Basically, I've used the Meike MK-EF-TR-A Auto Focus Mount Adapter for years and:
- It fits realy tight
- No light leaks
I don't think you need the pricey Canon Mount Adapter EF-EOS R for vintage glass!

Yo, quick reply while I have a sec! I went through this last year when I got my R6 and didn't wanna drop $100+ just for manual glass. Honestly, I've tried a few cheaper ones and: - Viltrox EF-R2 Mount Adapter - cost me like $50, the control ring is actually kinda cool.
- Commlite CM-EF-EOS R Mount Adapter - picked one up for $40, works basically perfectly. In my experience, no light leaks at all. Peace!

Before I drop my two cents, what kind of vintage glass are we talking about here? Any heavy telephotos or fast primes like the Takumar 85mm? The weight makes a huge difference for the mount's integrity, especially when you're stacking adapters. Since youre already adapting M42/PK to EF and then going to RF, youre basically doubling the chance for 'play' or wobbling between the connections. From a reliability perspective, you want something with tight tolerances so the lens doesnt sag. I'd skip the ultra-budget stuff and look at the Urth EF to RF Mount Adapter. Its really well-machined and feels like a solid block of metal. Since it doesnt have the AF pins, there is less to go wrong electronically if you're just shooting manual anyway. Another solid choice for build quality is the Fotodiox Pro Lens Mount Adapter EF to RF. They use hardened brass mounts which wont wear down as fast as the cheaper aluminum ones if you're swapping lenses constantly. Honestly, light leaks are rare these days, but a loose fit can ruin your focus plane. If you're using heavier glass, definitely prioritize a snug fit over saving twenty bucks. Basically, just make sure the EF-side of the adapter locks tight with your existing vintage-to-EF rings!

Good to know!

Saw this thread earlier but just now getting a chance to respond. I have spent a lot of time stacking adapters for my Helios and Super-Takumar sets on the R system. My journey started with just trying to save money, but I quickly realized that mechanical tolerances become the biggest headache when you are using three different mounts simultaneously. In my experience, even a tiny bit of play in the EF-to-RF connection gets magnified once you add the M42-to-EF ring. I ended up testing several units to find one that didnt feel like the lens was drooping under its own weight. A few things I noticed during that process:

• The internal matte finish matters more than the exterior build quality for keeping that vintage contrast.
• Some mounts feel tight initially but develop a wobble after a few months of use with heavier glass. To better understand what might work best for your R6 setup, I have a couple questions:
• Are you planning to keep a dedicated adapter on each lens, or are you looking for one master adapter to swap constantly in the field?
• Do you prioritize a lightweight setup for handheld shooting, or is absolute rigidity for tripod work your main concern?

honestly i spent way too much time researching the whole adapter market when i first switched to rf because i was terrified of light leaks ruining my shots or a heavy lens snapping the mount but basically what i found is that for strictly manual glass you really dont need to pay the premium for the big names. i ended up buying three different ones from various price tiers just to see for myself and it was interesting how the mid-range brands have really caught up in terms of build quality like the machining on the one i kept is basically perfect and i cant feel any play even with my heavier lenses. youll notice some brands are clearly just rebranding the same factory parts from overseas while others actually put effort into the internal flocking to prevent reflections which is probably the only thing i would really look out for besides the fit. so basically as long as you avoid the absolute bottom of the barrel stuff you should be fine because the mechanical side of things is pretty figured out by now and you arent paying for the autofocus communication which is where the cheap ones usually fail anyway.

Interested in this too

I have been messing around with these for a long time and honestly the main thing you gotta watch out for over the long haul isnt even light leaks but just the internal reflections... some of the really cheap ones have shiny paint inside that ruins the contrast of ur vintage glass which is super annoying and basically makes the photos look kind of washed out and hazy. If I were you I would just get any adapter from K&F Concept because they have been super reliable for me and the build quality seems to hold up even after years of constant swapping and use.

• Just look for the ones with the matte black internal coating
• Make sure the lens release button feels solid because those can snap off on the super cheap ones
• Check if the mount feels like it has a snug fit with no wiggle at all I actually wonder if anyone else has noticed the internal flaring on the no-name brands? Its something I only realized after using them for a few months and now I cant unsee it... basically I think you should just stick with a reputable brand that specializes in adapters and you should be fine without spending Canon prices!