One of Canon's niftiest new gadgets is the EF-to-RF mount adapter that can accept both variable ND and circular polarizing filters, allowing you to use them with any EF lens without having to worry about individual filter size, making it useful for a wide range of situations. This great video review takes a look at the adapter and how it performs in practice.
Coming to you from Christopher Frost Photography, this excellent video review takes a look at the Canon EF-to-RF drop-in filter mount adapter. When you adapt an EF lens to the RF mount, there is extra space due to the shorter flange distance. Instead of leaving empty air, Canon came up with the clever idea to allow the user to drop in variable ND and circular polarizer filters. A further advantage of this is that because the filter is inside of the adapter instead of mounted on the front of the lens, you do not need to carry different filter sizes to fit separate lenses. The adapter is definitely on the pricier side of things, but on the other hand, if you carry multiple filter sizes in your bag, it could end up evening out. Check out the video above for Frost's full thoughts.
I think the price is justified for these adapters. The amount of money I've spent on screw-on ND filters and adapter/step down rings is ridiculous, not to mention the time spent on shoots changing them out makes this adapter worth every penny.
There is only one thing I do not like with this concept.. since the flange distance is a bit different than with classic adapter you cannot just have an empty frame in it if you do not need any filters.
You need a clear glass filter. And introducing another glass in between the lens and sensor is not optimal.