TIL! Thanks for sharing

> I just picked up a Canon EOS R100 (RF mount) and I’m trying to add a budget-friendly telephoto for outdoor shots… kids’ sports and occasional wildlife… under $300–$400… fine buying used or using an adapter.

For your situation, I’d honestly go one of two ways depending on whether you value convenience or AF “hit rate.”

**1) Easiest/lightest (and usually cheapest):** Canon RF-S 55-210mm f/5-7.1 IS STM
- On the R100 it’s a super practical walkaround tele. AF is quiet, IS helps a ton, and it’s light enough you’ll actually bring it.
- Unfortunately, 210mm (on APS-C) is *okay* for daytime sports, but for small birds it’s not as good as expected unless they’re close.

**2) Best budget “reach + keeper rate” (my pick):** used EF 70-300 + adapter
- Look for Canon EF 70-300mm f/4-5.6 IS II USM + Canon Mount Adapter EF-EOS R.
- I’ve had issues with older micro-motor 70-300 variants hunting more, but the IS II’s Nano USM is snappy for kids running around.

If you can stretch or find a deal, the wildcard is Canon RF 100-400mm f/5.6-8 IS USM—way better for park wildlife. But within $400, EF 70-300 IS II + adapter is the safest bet IMO. Hope this helps!

Regarding what #2 said about “Hmm, not to disagree…” — I kinda disagree: for kids’ sports you’ll feel 210mm is short fast. I’d look used at:
- Canon EF-S 55-250mm f/4-5.6 IS STM: cheap, sharp, AF is solid. Great “safe” buy.
- Canon EF 70-300mm f/4-5.6 IS II USM: more reach + faster AF, usually still under your cap used.
- Canon RF-S 55-210mm f/5-7.1 IS STM: light/cheap, but reach is meh. Hope that helps!

Hmm, not to disagree, but I’d actually lean away from the smaller/shorter tele zooms if sports + wildlife are priorities on the R100. The R100’s APS-C crop makes reach king, and f/6.3 at the long end is still workable in daylight.

What I’d look at:
- Canon RF-S 55-210mm F5-7.1 IS STM: light, quiet AF, fine for kid sports in good light… but 210mm can feel short for wildlife.
- Canon RF 100-400mm F5.6-8 IS USM (used): this is the “cheap reach” sweet spot IMO. 400mm (640mm equiv) is honestly a game changer.
- Canon EF 70-300mm f/4-5.6 IS II USM (used): fast AF, good range; just note you’ll need some kind of EF→RF solution (already mentioned above).

I’ve been happiest with the 100-400 route—no complaints for parks/day games.

For your situation, the big “safety/reliability” win is avoiding bargain third‑party AF quirks—missed focus is basically lost shots. I’d suggest:
- Canon RF 100-400mm F5.6-8 IS USM (often ~$400 used): tons of reach, fast/quiet AF, solid IS—no complaints for daylight sports/wildlife.
- Canon RF-S 18-150mm F3.5-6.3 IS STM (if you want one-lens convenience): less reach, but super reliable.

If you can stretch a bit, 100-400 is the happy pick.