Originally posted by boriscleto The most common T4 to M42 adapter is marked PX. These have to be modified because although they have a switch kind of like an A-M switch on a Takumar, it doesn't stick in the M(anual) position. You need that to adjust the aperture with the ring. The modification is easy.
Soligor and Vivitar sold T4 adapters and lenses. The lenses might all be Tokinas, like this 300/5.5.
There's a T4 to M42 adapter with a better switch, marked ES. I never thought it was worth tracking down, but if you stumble across it, it's easier to use.
You can use the TX adapters too, they are only a little different, and there's a TX to K adapter. That would also have to be modified in the same way. The TX to K adapter allows a TX lens to be used like it was a Pentax K or M lens, but when it's modified and used on a T4 lens, the lens works like a typical M42 lens instead.
You can usually find the adapters on eBay, properly identified. Or if you like adventure, a lot of these lenses are sold by people who have no idea about T4, TX, etc. The rectangular bump is easy to pick out in a photo. In a few weeks you can have a modest collection of lenses and adapters.