I recently upgraded to the Canon R7 and Im super excited to start shooting wildlife, specifically birds in my area (Pacific Northwest, so lots of dark, overcast days and dense tree canopy) and I also have a trip to Yellowstone coming up in late September. But Im stressing out so much over what lens to get. My budget is pretty tight, absolutely cannot go over $800, and honestly I would prefer to keep it around $600 if possible because I still need to buy extra batteries and a decent bag.

Ive done a ton of reading online and Im just getting more confused. A lot of people recommend the RF 100-400mm f/5.6-8 USM. It seems perfect for my budget and super light, but that f/8 aperture at 400mm is giving me major anxiety. With the gloomy weather here and shooting under trees, is it just going to be a noisy, unusable mess?

The other option I keep seeing is adapting older EF lenses. I looked at the Sigma 150-600mm Contemporary, which is right in my price range used. But then I read on some forums that it has terrible autofocus pulsing issues on the R7 body and the autofocus misses a lot of shots. I really dont want to miss shots of grizzly bears because my lens is hunting. Some people say the Tamron 150-600 G2 is better but others say it has the same issue.

Here is what I need:

• Budget: $600 - $800 max (used is totally fine)
• Use case: Birding in PNW forests and large mammals in Yellowstone
• Good autofocus tracking that wont glitch on the R7
• Portable enough to hike with for a few miles

Should I just bite the bullet and get the RF 100-400 and just crank the ISO, or is there some adapted EF lens I missed that works flawlessly on the R7 without breaking the bank? I feel like I'm stuck in a loop of reading reviews and getting more undecided...