>>3310837It's about 6 inches long.. They are referring to this pair of 300mm f/4 lenses. The old "D" prime compared against the new "PF" prime.
Phase Fresnel and diffractive optics just refers to a certain element in the lens with a funky ridge pattern. If you've ever seen those magnifying sheets (you might have started fires with them when you were a kid), that's a Fresnel lens. You can make a lens out of a literal sheet of optical material, so naturally, when you correct for all the errors of the optics, that is going to result in a smaller overall package.
