I suppose the question here is why bother? Well for me, it's because Sony have nothing similar until the 28mm f/2 plus conversion lens becomes available and the Sigma f/1.8 a-mount lens I had for a while left a lot to be desired sharpness wise. Plus the possibilities of a fast wide-angle on the high resolution A7r and the low light high ISO capabilities of the lens on the A7r are tempting to say the least. With the FE cameras focus peaking, manual focusing isn't really a problem and for me this is a VERY useful lens for my FE cameras.
There are some raw file samples for download below.
NIKON 20mm f/1.8G Lens on Nikon Df. Raw File Samples For Download
PLEASE NOTE - ALL IMAGES MADE AVAILABLE FOR DOWNLOAD IN THIS POST ARE ©SOUNDIMAGEPLUS AND MUST NOT, UNDER ANY CIRCUMSTANCES WHATSOEVER, BE SHARED OR USED FOR ANY PURPOSE OTHER THAN TO VIEW AND ASSESS PRIVATELY AND INDIVIDUALLY.
First seven of the raw files above are taken with my A7s handheld, last three with my A7r, tripod mounted at f/1.8 and (approximately based on the shutter speed) f/5.6 and f/11.
The raw files are for you to see for yourself what the image quality is like. I'm perfectly happy with what I got and today I'm going to shoot a three way comparison between this lens, my Voigtlander 20mm and the 16-35mm zoom I've just bought. Should be interesting.