Honestly, grab the Canon RF 16mm f/2.8 STM. Over the years, I've realized that in Iceland, you really want that extra cash for gas and food. The prime is tiny, sharp enough for travel, and fits easily in a jacket pocket. The zoom range on the Canon RF 15-30mm f/4.5-6.3 IS STM is nice, but for that price, I'd rather spend the savings on a nice glacier hike. Safe travels!

Late to the party but honestly, I found the optical performance of both these lenses a bit lackluster for high-resolution sensors. If you go with the Canon RF 16mm f/2.8 STM, expect significant distortion that requires heavy software correction, which unfortunately eats into your effective resolution. The Canon RF 15-30mm f/4.5-6.3 IS STM offers more utility, but the variable aperture is not as good as expected for low-light shots at dawn.

• Prioritize a high-quality circular polarizer filter to cut glare from waterfalls.
• Bring a lightweight tripod, because the f/6.3 end of that zoom will struggle without it. If you really care about sharpness, try to find a used Canon EF 16-35mm f/2.8L II USM and grab an adapter. It is built much better than the current RF budget options.

Saw your post earlier and wanted to chime in. While that 16mm is a solid tiny prime, honestly, I think you should look at the Canon RF 15-30mm f/4.5-6.3 IS STM instead. Having that extra flexibility in Iceland is actually a lifesaver because you cant always physically move to frame the shot near those cliffs and waterfalls.

• The stabilization is a huge plus since you might be shooting handheld in some dim light.
• Youre getting 15mm, which is noticeably wider than 16mm for those epic landscape shots.
• It balances pretty well on the R8 body, so it wont feel too front-heavy. Check out the reviews on DPReview if you want to pixel peep the sharpness. Even if it costs a bit more, you really wont regret having the zoom range for landscapes. Totally worth the extra budget imo.