The T-mount is a standard lens mount for cameras and other optical assemblies.
The usual T-mount is a screw mount using a male 42×0.75 metric thread on the lens
with a flange focal distance of 55 mm and a mating female 42mm thread on a
camera adapter or other optical component. This thread form is referred
to as T-thread. The "T" is said to stand for Tamron or
Taisei, 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 mini
T-mount used a M37×0.75 thread; Tamron’s canonical M42×0.75 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, which is simply
the name that Soligor used for its version of the T-adapter. 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. Some T-mount lenses accept a T-adapter
that lacks its threaded inner ring, and simply slides onto the T-mount lens. It
is secured only by 3 set screws that match a channel on the lens barrel, such
that the barrel is not deformed by the set screws and the adapter 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 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 a standard way to mount a camera to a microscope to photograph pathological
specimen slides or to a 1.25″ telescope eyepiece.
Adapters Because the T-mount lenses have a long
flange distance of 55 mm and most 35 mm cameras have shorter flange distances, a
simple mechanical adapter called T-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 M42×0.75 metric thread on one side and a
corresponding lens mount of the camera body on the other side. A T-adapter
consists of an inner ring with T-mount thread and an external ring with
corresponding lens mount. The T-ring and the T-2 ring are identical to each
other. Both of them have the inner and outer ring. The two rings are held
together by 3 peripheral screws on the external ring. Loosening 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 T-rings usually have the same external diameter of their inner ring
because some T-mount lenses have their inner ring built onto the flange, thus
the external ring of the T-adapter can be directly attached, as described
above. Since some DSLRs won't meter without
electronic lens data, some adapters are now sold fitted with a Dandelion chip
which can be programmed to identify the focal length and aperture limits and
enable metering and in some cases focal confirmation.
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 T-Ring.
See also Tamron
M42 lens mount C mount
K mount ISO metric screw thread
References External links
Making Digital Camera Microscope Adapters. Various examples and machining
details of custom-made T-mount adapters.