The Canon EF-EOS R Filter adapter lets you mount EF DSLR lenses onto an EOS R mirrorless camera, while also using the spare space inside the adapter to accommodate drop-in filters. It’s sold in two versions, one with a circular polarizer and the other with a variable neutral density filter; both filters are also sold separately in case you buy the adapter with one and would like to have the other one as well.

It’s a neat solution, although one that carries a hefty price premium over the basic adapter or indeed a selection of traditional screw-in filters. But when you locate the filter inside the adapter behind the lens, it’ll work with any lens you attach, saving you from buying and carrying multiple filters or step-down rings, and more importantly, it’ll also work with exotic ultra-wide lenses that lack filter mounts at all. It’s cheaper for example than the massive third-party filter kits for models like the EF 11-24mm f4L, and will also work with fisheyes that typically don’t support front-mounted filters at all.

A word of warning though. You can’t use it as a normal adapter by just removing the supplied filter. First there’ll be a window leaking light, but even if you cover it, the filter has an impact on the optical path and removing it will prevent some lenses from focusing to infinity. The solution is to replace the polariser or ND filter with the optional Drop-in Clear Filter accessory which resolves the light leaks and fixes the optical path, but actually costs more than a plain adapter. And unlike some drop-in filters, there’s no easy way to make your own with gels. Oh and any of you with Canon super-telephotos? You can’t use those drop-in filters here either.

I’ve filmed a review about the EF-EOS R Filter adapter below which describes the pros and cons, shows the differences between the different adapters available and demonstrates what’s possible with the polarizing filter model.