Well, up until 2,8 the backfocus is pretty tolerable. In the 2,8-5,0 range it is the worst, making these apertures almost unusable. That is very sad.
The only "good" point is that I was right after all when I started this topic despite the claims of some reputable forum members ;-)
The Canon EF 35 2.0 does not suffer from this issue and is still pretty light with similar IQ + it is not STM, but USM. You may try it + converter. I sold my RF and bought EF.
Yeah, I had no trouble with f2.0 - f2.5 at closer distances. Focus accuracy was very good. When I got a bit further out it wasn’t too happy. Funnily enough the lens actually reminds me of my old EF 50mm 1.4 in terms of focusing motor behaviour.
At a slightly further distance the RF 35mm f1.8 autofocus tends to jitter back and forth a bit while locking focus. With my L lenses once it has locked focus in single shot AF mode it just stays there. Even if I half press the shutter again, the L lens will only ever make the slightest of movement. I would assume this is because the initial acquisition of focus is more confident /accurate. With the RF 35mm 1.8 there are constant little movements all the way up until the focus locks. If I half press the shutter again without the subject moving it will start jittering back and forth a bit before locking focus. I would assume all of the missed focus were back focused because the lens tends to move in that direction when stopped down a bit anyway. So if the the focus is slightly off, the problem is made worse by using a slightly stopped down aputure?
Did you ever experience more focus motor movement like I described when trying to aquire focus from a slightly further distance?
EDIT: some further details after reviewing a batch of ‘problem images’ taken at a full length body distance at f2.0 and f2.2. Out of 62 photos 18 were backfocused to the point of being practically unusable. A further 5 were backfocused to the point were some sharpening made them acceptable. The remaining 39 were quite good and usable without the need for sharpening.
I should note that all 62 of these images were partially backlit. No direct sun or anything, just an open window with a partial sheer curtain covering. Contrast on the front of the subject was fairly low and settings ranged from ISO 400, f2.2, 1/200 to ISO 400, f2.0, 1/320. Just to give an idea of the light level. In essentially the same lighting conditions my RF 50mm 1.2L and RF 85mm 1.2L did perfectly fine with focus accuracy.
The frustrating thing with the AF inaccuracy on this lens in this particular situation is that I had 8 in a row that were backfocused to the point of being unusable. This meant I missed 3 different poses/expressions from the model (including one I was hoping to use as a final image).