Tamron's budget Adaptall-2 telephoto prime — the 300mm f/5.6.
The Tamron 300mm f/5.6 in Adaptall-2 mount is a budget telephoto prime with universal mount capability. At 300mm f/5.6, it provides basic telephoto reach at a very slow aperture. The Adaptall-2 enables cross-system use. The f/5.6 severely limits low-light capability. A basic telephoto at minimum specification.
Basic optical quality. The f/5.6 is very slow at 300mm — limiting use to bright conditions. Sharpness acceptable stopped down. The 300mm provides useful telephoto reach. The Adaptall-2 versatility is the main feature. Build quality basic. Not recommended when faster alternatives exist.
Adaptall-2 universal mount. Filter thread varies. Weight varies. Build quality basic. The f/5.6 makes this impractical in anything but bright daylight. Ensure correct adapter is included. Tripod recommended at 300mm without stabilisation.
Very cheap. Very slow f/5.6. Not recommended. The Adaptall-2 versatility is the only feature. Better 300mm options exist in most mounts. Only if essentially free with the correct adapter.