The "T" is said to stand for Tamron, a Japanese manufacturer, who in 1957 made a line of aftermarket camera lenses all using their T2 T-mount as a universal fit to
35mm SLR cameras of various manufacturers. The proprietary lens mount of each competitive manufacturer was adapted to the T-mount thread with a simple adapter.
With an adapter a retailer could stock a small number of expensive lenses that would fit a large number of camera brands using a selection of inexpensive adapters.
T-Mount Adapter Ring for Canon EOS Cameras
T-mount is a standard lens mount for cameras and other optical assemblies. The T2 version is a screw mount using a male M42x0.75 metric thread on the lens with a flange focal distance of 55 mm, and a mating female thread on a camera adapter or other optical component. This thread form is referred to as T-thread. (This should not be confused with the
M42 lens mount which is also 42mm, but has a 1mm thread pitch.)
The original T Mount slides onto a T mount lens. It is secured only by 3 set screws that match a channel on the lens mount so that the lens barrel is not deformed by the set screws, and therefore
it can be removed without problems.
The T-mount connection is purely a mechanical specification. Electrical or mechanical connections (such as for
autofocus) are generally not provided, however Sigma's YS Mount featured an automatic diaphragm coupling.
Besides cameras, applications include optical breadboard prototyping components, and telescope and microscope attachments. As a common mechanical interface, T-mount allows components of various manufacturers to be interchanged and assembled.
The T-mount is the standard way to mount a camera to a microscope to photograph pathological specimen slides.
Because the T-mount has a long flange distance of 55mm and most
35mm cameras have shorter flange distances, a simple mechanical adapter called T/T2-Ring is needed to adapt a T-mount lens to any camera body without optical correction to achieve infinity focus.
An adapter ring has a female M42x0.75 metric thread on one side and a corresponding lens mount of the camera body. The differences between T-Ring and T2-Ring is that a T2-Ring consists of an inner ring with T-mount thread and an external ring with corresponding lens mount. Both rings are held together by 3 peripheral screws on the external ring. Slackening these screws allows rotation between the lens and the camera body. This is especially useful when the camera body is attached to telescopes or microscopes.
The thickness of the external ring is equal to the difference of flange distance between T-mount and corresponding lens mount in order to achieve infinity
Standard T2-Rings have the same external diameter of the inner ring. The reason is that some T-mount lenses have their inner ring built on the flange. Thus the external ring of T2-Ring can be directly attached.
Other large format camera lenses have flange distance longer than 55mm. They can be attached to smaller format cameras with a combination of corresponding lens-to-T-mount-adapter and a T2-Ring.