I've used my EF 100-400 with an RF adapter. It's good but as someone older, this RF lens is light, compact, and is optically sound for my purposes. I also own a 150-600 Sigma which usually needs a pod to hold steady and I've missed more shots than I've captured with it. This RF 100-400 is great for me; it so easy to pan while shooting birds in flight!

I think the picture quality is some times soft with using and extender, but not to the point it is overly noticeable, or makes picture unacceptable. Doubles your lens and is compatible with EF mounts. In the photos attached the first two were taken with a 5D mark III 100 - 400 zoom lens with 2x adapter for 800 MM the third was with a 5D S R 100 - 400 mm zoom lens at 400 mm no 2x adaptor

I was looking for a way to extend my reach beyond my 100-400L lens. I have an extender but I loose light using it. Thought of getting a Sigma 100-600 sport = kind of expensive and picture I didn't think it would be as sharp as my canon lenses, so I got this with adapter ( Vello brand) and slapped on my 100-400L canon lens. Well it's pretty darn sharp. Here is my insight. Wow 14fps = something will be in focus. It tracks very well. Maybe not as good as my 5D mark iv. Not all of the 14fps are in focus. Using it with a long lens works just fine and is not awkward. It's sort of like using the lens with no camera. You can not move the focus point without touching the screen. I can live with that because the weight of holding it up is on the left hand with the lens weight in it leaving your right hand available to touch the screen. With the 3 dials it is easy to change F-stops, Speed, and ISO along with the Q touch screen. It does struggle with low light, so mornings and fast birds in flight may be a challenging. I find iso 1200 about the limit without loose sharpness if you crop. For non cropped city or landscape high is fine. So far all of my 5 EF L lenses work just fine and my EF-s 60mm. So far they all work with focus bracketing. NO CR3 raw file support in lightroom is a pain. Pop up flash causes shadows when an EF or EF-s lens is used.UPDATE: I was not happy with the Vello. It was loose and cause my longer lenses to not focus and the loose connection caused other weird issues. Got the Canon lens converter and my 100-400 works like a charm. It focuses everytime and the focus is better and I swear the noise is much better. Wish I could post the macros I have been getting but for now Amazon doesn't have that feature. Found that my Canon 600EX-RT flash works with TTL and even Fast Shutter sync! So Macros are not awesome as the large flash does not get blocked by the Ef lenses!

I read a good review on the RF 100-400 IS lens and, having already experienced the high quality of the RF 600, decided to give it a try. My initial impression is that this lens will be producing some fine quality images with it's stabilization and lighter weight compared to my EF 75-300 L, especially when chasing butterflies without a tripod. The RF 100-400 can focus much closer to a subject compared to the RF 600 and is a welcome addition to my collection. Thanks

I use this with 400 and 600 mm lens and it works great.

Returned because I put it on my 100-400 4.5 and lost too many stops.

I'd originally planned to get the RF 100-500 mm L lens, but it costs nearly $3,000. At well under 20 percent of that price, this is a great value. Sure, it's a little slower and only 400 mm at most, but it's a lot lighter and takes very good pictures. I haven't compared it directly against that L lens, but pictures are comparable to or slightly better than the (e.g.) Tamron 150-600 mm G2 EF, although with less reach (but a delight to carry around). I'm very glad that I bought this (100-400 mm) lens.

I’ve been known to be a Canon L lens snob. I picked up this lens for my daughter’s kit but decided to test it during a local soccer game. Besides losing a 100mm of range from my Canon 100-400 L II lens, I noticed no significant visual difference in the photos compared to the significantly more expensive L. In the end, I post process every pic anyway so they only thing that matters to me is sharpness, focus speed and accuracy. This little lens does the trick in all those areas and is great for video. In fact, I’m keeping it and selling the 100-400. It is a very nice lens and I highly recommend it.