Sony FE 28mm f/2 with Ultra-Wide adapter: Shading
With the Ultra Wide Converter in place, we see moderately strong shading wide open at f/2.8, with around 1.2 stops fall-off in the corners. This drops to around 0.9 stops at smaller apertures. With shading compensation turned on, the camera will correct for it automatically.
It may look surprising that the ultra-wide adapter gives less vignetting wide open than the 28mm f/2 lens alone, but this is simply because the maximum aperture is restricted to f/2.8. If you compare the graph below to that for the lens at f/2.8, you’ll see they’re near identical. This means that the adapter itself introduces no extra vignetting – an advantage of its large domed front element.
Sony FE 28mm f/2 with Ultra-Wide adapter: Distortion
Here we see the benefit of software-aided lens corrections. The Ultra Wide Converter brings distinctly strong barrel distortion when we examine uncorrected images, but because the camera knows all about it, it is able to correct the JPEG output such that only a little residual distortion is left.