|External diameter||42 mm|
The 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 42 mm, but has a 1 mm thread pitch.)
The "T" is said to stand for Tamron, a Japanese manufacturer that released in 1957 the first of a line of aftermarket camera lenses that fit 35 mm SLR cameras built by various manufacturers using their universal T-mount. On the first model, the T-mount used a M37x0.75 thread; Tamron's M42x0.75 T-thread didn't appear on the market until about 1962. The company referred to it variously as a T-mount, T-thread, T-adapter, or a T-400, but not as a T-2. The proprietary lens mount of each camera manufacturer was adapted to the T-mount thread with a simple adapter. Thus 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.
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, such that the lens barrel is not deformed by the set screws and can therefore be removed with no damage.
As a common mechanical interface, the T-mount allows components of various manufacturers to be interchanged and assembled. The T-mount is a solely mechanical specification. Electrical or mechanical connections (such as for autofocus) are generally not provided, although Sigma's YS Mount featured an automatic diaphragm coupling.
Besides cameras, T-mounts are used in optical breadboard prototyping components as well as telescope and microscope attachments. The T-mount is also the standard way to mount a camera to a microscope to photograph pathological specimen slides or to a 1.25" telescope eyepiece.
Because the T-mount has a long flange distance of 55 mm and most 35 mm 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 a 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.
Standard T2-rings have the same external diameter of the inner ring because some T-mount lenses have their inner ring built on the flange, thus the external ring of the T2-ring can be directly attached.
The thickness of the external ring on an adapter is equal to the difference of the flange distance between the T-mount and the corresponding lens mount in order to achieve infinity focus. Other large format camera lenses have a flange distance longer than 55 mm. They can be attached to smaller format cameras with a combination of corresponding lens-to-T-mount-adapter and a T2-Ring.
See also 
- Making Digital Camera Microscope Adapters. Various examples and machining details of custom-made T-mount adapters.