T-Rings have a bayonet fitting specific to your camera model on one side. On the other side they have a T2 thread standard to most telescope T-adapters.
After choosing your T-Ring, be sure you also have a T-adapter to suit your telescope or spotting scope.
Canon T-rings fit all Canon DSLR cameras in the EOS range. These T-Rings do not fit Canon Mirrorless cameras (see below)
(Canon mirrorless system cameras require an adapter to convert the bayonet fitting to an EOS equivalent – this is usually supplied with or is available from camera outlets)
Nikon T-rings fit all Nikon DSLR cameras
Micro Four Thirds T-rings will suit many Olympus and Panasonic cameras (older cameras may have Four Thirds mount – larger than the new micro four thirds)
Sony Alpha T-rings – older Sony Alpha SLR and DSLR cameras use Minolta mounts. The Swarovski Sony mount will fit these cameras.
New Sony DSLR cameras and mirrorless cameras use the new E-mount.