Like Vivitar, Spiratone does not make its own lenses. But unlike Vivitar, Spiratone didn't design any lenses either. They were a company that slapped on their own label from OEM lens makers like Sigma, Tokina, Kiron, etc.

For the Spiratone 105mm f2.5 T-mount lens, I am not sure who makes it for them. It's a very compact lens. Single coating and horrible at flare/reflection control. Shooting backlit is asking for trouble. A hood might help, but I don't have one for it. Lens is of pre-set type aperture, and has a 49mm filter thread. Wide open at f2.5, it's not very sharp, but I guess as a portraiture lens, it's usable. It's quite respectable whens stopped down to f4 or smaller.

In short, this is not an exotic lens, but does create memorable pictures if you spend sometime to know it. Not as nice as the Takumar SMC 105mm f2.8, but not bad either, especially if you can get one under $10.