Eye-AF works pretty well with this combination. Focus is slow, but it’s able to find and lock onto an eye. Because of my experience with the X-T3, sometimes my expectations can be a bit skewed; face and eye detect on the 2018 X-T3 is quite good, especially on distant (small) faces as Fujifilm has improved by leaps and bounds in this area. How quickly I forget the GFX 50S is over a year and a half older, with an imaging processor based on the X-Processor 3 design in the X-T2 and not the current X-Processor 4. Combined with other hardware limitations, plus the fact that the latest firmware update was over a year ago, eye AF on the GFX 50S (or 50R, for that matter) cannot be expected to perform anywhere near that of my X-T3.
As such, eye AF struggles with most non-Fujifilm lenses, but as I discovered with the Canon EF 135mm ƒ/2L USM, does work pretty well because the lens allows faces to appear closer (larger) on the imaging plane. Detecting faces at a distance was something even the X-T3 struggled with, resolved only recently with their latest firmware update.
With its wide angle of view however, the Canon EF 28mm ƒ/1.8 USM struggles to lock onto eyes, even at close distances. It can locate them but drops back into face detection only in over 90% of the attempts I’ve made. I’ll post an update on that lens if I’m able to find a combination of settings that happens to improve that performance.
Just make sure you pop the shutter as soon as you get a lock, though. AF-C performance is a little better on the 135/2L compared to other non-Fujifilm lenses but it continues to underwhelm and is mostly useless.
Regarding the Canon EF 85mm ƒ/1.8 USM, I’m on the lookout for an 85mm lens in general. While the Canon is recognized for use up to 50MP and is attractively priced, there’s also the Tamron SP 85mm ƒ/1.8 Di VC. Despite being nearly twice the price of the Canon, the inclusion of optical stabilization is highly attractive. It’s just a question of whether the lens will work to full capacity when adapted to the GFX. I’d like to have an 85mm in my bag by the beginning of next month (March 2020) to close the gap between 45mm and 135mm in my lens collection. The other issue concerns cost; if I go with Tamron, it’s all I’ll be able to afford but Fujifilm is currently selling the GF 50mm ƒ/3.5 for only $500. I could easily get the Canon 85/1.8 and the GF 50/3.5 at the same time as their combined cost is $800, but not the Tamron and Fujifilm lenses as the combined cost exceeds $1250. So, why the Fujifilm lens when I already have the GF 45mm ƒ/2.8? The simple answer is value: I can get it for half price and resell it for more than I paid once the sale ends after March. From there, the profits can be applied toward the GF 63mm ƒ/2.8 or 45-100mm ƒ/4, when those lenses inevitably go on sale, focal lengths I truly need to help fill the gap.