There's been some discussion on the merits of cheaper options for ultra wide angle lenses. I've the Samyang 12 mm f2 fully manual lens for about three years now using it both on Sony APS-C (A6000) and Sony full frame (A7 and A7R11).
I've found it quite a useful lens probably best matched to crop sensor, but found it quite ok on full frame too given the width and extra latitude for cropping the image.

Asking for un-cropped samples caught me out a bit as I only keep/store edited images so I had to duck out to the back yard and get a full frame sample to show.

The shot of the garden is full frame (Sony A7R11) with the Sam. 12 mm

The fireworks is with APS-C (Sony A6000) with the Samyang 12 mm.

Garden r.jpg FW r.jpg