[davidmanze]

There are two models of the Sigma 10-20mm, the F4-5.6 and the F3.5 constant. There is slightly differing opinion but the majority vote gives the cheaper F4-5.6 the nod saying it's sharper at the edges.....I have that lens and it gives sharp results but not stellar,  needing F8-11 at the 10mm end to have sharp borders (forgivable at that focal length) ..above 12mm it's fine even at larger F stops......low distortion (moustache 10mm)...good build....flare resistance is fine and it's not expensive!... a useful additional ultra wide lens.

 

  I tend to think FF lenses are very bulky on APSc and often don't achieve their full potential.