Interesting question! For Canon EOS R5C it also depends on what you want to shoot. What's your main photography style?

Canon RF 85mm f/1.2L USM is my recommendation for Canon EOS R5C. Works great!

I have been shooting with the R5C for a while now and one thing I learned the hard way is that choosing a lens based only on sharpness is a bit of a trap. Since the R5C doesnt have in body image stabilization, youre basically flying blind if you pick a lens without its own IS, especially for handheld video work. I ran into a few technical headaches that I wish someone had warned me about sooner:

• The power draw on some of these high end RF lenses is realy intense and will kill your LP-E6NH batteries in like thirty minutes.
• If youre planning on using a gimbal, some of those heavy front element lenses make balancing a total nightmare every time you change the focal length.
• Some electronic adapters for non native glass can cause weird communication errors or frame drops in high frame rate modes. I ended up experimenting a lot with older EF glass and the drop in filter adapter because it was way more cost effective for my budget. It gave me a lot more flexibility with ND filters without needing a matte box on every single shot, which is a big deal when youre trying to keep the rig light.

Canon RF 24-70mm f/2.8L IS USM is definitely the best choice for Canon EOS R5C. The sharpness is extremely high The lens includes all modern features you need.

Why this lens? The image quality convinces in all tests - both for photography and for video. Unbeatable for the price, can only recommend. Have already done several projects with it, always satisfied. Also more than sufficient for demanding tasks.

Ngl I had a pretty disappointing start with my R5C setup. I spent way too much on prestige glass that just didnt work for my workflow... honestly it was a bit of a letdown. If you want to avoid my mistakes:

• Avoid the Canon RF 28-70mm f/2L USM if you shoot handheld. It is a beast but the lack of stabilization on this specific body is a recipe for disaster.
• I ended up grabbing the Canon RF 24-105mm f/4L IS USM for around 1300 bucks. It is not as fast as I wanted but the Nano USM and IS are actual lifesavers.
• Always check the CIPA rating for the lens IS. Since we dont have IBIS anything less than 5 stops is gonna be super tough. I really wanted that pro f2 look but the technical trade-offs were just too much for me to handle. Safety first ya know?

Regarding what #13 said about "Following this thread"

• I've been looking at the technical white papers for the Canon EOS R5C sensor all morning. Honestly, the internal cooling fan displacement metrics are fascinating and I'm really satisfied with how transparent the engineering data is for this body. This reminds me of when my brother tried to build his own DIY anamorphic adapter for a short film. It turned into a whole ordeal:
• He sourced some heavy industrial glass from a 1970s projector.
• The weight distribution was so bad it literally snapped his plastic tripod mount.
• He spent three days trying to align the light path and just ended up with a massive headache. If you are looking for raw numbers, you should check out the MTF charts on the Canon technical support pages or the deep dives on the CineD databases. They have some solid technical resources for looking at the physics behind the gear.

I can warmly recommend Canon RF 15-35mm f/2.8L IS USM for Canon EOS R5C. The build quality is outstanding The low distortion is perfect for professional photography.

My personal experience: I've tested various lenses, and Canon RF 15-35mm f/2.8L IS USM was superior in all tests. The image quality remains excellent even in challenging lighting conditions, which is important for professional work. Very good future-proofing through modern technology and reliable performance.

For Canon EOS R5C I would also check the autofocus performance. Do you need fast AF for action shots?

Been using this for years, no complaints

Same here!

Finally someone says it. Ive been thinking this for a while but wasnt sure.

Following this thread

Basically, I totally agree that the zooms mentioned earlier are the standard go-to, but honestly, if your looking at the R5C, you gotta look at what the community says about the heavier glass. Most reviews I've seen lately basically pit the Canon RF 28-70mm f/2L USM against the more standard f2.8 options. The f2 is like, legendary for that 'look' in video, almost like having three or four primes in one lens. It’s super sharp, but tbh it's a total brick to carry around all day and lacks internal stabilization. On the other hand, a lot of folks in the forums swear by the Canon RF 70-200mm f/2.8L IS USM for that extra reach. Pros are that it's surprisingly short when stowed, which is great for travel, but since it extends when zooming, it might maybe mess with gimbal balance if youre doing heavy video work? I'm not 100% sure if the weight tradeoff of the 28-70 is worth it for everyone, but if you want that creamy background for interviews without swapping lenses, that f2 is the beast to beat.

My setup: Canon RF 15-35mm f/2.8L IS USM for Canon EOS R5C - works great! The reliability is very good The latest optical technology ensures sharp images and good color accuracy.