Next in my series of "why is the world like that" questions concerns the screwmount Takumar lenses. By the time the lenses were at the Super / Super-MC stage, I am trying to figure out why Pentax would produce a pair of lenses like the f1.4 50mm and the f1.8 55mm. In terms of aperture, the difference is negligible, and likewise in terms of focal length. I wouldn't see a reason for someone to own both. I am sure most people chose either of these. Ditto for the 35mm Takumar: they exist in a f3.5 and an f2 version. But why did Pentax produce such apparently nearly identical lenses, and above that sell them at different prices? The one thing I know is that the f1.4 50mm and the f2 35mm had thorium which cause them (like my 50mm f1.4) to yellow slightly after age.