Unfortunately, I had issues with the low light on the Canon RF 800mm f/11 IS STM. It was not as good as expected for the R5 sensor. Since your budget is tight, look for a used Canon EF 100-400mm f/4.5-5.6L IS II USM. It still holds up great. Quick tip: bring extra batteries, that sensor eats power like crazy when you're tracking wildlife... honestly dont leave home without a spare.

Unfortunately, you will likely find the Canon RF 200-800mm f/6.3-9 IS USM quite frustrating on a high-resolution sensor like the R5. I had issues with the overall sharpness, and it is simply not as good as expected when the light drops during those early morning wolf sightings in Yellowstone. It is a massive beast to lug around on hikes too, which gets old fast. The Canon RF 100-500mm f/4.5-7.1 L IS USM is technically the superior lens, but honestly, I was disappointed that it hits f/7.1 so early in the zoom range for such an expensive piece of glass. It is definitely the better choice for hiking since the weight is manageable and the autofocus is snappy. If the budget is tight, even an adapted Canon EF 100-400mm f/4.5-5.6L IS II USM might serve you better. It is older tech, but the glass is legendary and handles the R5 sensor much better than the consumer-grade 800mm does imo.

Lugging a massive setup through Lamar Valley last year was a mistake, my shoulders still hurt. For the R5, I'd suggest the Canon RF 100-500mm f/4.5-7.1L IS USM because the glass is way better for those pixels. Be careful with that 200-800 tho, f/9 is super dark for early morning wildlife. If budget is tight, maybe find a used Canon EF 100-400mm f/4.5-5.6L IS II USM and a Canon Mount Adapter EF-EOS R.

Been thinking about your Yellowstone trip and honestly... quick question before I dive in: are you doing mostly roadside spotting or actual long-distance trekking? It makes a massive difference for how much weight you can realistically carry without getting exhausted. I would suggest you stay cautious about the 200-800mm because it lacks the robust weather sealing of the L-series glass. Yellowstone weather is totally unpredictable and you dont want dust or moisture ruining a once-in-a-lifetime trip. For total reliability on the R5 sensor, the Canon RF 100-500mm f/4.5-7.1 L IS USM is really the safest bet. If you find you're desperate for more reach once you're there, maybe consider looking into a Canon Extender RF 1.4x but keep in mind it limits your zoom range on that specific lens. Honestly, make sure to check out rental options too if the $2500 is a stretch right now. It might be better to rent something like the Canon RF 600mm f/4 L IS USM for the week if you really want those wolf shots, rather than settling for a lens you might regret later. Let me know about the hiking situation and I can try to help you narrow it down more!

Re: "Been thinking about your Yellowstone trip and honestly......"

• Uscal hit the nail on the head regarding the weather sealing. Yellowstone can be brutal and having that L-series protection gives me so much peace of mind. I've been using the Canon RF 100-500mm f/4.5-7.1 L IS USM for a while now and I'm super satisfied. The focus is snappy and it just feels right on the R5 body. The Canon RF 200-800mm f/6.3-9 IS USM is definitely tempting for the reach, but like people said, it's a lot of glass to carry. If you find yourself needing more reach on a budget, I actually think the Canon RF 600mm f/11 IS STM is a fun little lens to have in the bag. It's light enough to forget you're carrying it, tho obviously it isn't as good as the 100-500 when the light gets dim. Honestly, I'm happy sticking with the 100-500 because the quality is just top-tier. You're gonna love the R5 regardless!

Building on the earlier suggestion, I'm totally with Uscal on the weather sealing worries. Honestly, I've had issues with gear getting trashed in the park because I was overconfident. It's just not as good as expected when the wind picks up and that fine volcanic dust starts flying... it gets everywhere. Since you're on a budget and heading into the wild, think about these DIY safety steps:

• dont trust the factory gaskets alone, bring a pack of heavy-duty plastic bags and some rubber bands to make emergency rain sleeves... looks cheap but saves thousands.
• avoid swapping glass while you're standing in the wind or near the thermal vents cuz the sulfur and grit will wreck your sensor faster than you can blink.
• maybe grab a simple blower brush and use it every single night to clear the mount area... I forgot once and it was a total disaster. Yellowstone is gorgeous but it's brutal on electronics, so just be extra cautious out there. Nothing worse than a gear failure on day two of a dream trip.