summicron-M 50mm f/2 (v4) guide →Choose Canon FD 50mm f/1.4 S.S.C.
Pick the Canon FD 50mm f/1.4 S.S.C. if you want a fast, sharp, dependable standard lens for very little money, especially paired with an AE-1, A-1, or F-1. It is the better call for low-light SLR shooting, for close framing where you need to see exact focus, and for anyone building a film kit on a budget or just starting out. Mirrorless shooters who like adapting glass also get more shallow-focus character per dollar here.
Full Canon FD 50mm f/1.4 S.S.C. guide →The verdict
Different systems, so the honest answer is mostly which body you own or want. The Canon delivers most of the image for a fraction of the cost and adds a stop of speed; the Leica gives you rangefinder handling, a smaller package, and a touch more wide-open polish. If budget matters at all, the FD is the sane buy. If you live in the M system, the Summicron is worth it.