"It seems like Pentax (and maybe Olympus?) are the only ones to make pancake lenses. Is there any reason why the other manufacturers don't as well?" I just want to put in a good word for Cosina here. The Cosina/Voigtlander 40/2 Ultron SL II certainly qualifies as a pancake lens and is available in both Nikon and Pentax mount chipped to meter and have aperture controlled from the camera. In my experience it performs better than my Pentax DA 40 and my Nikon 45 2.8P and is a stop faster than both lenses. The newly announced to be delivered Cosina Voigtlander 20/3.5 Color Skopar is also a pancake and a lot of us are eagerly waiting to see how it will perform