How effective is R6's IBIS with EF Lenses?

ArtHeals wrote:

I use a couple of Canon EF portrait lenses (135/2L & 85/1.8 USM) with EOS R and I don't have any major problems as i keep the shutter speed around 1/200 most of the time. But I was wondering how effective is the IBIS on R6 with these EF lenses. Is it still 5-axis with EF lenses or downgrades to 3-axis?

Careful what you wish for.  The extra axes that the R5 and R6 give with IBIS are something that some of us wish that we could turn off, as the implementation seems to be buggy.

Typical lens IS is 2-axis, Pitch and Yaw, the motions that one could easily and freely make with a camera mounted on an otherwise rock-solid tripod with a loose head.  These are the core part of any stabilization system.

IBIS adds X/Y translation correction, which is for motions like what you'd get if you pressed the front of a lens flat up against a window, and moved it left and right and up and down (but always horizontal),  In my experience with the R5, this correction, when added to an EF IS lens, actually ruins stability to a very small degree, causing waves of slight added instability over a burst that I would not have with the same EF IS lens and the same shutter speed with my previous DSLRs which had smaller pixels, which should have made any instability easier to detect.  I don't need X/Y translation most of the time, as I never needed it with my DSLRs, shooting distant subjects.  X/Y translation affects mainly close-focus shots, and I wish, therefore, that I could only enable it when I need it, for close focus.  I suspect that for EF lenses, Canon may have dumbed down the precision of added IBIS, causing errors of added small twitches that amount to an added blur of up to a half pixel in size or a little more, which is great if it would have been 5 pixels without it, but a loss, if it would have been only 0.2 pixels without it.

"Roll" correction is also added by IBIS.  It corrects the roll or twist of the camera on the lens axis, a circular path that has nothing to do with focal length.  The need for this correction depends only on the length of exposure, so it may be very useful if you want to shoot a 20mm lens at 1/4s, but not as needed for 400mm at 1/80.