A T-MOUNT is made up of an inner ring with an internal M42x0.75 thread, and an external ring which is the camera fitting. The two are held together by 3 peripheral grub screws. Slackening these screws allows orientation of the T accessory relative to the camera. T MOUNTS fit to the T thread on the back of the accessory just like a screw bottle top. Each T Mount is a different thickness to accommodate the difference in film plane/lens flange register lengths of the various camera systems. In this way accessories designed for infinity focus or parfocality will focus correctly.
They are commonly used to fit film and digital SLR cameras to microscope and telescope adaptors, slide copying units, mirror lenses etc.
Please note the T thread is M42x0.75, not the Pentax/Praktica/Zenith M42x1 thread.
