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The Samyang is a fisheye lens so the extreme amount of barrel distortion is actually a feature and not a bug. Unlike its in-house cousin it has an equisolid or equal area projection just like most other fisheye lenses (although the projection type is somewhat disputed).
It's basically impossible to provide really valid vignetting data for a fisheye lens but as the fisheye effect overshadows any amount of vignetting this is almost meaningless to viewers anyway.
Testing a fisheye lens is a bit of an adventure rather than factory-style measurements. It's fairly easy to get meaningful values at the image center but the borders are already far out there so we had to use a quite non-standard setup here and it was really impossible to measure the extreme corners in a meaningful approach.
Anyway, the center resolution is excellent straight from f/3.5 and even the borders are very good. Stopping down reduces the quality due to diffraction effects but it is still on a very good level up to f/11. Based on our field image we'd say that the extreme corners are also good.
Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Chromatic Aberrations (CAs)
Fisheye lenses are extreme lenses and their edge projection characteristic isn't exactly an easy task for today's image sensors.
Consequently there's quite an amount of lateral CAs (color shadows at the image borders) at wide open aperture of around 1.2px (width) at the image borders and some more in the extreme corners. However, CAs drop below 1px from f/5.6 onwards and this is hardly something to worry about compared to other fisheye lenses that we've tested so far.