They are the same lens. They have a aperture limiter that opens up to f2. They were made to be sold with the cheaper SP500 and instead of establishing a new production line to produce a budget lens, they added a limiter to the f/1.8.
Yes I believe that's the case too. It was a bit like what they did with one of their early pre-Spotmatic cameras - they "deleted" the 1/1000th of a second shutter speed leaving the top speed as 1/500th then sold that camera as a cheaper alternative for the lower end of the market. But they did this by the simple expedient of swapping out the shutter speed dial with 1/500th as the apparent top speed. But for those in the know, you could in any event, move the shutter setting one more click to an (unmarked) position of 1/1000th and use that speed as the internals were still intact (It was much cheaper to do it this way than to re-engineer the guts of the camera). 🙂
I can't remember it this lens was Pentax's copy of the Biotar or the Sonnar. One or the other.
maybe my copy have some problem, I found it less sharp than the 1,8 version....
There was an early Sonnar but not the one I am referring to. I am pretty sure that was manual stop down whereas mine is an auto Takumar and has a different design. I have a feeling I read somewhere that it was Biotar though but I have not done enough research to find out to my satisfaction. I have read posts with people stating its identical to the f2 lens but I still am unsure.
