RF mount is the new mount from Canon that is used on their lineup of full frame mirrorless cameras.
EF mount is the older mount used by Canon in their Film SLR and Digital DSLR cameras (cameras with mirrors in them).
Can lenses from one mount be used on the other mount?
While EF and RF mount are similar, they are not interchangeable. In addition to different physical design of the flange which prevents the lenses from being mounted directly on a different mount, there are some other differences such as the number of electrical pins that make a connection between the lens and camera body.
Thankfully, EF and EF-S mount lenses can be used on RF mount with an adapter like these:
However, RF mount lenses cannot be used on EF mount at all at the moment.
What cameras can use RF mount lenses?
As of writing, the EOS R, EOS RP, EOS R6, and EOS R5 are the only stills cameras that use RF mount. In addition to those the Cinema EOS C70, and Red Komodo 6k are also able to use the new RF mount lenses. With the high quality of Canon RF lenses I think you can expect them to be quite popular with everyone including filmmakers.
Why did Canon make a new mount for mirrorless?
One theory I have is that RF is the first “digital” lens mount for Canon. A mirror is not necessary in modern digital cameras, so it has been removed, this is progress. With EF you could remove the mirror but the larger distance between sensor and flange would remain from the old film days, therefore the first truly “digital” mount would have to be all new.
Of course, in removing all that extra space the camera is made smaller, and in removing the mirror assembly and associated mechanical bits as well as the mirror-based phase detect AF system, the camera is made substantially lighter and less complicated.
From an engineering standpoint it is an obvious move to both decrease the cost of design and manufacture as well as increasing the quality of the end product.
Getting rid of the mirror may seem like a minor thing but it is pretty major. The mirror and the systems bound to it, such as the old-style phase detect AF are a significant limitation in how fast the camera can operate.
Think about how the mirror-based AF works, when the mirror flips up, the AF shuts off, and it cannot turn back on until the mirror is brought back down and rested in position. When shooting rapidly at 12-20 frames per second, the mirror spends most of its time moving in and out of the way, which means the mirror is severely limiting how well the AF can track a subject.
By taking the mirror out of the camera, not only is the relatively slow movement of the mirror eliminated from the equation, but the AF system is now able to function during more of the time before and after image capture occurs.
This is why cameras like the EOS R5 are able to shoot at 20 frames per second in electronic shutter mode and still maintain excellent subject tracking while doing so.
I also do not know about you, but I really do not want the buzzsaw like sound of a mirror flipping around at insane speeds coming out of my camera all the time!
Is RF mount better than EF mount?
I hope you will agree after reading the above that removing the mirror from the camera allows the camera to be even more powerful than ever before. The fact that both the EOS R6 and EOS R5 can both shoot full resolution stills faster than any DSLR ever made should be proof enough. Problems that people point to with mirrorless like battery life and EVF quality will get even better in time even though they are already excellent in the current cameras. The really good news for anyone who owns EF lenses is that they work great on all the RF cameras when using the Canon adapter. So, in actual fact you should not even worry about the question of which mount is better. You can simply buy and use an RF camera as you always would with your current EF lenses.
Will Canon abandon EF mount?
It might be too soon to say when EF mount is officially mothballed, but I have to believe that the writing is on the wall for EF mount. The one thing that might keep EF mount alive longer is that EF mount has a huge established user base, and EF mount lenses can naturally be adapted to most if not all mirrorless cameras. That fact might encourage Canon to make a new EF lens or two in the future as well as to continue producing existing EF lenses. And who knows, people of the future may demand mirrors make a return someday, I guess anything could happen at this point.
What did I do?
When RF mount was announced I was naturally excited about it. Canon had been rightfully accused of not doing enough with their camera lineup in the recent past. But when they released the EOS R it hit the market with all the enthusiasm of a wet mop. Basically, people were not impressed on the whole. And while it was certainly a serviceable camera it was also very much a bit of a WTF moment. It even had some people questioning if Canon was serious about the RF mount. Then they released the EOS RP and people were even more confused. So now we had a bunch of really high-end expensive lenses, and two barely serviceable camera bodies to use those lenses with… WTF indeed!
Whether by design or by accident the question marks turned into exclamation marks when the R5 was announced. People piled onto waiting lists to own it, and I excitedly added my name to the list as well.
As of now I am shooting exclusively with the EOS R5 and I have no qualms about it. It is a great camera. Not a perfect camera at all. It solves lots of problems and introduces some others, but ultimately it allows a new level of photography. I have pretty much missed nothing from EF mount, and I think most photographers will find the same holds true for them.