Reproduced from a reply in the DPR Canon Full-frame forum:
However, the small flange distance on the Sonys creates difficulties for lens designers because it sharpens the angle at which light strikes the sensor, creating loss of edge and corner sharpness and more vignetting from more light fall-off--all because of the small flange distance and the sharp angle of incidence. This puts pressure on Sony's lens designers to raise the exit pupils of their lenses to male the angle of incidence less acute. But this makes Sony's lenses longer and bigger--many on dpreview have noted that the Sony FF lenses often seem longer and bigger than one would like or expect.
This is simply and completely wrong. I've read this meme dozens of times, and it is provably untrue.
Lens designers with the E-mount have the option to move the rear lens element as far away from the sensor as desired. This is indeed how Sony E-mount cameras can mount almost any other type of lens - they simply mount the lens further back away from the sensor via a metal tube (adapter). And that is how Rokinon delivers their E-mount lenses - they take their SLR designs and add an extension tube to the lens body, so they have the correct mount distance on E-mount cameras.
The short flange distance simply adds options to lens designers, it does not take any away. They can place lens elements as close at 18mm from the sensor, or as far away as needed. Any suggestion otherwise is simply incorrect.
