[Guest]

Sure about the focus or the resolution?  If it's the latter I find the corners are really not there at F5.6, maybe the field of curvature?  most sites refocus so FOC is not  always representative of real life shots.

 

Resolution, I was mainly thinking of center, which peaks already at f5.6.  At the very corner, it's not measured in the reviews, as also here on photozone. As to field curvature, you can have a look here:

http://michel.thoby.free.fr/SAMYANG/Earl...port.html,

it presents a focus map

http://michel.thoby.free.fr/Fisheyes_Foc...us_Web.swf

you can click on the animation to stop it select different distances.  (Edit, oops, did't realize that this is embedded, only wanted to give the link. Anyway, the link is contained in the first link (the report page)).

 

I suspect most fish-eyes (except perhaps the new canon zoom) will not be that great in the very corners. I found the Samyang to have CA, which can be corrected in postprocessing.