Big warning: dont cheap out on a random third‑party adapter or bargain telezoom… you’ll get flaky AF/stabilization and it’ll drive you nuts for wildlife/sports.

For your situation, I’d suggest going RF if you can—AF just feels more “locked on” on the EOS R, and the stabilization/handling is usually nicer. I’ve shot kids’ soccer and birds for years, and what’s kept me happy is:
- A mid‑priced RF telephoto zoom that’s sharp wide open and has solid IS
- Or an adapted EF 100–400-ish zoom with the official Canon adapter (still works well, no complaints)
- Skip the super slow variable‑aperture stuff if you’re shooting evening games

Also… weight creeps up fast. Try renting for a weekend if you can. cheers

Ive been shooting with the R for a couple years now and native RF glass is just so much easier to deal with if you hate lugging around heavy gear. Since youre looking at kids sports and wildlife, here is what I would look at for that budget.

• Canon RF 100-400mm f/5.6-8 IS USM: This is probably the best value lens Canon has right now. Its tiny and light, so you wont get tired holding it at a long game. AF is very quick.
• Canon RF 70-200mm f/4L IS USM: If you want that pro L-series sharpness, this fits in your budget and is incredibly compact. You lose some reach for birds, but for sports it is killer.
• Canon EF 100-400mm f/4.5-5.6L IS II USM: You can find these used for a great price. It is definitely heavier than the others, but the glass is legendary. Just gotta use the adapter. I am super happy with the RF 100-400 for most stuff... it is just so convenient. If you dont mind a slower lens, its a total win for the price. Let me know if you want to know more about how they feel on the R body!

Bump - same question here

To add to the point above: the Nano USM motors in the newer RF glass are an absolute game changer for tracking speed! Honestly, i was skeptical about the slower apertures at first but the AF reliability on my EOS R has been totally flawless even when my kids are sprinting across the field. I used to lug around an old adapted EF telephoto and my hit rate for sharp shots went up noticeably once I switched to native glass. The data throughput between the lens and sensor is just way faster which matters a ton for wildlife. For your budget, you really should check out the Canon RF 70-200mm f/4 L IS USM. Its incredibly compact and sharp as a tack! I love how it fits in a small bag without weighing me down during a long hike or a soccer tournament. The 5-stop stabilization is amazing and lets me get away with much longer exposures than I ever could on my old DSLR setup. Quick tips for the EOS R:

• Stick with native RF lenses to maximize the dual pixel AF performance for fast moving targets.
• Make sure your firmware is updated to version 1.6.0 or later to get the best eye-tracking results for sports!

Regarding what #4 said about "To add to the point above: the Nano...", that tracking speed really is phenomenal because the Nano USM tech moves those focus groups with such high frequency! Basically, everyone here is right that native is the move if you want to stay light. If you compare the Canon RF 100-400mm f/5.6-8 IS USM against an adapted Canon EF 100-400mm f/4.5-5.6L IS II USM, youre looking at a massive weight difference. The RF is only 635g! Thats insane for a 400mm reach. The old EF is a tank at 1.6kg plus the adapter weight... my arms would be shaking halfway through a soccer game lol. Another fantastic contender is the Canon RF 70-200mm f/4 L IS USM. Its incredibly sharp and that constant f/4 aperture is amazing for separation, but 200mm is usually way too short for birds or wildlife. If you can handle the slower f/8 aperture at the long end, the RF 100-400 is the technical winner for portability and AF response on your budget. Love seeing Canon push these light designs!

Solid advice 👍