What are the photos like? I've always preferred using the
sweet spot advantage of FF lenses on APS-C cameras.
Focal length reducers tend to negate any of the desirable
optical qualities of the lens.
First of all, use of a focal reducer is not my go-to mode.
Given that the AF and OIS don't work, and given other
options in my hoard of gear, the FL reducer comes into
use mainly when a peculiar advantage reigns supreme
for certain circumstances.
Since the "most supreme" advantage, IMNSHO, is the
increased optical speed, and since the increase only
matters when at wide open, my application is for rather
dim light where gaining an f/stop matters; and since it's
also a job for high ISO, there's no visible loss of optical
performance. Also, given the lack of DoF at wide open,
there's no issue of corner sharpness. It's very unlikely
that the focus plane intersects anything other than the
main subject anywho.
So much for "sweet spot" miracles. If I use a 50/1.4 at
wide open without a FL reducer, there is still no sweet
spot advantage, cuz for me working at wide open goes
hand in hand with dim conditions and high ISO.
FWIW the lack of useful AF is a nonissue since in such
circumstances I can't trust AF anywho. Even when it's
working very well ... fully as well as it was engineered
to work ... I can't trust AF's discernment for placing the
focus plane exactly where I want it when the dearth of
DoF demands extra careful placement.
Yet I can state that the optical qualities of the adapter
shown above are very good. Given that these things
cost a few bucks, I do check out the IQ in a practical
manner. If the corners look decent at one-stop-down
thaz very good. I check for curvature of field and if it
holds very near to center focus across all, or at least
nearly all, of the frame, thaz very good. I also check
for reflections and veiling, and for vignetting. Oddly,
the particular adapter above seems to actually "cure"
vignetting. The corner illumination is noticeably better
than the lens alone [lens alone checked on FF]. I'm
aware that sounds dubious, but thaz my observation.