AF-area using 600mm/f11 and 800mm/f11 with EOS R6 II surprises a bit...

koenkooi wrote:

Stig Nygaard wrote:

I just thought some would fine this interesting.

The AF area using RF600/11 and RF800/11 on EOS R3 are larger than when used on the R6 or R5. Since they are all fullframe cameras, there are no crop-factor to explain that. My theory was it had something to do with R3's sensor being stacked. But apparently there are other factors involved. It turns out the R6II offer same 80%x80% AF area as the R3 when used with those lenses...

HxW:

R5/R6: 40% x 60%
R7/R10: 60% x 80%
R3: 80% x 80%
R6II: 80% x 80%

https://cam.start.canon/en/H001/supplement_0110.html

My guess would be that the angle of the microlenses plays a part in it. When Canon reused 'old' sensors in the R and RP, they didn't have 100% coverage. More recent sensors expanded that to nearly 100%. Again, this is just a guess: I think the microlenses near the edge are angled in to deal with wide angle lenses with a short backfocus distance. On EF the lightrays would be a lot closer to perpendicular due to the large flange distance. On RF the lenses can be a lot closer to the sensor.

The angle would make the pixels below them slightly less sensitive, which becomes an issue with f/11 lenses. And Canon has tradionally erred on the "It really needs to work most of the time" side of features, so we get a really small AF box.

Not sure that explains why the EF 100-400 II with 2X can still use the entire area for AF though.

I do find that the centre box for the 800mm f11 seems more reliable than having the entire focus region of my RF 100-400mm in low light, even though the latter lens focuses much faster it seems to get confused easily in this situation (eg a bird with branches/leaves in foreground).