I have decided to get one of those as my ultra wide lens to upgrade from my current SEL16F28 with ultra wide adapter which has nowhere near good corners and wide open is pretty horrible.
You can see the resolution test here:
Sony Wide Angle Converter VCL ECU1 : ERPhotoReview
and the 20mm does a lot better:
Canon FDn 20mm f/2.8 on Sony A7 : ERPhotoReview
Unfortunately I cannot find the same test for the 17mm F4 so I am left wondering how it will compare. My SEL16F28 with the ultra wide adapter was giving me focal range of about 18mm (35mm equivalent). Either Canon lens will be used with a Lens Turbo 2 adapter, meaning it will act almost as a FF instead of having a crop factor of 1.5 thus I will end with about 18mm for the 17mm and about 21mm for the 20mm.
So my preference would be the wider lens.
I currently have an FD 24mm and and a 35mm F2.8 (amazingly sharp!), so likely I will sell the 24mm and stick to one super wide lens so MAYBE the 20mm will be wiser choice as to not have such a huge gap in my lenses from 17mm to 35mm or maybe I will keep the 24mm IF I buy the 17mm.
In any case, anyone has seen any resolution tests between the 17mm and the 20mm or have actual real life experience between the two?
The other option will be a more expensive FD 20-35mm L lens but will that be better than either the 17mm and 20mm in IQ? Usually zooms cannot match primes with exceptions like the 80-200mm F4L of course.
Finally maybe a modern lens like the Rokinon 12mm F2 will be a better choice for me.
Any comments will be appreciated.