Many photographers have made the switch to Canon's recently fleshed-out ecosystem, but that might not mean they have sold all their glass. Instead, there are many options for adapting Canon EF lenses to the new Canon RF mount. Here is a comparison of the three official Canon adaptors.
Whenever you move to a new camera system and mount, you have the dilemma of whether to sell all the glass you have acquired that no longer fits and buy lenses that do, or to adapt the old lenses. I have had this dilemma a number of times over the years and I'm confident that I made the right decisions each and every time: adapt.
If you can afford to lose a significant wedge of your hard-earned cash, then by all means go ahead. However, be aware you are almost guaranteed to lose money, even if you bought the lenses second-hand. This is particularly true if there is a sudden mass migration towards a new mount of the same manufacturer, as has been the case since the release of Canon's R5 and R6. Their first two mirrorless bodies were by no means bad, but the latest two have seen a lot of interest from professional and enthusiast photographers alike.
If you decided you can't quite justify parting with your old Canon EF-fit lenses, or you can't bring yourself to part with your favorite glass (for me it was both!), then you need an adaptor. While there are multiple EF to RF adaptors, Canon themselves have three (technically four), though they have been infamously tricky to get your hands on. Here are your options:
Canon Control Ring Mount Adapter EF-EOS R
Canon Drop-In Filter Mount Adapter EF-EOS R with Circular Polarizer Filter
Canon Drop-In Filter Mount Adapter EF-EOS R with Variable ND Filter
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For the first option (pure adapter, no additional functions) there are several third party options already available. As usual: they do the same job for half the price.
You can use the 'basic' EF to RF adapter instead of the clear drop-in.
Honestly the drop-in filter (with either filter) for the EF to RF adapter is pretty amazing, mostly b/c you don't need to get different filters for each lens (including for lenses that don't have easy ways to attach filters).
My experience with the 3rd party filters is that they are no where near as mechanically precise as the official canon adapters.
Unless you want filters, which I don't use, the basic Canon adapter is all you need. The R5 and R6 already have enough dials to operate iso, aperture and shutter. You don't need another ring.
Nice presentation. By the way, do you know whether the adapters will work with EF-mount lenses from Tamron and Sigma as well as Canon lenses?