Here are my four Pentax 15mm lenses - the SMC Takumar 15mm f3.5, K 15mm f3.5, A 15mm f3.5, and DA 15mm f4 Limited. They span over 30 years of time; the first being introduced in 1975 and the last in 2009. This is a side view; as you can see, the first three are essentially the same in size (and optical design), while that last is tiny by comparison. Interestingly, the first one (SMC Tak) is the sharpest of the three large lenses. The modern DA 15 design has the highest contrast, best sharpnes across the full image, and the lowest distortion, but it is only a little sharper in the center than the SMC Tak!