Originally posted by JanG Well, these are three 50 mm lenses of quite different nature. Any relation to why a Q-mount macro should not be faster than f/2?
I don't there there can be any reason except cost. But even so if anything it should be cheaper to make than a 50mm F2 for APS-C is. Making the same focal length/F-number for a smaller sensor can only be easier and cheaper than making same for a large sensor as it is optically identical except that it can get away with being sharp over a much smaller image circle. Same for vignetting and CA.
Incidentally I do have a 50mm F1.4 c-mount lens which I use with my Q10. I don't see any reason why Pentax couldn't make a similar one but with better IQ and add the mechanisms required for shutter, diaphragm, focus and so on.
(Here it is shown with 12mm extension tubes for macro.)