The new RF 100mm f/2.8 L Macro IS USM lens from Canon does what it really needed to do: provided a faster-focusing, excellent macro that is sharp across the frame. It also added a couple significant new features: it now magnifies to 1.4x the actual size of the subject, rather than going to just 1.0x.
It also provides a gimmicky aberration control ring that in most cases will make your images look worse, but in some cases may make your bokeh more “special” in some way that appears to appeal to some Japanese lens engineers. The effect is much like looking at an image through the glass of a shower door. The good news: you can leave the dial alone.
Bryan Carnathan, over at The Digital Picture did one of his exhaustive reviews. The sharpness upshot: the new RF lens is just as sharp as the very impressive EF version, but the RF lens extends that level of sharpness across the frame, showing significantly better corner sharpness.
Unfortunately, he found that it commits focus shift problems if you’ve set the aperture for narrower than f/2.8 and wider than f/11. This focus shift is particularly hard to compensate against, as the newer R-series cameras do not autofocus while you press the depth-of-focus preview button. This means you can’t both check out the depth of field, and have the camera leave the focus at the desired spot in that view.
The lens isn’t free of color aberration, and this is somewhat corrected at narrower apertures. Focus breathing is quite a bit worse than the older EF mount lens, and this could take a toll on all those macro photographers who like to focus stack.