[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.

[dpvideo]

I will give you a choice. You can pick Samyang Fisheye for an incredibly high performance and a totally affordable price. Or, You can go ahead and spend 3 times as much of your precious money on Sigma or at least 4 times as much on Canon. For the quality of its optics and build quality, Samyang is the best bargain. I mean, after all, it's your choice, but I would say go with Samyang. Also, Samyang is the mother brand for Rokinon, so you might as well go with Samyang. If you are still struggle, you can always find what amazing things they said about Samyang here on Photozone or on any other forum. Samyang's got its own facebook page, and they are also holding this big photo/video contest. If you choose to go for Samyang, you might want to look into it.
https://www.facebook.com/SAMYANGOPTICS
 
As far as I know, you can get the best deal for Samyang at B&H or Adorama.