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.
--
Alfred Molon

Olympus E-series DSLRs and micro 4/3 forum at
http://tech.groups.yahoo.com/group/MyOlympus/
http://myolympus.org/ photo sharing site