A T-Mount (or T2 mount) is an adaptor that enables an accessory such as a microscope/telescope adaptor, slide copier, or mirror lens to be fitted to a wide range of 35mm and Digital SLR camera bodies.
They also have a variety of other applications
T-Mount Inserts
Fittings for T-Mounts to turn them into macro mounts
T2 Mounts
Fit A T-Mount accessory such as a microscope adaptor to a film or digital SLR body
T Mount Extension Tubes
Use with T-Mount microscope adaptors to increase magnification. Use as a macro set up with an enlarging lens.
