Ultra Wide and Fisheye Converters: Image Quality
Front-of-lens adapters tend to get a bad press for image quality, mainly because most are budget add-ons that are often used on already-inexpensive lenses. But with this pair, Sony has been able to match the optics specifically to the prime lens, giving combinations that work very well.
Looking first at the Ultra Wide Converter, this is noticeably a bit soft at f/2.8, but improves quickly on stopping down. At f/5.6, only the extreme corners of the frame are soft, and I’d be happy using it for highly detailed landscapes at apertures of f/8 through to f/16. The Zeiss FE 16-35mm f/4 would surely do better, but I doubt that you’d see much difference in an A3 print.
The story is broadly similar with the Fisheye Converter, although with its more extreme optics, smaller apertures have to be used to get the best results. Edges and corners are rather soft and blurry wide open, and the lens has to be stopped down to f/8 at least (and preferably f/11 or f/16) for them to really sharpen up. But at this point, images are impressively crisp across the frame.