On 25/11/2014 9:57 a.m., Alfred Molon wrote:
I was browsing photozone.de and couldn't find any full-frame rectilinear
12mm lens. The widest was 14mm. Why is that the case?

In the APS-C world there is the Sigma 8-16, which has an equivalent FF
focal length of 12mm. A bit strange that for APS-C there are wider
rectilinear lenses than for FF.

Not really correct as Sigma have made a rectilinear FX 12-24 lens for a
long time, even updated it a few years ago to a new version.
It's tested on Photozone (on Canon 5D II full-frame camera).
Most lenses (even primes) at this extreme focal length suffer from
either/and field curvature, wavy or considerable barrel distortion,
softness on the edges, CA, as well as flare - exacerbated by the large
bulbous front element on fast (ie F2.8) lenses, and the practical
difficulty on shooting outdoors without the sun in the frame.
As a Dx and FX user, IMO FX has a massive advantage over DX at these
focal lengths. The Nikkor 18-35 G is less expensive than the Nikkor DX
12-24, it weighs about the same, covers approx the same field of view,
but is much, much sharper. Nothing on DX format comes close, and for
more $, the 14-24 Nikkor optical performance steps up another level.
The Sigma 8-16 DX lens is extreme and unique (as is the Sigma FX 12-24).
The widest alternatives are Nikkor/Tamron co-designed 10-24, and Sigma
10-20, with widest "equivalent" focal length of 15mm.