T-mount to Canon RF adapter compatibility

T-mount is the one entry in this matrix that is not really a camera mount at all. It is a lens-side interface — the M42 × 0.75 thread Tamron introduced in 1957 and standardised as T-2 in 1962 — with a fixed 55 mm spacing from the thread shoulder to the focal plane. The lens carries no bayonet of its own; it screws into a body-specific T-X adapter ring, and it is that ring which carries the camera mount. For an EOS R body you fit a T-Canon-RF ring: the lens threads into the front, the RF bayonet sits on the back, and because Canon RF's flange is only 20 mm the ring is built as a roughly 35 mm tube that makes up the difference. That leaves a full 35.0 mm of clearance, so infinity focus is never in doubt — the optics always sit at their designed 55 mm regardless of which RF body the ring adapts to. The appeal is exactly that one-lens-many-bodies design: buy the long glass once, change the cheap ring when you change systems, and screw the same ring onto a telescope when the scope comes out.

What you actually mount is all long glass, because that is what T-mount has always been for. The Vivitar 500 mm f/8 Mirror and the Samyang 800 mm f/8 Mirror MC are catadioptric (mirror) lenses — astonishingly compact for their reach because the light path folds back on itself — and the Soligor 400 mm f/6.3 Tele is a conventional refractive long lens. None of them autofocuses; all three were built as budget reach for wildlife, the Moon and the Sun. Canon RF is a particularly fitting destination because Canon has the deepest astrophotography pedigree of any maker in this matrix: it shipped the EOS Ra in 2019, an RF-mount body whose modified infrared-cut filter passes roughly four times the hydrogen-alpha (656 nm) light of a standard sensor for emission nebulae, with 30× live-view magnification to nail focus on a star — the spiritual successor to the EF-era EOS 20Da and 60Da astro DSLRs. A shooter who bought an EOS R5, R6 or the astro-leaning Ra for native RF glass can add a T-Canon-RF ring from Fotodiox, K&F Concept or Gobe for less than a memory card and reach 500, 800 mm and a telescope's focuser without rebuying anything.

The verdict reads Mechanical · MF · no IS · Ap. ring, and every term is honest for this glass — with one nuance the badge cannot show. Focus is manual, set on the lens barrel and confirmed through the EOS R body's focus peaking and 5×/10× magnified view, because there are no electrical contacts anywhere in a T-mount lens or its ring. 'Ap. ring' is true of the refractive Soligor 400 mm f/6.3, which carries a real preset diaphragm — you dial the working f-stop on one ring, then a second ring stops down in a quick turn — but it is not true of the two mirror lenses. A catadioptric design has no iris at all: the Vivitar 500 mm and Samyang 800 mm are fixed at f/8, so you control exposure with shutter, ISO and screw-in or rear drop-in ND filters, and you accept the signature ring-shaped 'donut' highlights a mirror lens renders out of focus. None of the three passes EXIF, electronic aperture or stabilisation data to the body.

Stabilisation is available on the bodies that have it, but you have to ask for it. Canon's in-body IS works with a fully manual lens, yet a dumb T-X ring reports nothing, so on an IBIS-equipped RF body you open the IS / 'lens info' setting and enter the focal length by hand — 500 mm for the Vivitar, 800 mm for the Samyang — before sensor-shift will engage, and even then it only tames so much shake at those lengths, so a monopod or tripod stays in the kit. Mind which body you are on: the full-frame R5, R5 II, R6, R6 II, R3 and the APS-C R7 carry IBIS, while the R8, R10 and R50 have none and lean entirely on a tripod. Image circle is the easy part — all three lenses cover full-frame, so they shoot clean on a full-frame R5, R6, R8 or Ra, while Canon's APS-C RF bodies (R7, R10, R50) read the centre and apply a 1.6× crop, turning the 500 mm into an 800 mm-equivalent and the 800 mm into a 1280 mm-equivalent. The 32.5-megapixel R7 is the standout reach body here: 1.6× crop plus high resolution makes the mirror lenses a genuine budget birding and lunar kit.

The honest summary: T-mount → Canon RF is the budget-reach, wildlife and amateur-astronomy route onto an EOS R body, not a precision-AF or cine path. Fit a generic mechanical T-Canon-RF ring, focus by hand on the EVF, accept fixed f/8 and donut bokeh on the Vivitar 500 and Samyang 800 mirror lenses, reach for the Soligor 400's preset ring when you want a conventional rendering, and register the focal length in the IS menu so IBIS helps at the long end on an R5/R6/R7. And because T-mount is the de-facto standard for astronomy optics, the same T-Canon-RF ring threads straight onto a telescope's focuser for prime-focus lunar, solar and deep-sky work — which is exactly where Canon's H-alpha-sensitive EOS Ra heritage pays off, so one inexpensive ring serves both your mirror lenses and your scope on the Canon body you already own.