Head to head
Tamron SP 90mm f/2.5 Macro (52BB) Adaptall-2 vs Canon EF 100mm f/2.8L Macro IS USM
Both land in the same spot in a bag: a short telephoto that does life-size macro and doubles as a portrait lens. People weigh them against each other because the Tamron 52BB is the cheapest path to a legendary macro rendering, and the Canon is the do-everything modern version with autofocus and stabilization. The single biggest difference is era and automation. The Tamron is a manual-focus adapt-anything lens from the film days, the Canon is an L-series AF macro built for digital bodies with IS.
How they differ
In use, the gap is mostly about focusing and reach. The 52BB is manual only, focuses by feel, and the 90mm focal length sits a hair wider, which matters for working distance when you are close on bugs or jewelry. It is famous for its rendering, smooth out-of-focus areas and a creamy color signature that a lot of people chase specifically for portraits. The Adaptall-2 mount is the trick: swap the cheap mount adapter and the same lens fits Nikon, Canon FD, M42, Pentax K, and modern mirrorless via a further adapter. Build is metal and dense, and it reaches 1:2 on its own (the original 52BB needs a matched extension or the later 52E reaches 1:1).
The Canon brings hybrid IS, ring USM autofocus, full-time manual override, and weather sealing. It does 1:1 natively, the 100mm gives slightly more standoff, and on Canon bodies you get metering, AF confirm, and EXIF without thinking about it. It is also a lot more money and a fair bit bigger. The Tamron is cheap on the used market and built to outlive everyone, but you are doing everything by hand and metering stop-down or with an adapter chip. The Canon costs several times more and ties you to the EF mount (which adapts cleanly to Canon RF, less cleanly elsewhere).
Choose Tamron SP 90mm f/2.5 Macro (52BB) Adaptall-2
Pick the Tamron 52BB if you shoot film, want one macro that moves between bodies via Adaptall mounts, or just want that specific drawing for portraits without spending much. Manual focus does not bother you, you like a metal lens with real heft, and you would rather put the saved money elsewhere. It is the smart buy for adapting onto mirrorless too, where focus peaking makes the manual focus a non-issue.
Full Tamron SP 90mm f/2.5 Macro (52BB) Adaptall-2 guide →Choose Canon EF 100mm f/2.8L Macro IS USM
Pick the Canon 100L if you are on a Canon EF or RF body and want autofocus, image stabilization, weather sealing, and native 1:1 in one lens. It earns its keep handheld, for tabletop and product work, event detail shots, and as a sharp 100mm portrait lens where you do not want to chase focus manually. If macro is a working tool rather than a slow deliberate craft, this is the one.
Full Canon EF 100mm f/2.8L Macro IS USM guide →The verdict
Close call that comes down to how you work. Want autofocus, IS, and 1:1 on a Canon body, pay for the 100L. Shoot film, adapt across systems, or just want that storied rendering for little money and do not mind manual focus, the Tamron 52BB still punches far above its price. Neither is a wrong answer.