Hi Alex
XF was only ever made in a 8.5 and 12 fl. If memory serves me correctly the earlier incarnation of the Pentax widefields (ie the XL's) was 21 mm and the current XW range only features a 20mm. However, obviously, double check this with Lewis.
Whatever the case you simply can't go wrong with any of the Pentax EPs - grossly under-rated - which is perhaps a good thing for some of us as it makes buying them slightly easier
