So, my experiment with Canon lenses didn’t go so well. Preliminary tests were acceptable but didn’t perform well at all during practical use. While the 135mm ƒ/2L was ok, the actual output was less than stellar due to significant vignetting. While the 135mm was borderline, the 28mm ƒ/1.8 was atrocious. Both seemed to be affected by the baffles in place over the rear element. That baffle seems to be designed to prevent reflections from stray light within the mirrorbox but were obviously cut to fit a 3:2 sensor. That means they interfere with a taller sensor, like the 4:3 ratio of a 645 medium format sensor.
I decided to send the lenses back and go with Sigma. Their reputation for compromising size and weight for image quality, plus their use of a slightly larger imaging circle to ensure a larger sweet spot hitting the sensor gave me more confidence in their ability to fill a 645 sensor completely. To be sure, I tested out all of the desired focal lengths at Glazer’s Camera to ensure any vignetting is acceptable to me. From there, I chose the 35/1.4, 85/1.4 and 135/1.8, with Fujifilm GF 23/4, 45/2.8 and 63/2.8 filling in the holes affordably. To make this happen as quickly as possible, I made the difficult decision to trade in a few XF primes I hardly use, keeping a range of zooms only for the X-T3. But I digress…
The Sigma 85mm seems to cover all of the imaging sensor with no hard vignetting. Fitment of a Wine Country Cameras 100mm filter holder does cause some very light vignetting at the extremes but is easily correctable. Image quality at the extreme corners and edges is a bit compromised at wide open but shapes up when stopped down to ƒ/4.
Latest firmware installed on lens.
Autofocus works quickly throughout range.
Continuous AF is nervous and fails to lock; completely useless.
Eye AF does work but is less confident than a GF lens, often dropping back into face detect.
Only you can decide if an adapted lens will work for you but in the case of the Sigma 85mm ƒ/1.4 DG Art for Canon EF, this is about as good as it gets for image quality and sensor coverage. Autofocus isn’t perfect, far from it, but for landscapes and studio portraits, it should work well, especially in situations where you have the time to manually focus or punch in for critical focus.