I have read a number of articles that discuss the focus shift problem with this lens. The only response I have seen from Canon was extremely brief and said that it was a design decision. I must be missing something because I cannot imagine any reason to buy any lens that exhibits focus shift as the lens is stopped down. The only work around that I have seen is to take a photo and, if it is out of focus, then manually focus in front of the desired focus point and use focus bracketing to take a series of photos and pick the one that has the desired point in focus. Why would I (or anyone) buy this lens instead of keeping their EF-100mm macro and sticking an adapter on the back. What am I missing?
All of which has nothing to do with the question. I have never read a review or article about the original EF 100MM f2.8 macro lens or the EF 100mm f2.8L macro lens that mentioned focus shift. However, there are numerous articles about the RF 100mm f2.8L macros lens that describe the focus shift problem with sample images. To me this suggests the focus shift of the RF 100mm f2.8L macro lens is severe enough to be very obvious and, therefore, a problem that no one would want to deal with. I certainly would not. I cannot understand why Canon would think anyone would want this lens given the reviews I have read. Clearly Canon engineers could have designed a lens without obvous focus shift since they have done so in the past. Please understand that I am not trying to troll the forum. I am a serious photographer looking for a factual answer.
I'm not following. Are you saying you are acquiring focus at some aperture, then while that focus is locked in place, you are changing the aperture before you take the shot?
Or are you speaking to the lens back or forwards-focusing?
If the latter, does your camera allow for AF micro adjustments?
I am shooting a R5. I do not own the RF 100mm f2.8L macro but considered buying it until I read several articles complaining about focus shift. According to my reading the problem is that the camera focuses with the lens wide open, in this case at f2.8. As always, when you trip the shutter release the lens stops down to the aperature set either by the automatic exposure system or by the user in manual mode. The problem accourding to the articles I have read is that when the the lens stops down the focus point shifts to a point behind the original focus point. This is more common at small f-stops (f8.0 and smaller) and high magnifications (close focusing distances). If you want more information do a search for "Canon RF 100mm macro focus shift" (without the quotes) and you will find serveral articles describing the phenomenon.
See the article at https://www.the-digital-picture.com/Reviews/Canon-RF-100mm-F2.8-L-Macro-IS-USM-Lens.aspx posted by jr_hoffman. It explains the issue better than I did. I am shooting an R5. Mirrorless cameras do not have micro-adjustments because they do not need them since they focus on the sensor.
"All lenses have a focus shift to some degree." "... which has nothing to do with the question."
Ah, but it does. It explains why and what is happening. The original EF version has focus shift. Apparently it isn't as bad as the the new lens but it's there. Lenses that are prone to focus shift problems have more faster maximum apertures. Prime lenses with uncorrected spherical aberration will always have focus shift problems. All fast lens with aberration problems will exhibit some focus shift at different apertures. This is not new nor is it solely an issue with the lens in reference. Most, or I might say, all of the time the focus still lies in the acceptable focus range although perhaps not at the critical focus point.
If the EF 100mm f/2.8L IS USM has focus shift, then I have never noticed it. More times than not, I am manually focusing from a tripod. When I do shoot handheld AF, then I am almost always using a crop sensor camera body, shooting at around f/4.