For us laypeople, what do you mean by a retrofocus design, and why do you say a 20 to 30mm lens would have to be that? Sorry to show my ignorance...
Think about the simplest possible lens, with just a single element (eg a magnifying glass). Its focal length is the distance between the centre of the lens and the plane where it focuses light from a distant object. So a single element 20mm lens for cameras would have to be located 20mm from the film/sensor. This is impossible in an SLR camera, because this is the region where the mirror lives.
To resolve the problem, lens designers use a more complex configuration of concave and convex elements (etc) to allow the lens to be moved forward out of the mirror region while maintaining a short focal length. Such lenses are known as "retrofocus" designs.
The downside of a retrofocus design is that the size, weight and complexity of the lens increase. Lenses tend to get smaller with decreasing focal length down to around 50mm, and thereafter remain around that size or start to get larger again due to the increasing complexity required in the optical design. That's why I wait with interest to see what Pentax come up with in the way of a 20-30mm pancake.
Personally I'd be quite happy with a 20mm/2.8 about the same size as the M20/4. I'd like to think that lens design techniques have improved sufficiently in the last 20 years to give the extra stop of light and reduce the distortion a little. :-)
S
