T and Pentax adapters
A T adapter is a ring for connecting lenses or accessories with a male screw mount to a camera body with a different lens attachment. A T adapter has a female M42 x 0.75 screw for the lens or accessory, and a proper connection for the camera (a male Nikon F bayonet in the case of Nikon SLR and DSLR bodies). This allows the use of lenses and accessories with a T male screw mount on Nikon bodies. The T2 adapter is the successor of the T adapter. It uses the same thread, but it consists of a ring with three grub screws that let you rotate the threaded portion with respect to the rest of the mount. At present, virtually all T-thread adapters are of the T2 type.
A T2 adapter is a very simple accessory. Usually, it consists of two concentric metal sleeves that can be rotated and locked with set screws within each other (to allow the scales and controls of the lens to be placed in an appropriate orientation). Thus, T2 adapters are relatively cheap. Their popularity is due to the fact that manufacturers of telescope, microscope and accessory equipment can provide a single adapter for connecting their equipment to a female M42 x 0.75 screw, instead of multiple types of adapters for various camera brands. A few decades ago, several film SLR brands, foremost among them Pentax, used a similar (but not mutually compatible) screw (M42 x 1.0) on their camera bodies (hence the alternative name Pentax screw for this type of attachment).
These days, T2 adapters for connecting to a camera body are usually manufactured by third-party companies. Consequently, their quality is much variable. I have not been able to find one with a good quality Nikon bayonet made of nickel-plated brass, and I was forced to choose among types with an aluminium bayonet.
The above picture shows two adapters with Nikon bayonets. The one at the right (T2) does look peculiar - this is the result of the adapter falling from a work table onto a wood floor. The modest impact was enough to shear the very thin aluminium (because of a thread cut into the inner side, less than 0.2 mm) holding the bayonet flanges attached to the adapter. I was extremely lucky that I had not used this ring yet. The bayonet would have broken off under the weight of a camera, and the camera would have fallen off.
This particular model of adapter is an example of shoddy manufacturing. Its surface feels rough and the black anodizing is easily scratched. The index marking on the barrel for mounting the adapter to a Nikon body looks like a hand-made scratch. I discuss here a different adapter ring with comparable manufacturing faults. Good riddance.
The adapter on the left (with Pentax thread) is considerably thicker where it counts, machined to good tolerances, and its surfaces feel smooth and well anodized. It is also slightly thinner front-to-back, which is an advantage. The prices of the two adapters were roughly the same.
My principal use for a Pentax adapter is to connect a Nikon DSLR to a microscope. The above picture shows the surviving adapter mounted on an old Pentax microscope adapter that works perfectly well with current DSLRs. This type of microscope adapter contains no optics and mounts onto the end of a standard microscope photo tube, with or without a projection ocular.