It converts a camera bayonet into the T2 thread that almost every astronomy accessory uses, so a camera body can sit where an eyepiece would. You need one that matches your exact bayonet, and mirrorless and reflex mounts of the same brand are different fittings.
The T-ring is the smallest and least interesting part of an imaging train, and it is the one that sets the reference for everything else.
What it converts
A camera bayonet on one side, the T2 thread on the other. That thread is a 42 mm diameter with a 0.75 mm pitch, and nearly every astronomy accessory behind the focuser carries it.
Buying for the brand instead of the bayonet
Buying for the brand rather than for the mount. A maker’s mirrorless bodies and its reflex bodies take different bayonets with the same brand name on the listing, and a ring for one will not fit the other.
The second mistake
Assuming the old M42 screw mount from vintage lenses is the same thing. The diameter matches and the pitch does not — that one is a 1 mm pitch — so the two will start on each other and then bind.
Why an inexpensive ring is a poor economy
Because the flange distance it establishes is the reference for the whole train. A ring a millimetre out of specification produces corner aberrations that look optical and are arithmetic.
The questions that keep arriving
Does the ring affect image quality?
Only through the distance it establishes. A ring that is out of specification shifts every calculation downstream — the spacing of a reducer, the reach of the focuser, the flatness at the corners.
What else do I need besides the ring?
A way to reach the focuser: either a nosepiece that slides into it like an eyepiece, or a thread that screws directly onto the back of the telescope. Which one depends on the telescope.
Last reviewed 17 September 2026