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So Canon finally got into the mirrorless league — but they also made a new lens mount for their mirrorless cameras!

Is there anything better in RF lens mount or lenses that makes them better than older lenses like EF and EF-M? I have a suspicion that they really only made the new lens mount to sell more lenses. What am I missing?

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Like most mirrorless systems, the RF mount has relatively short flange focal distance, of only 20mm, as opposed to 44mm of EF (and EF-S). This has a number of optical advantages, better allows for adapted lenses, and is just plain smaller.

Additionally, the new mount has a slightly wider throat diameter, about which Canon says:

With a large 54mm lens mount, large diameter elements to match and placement closer to the image plane, RF lenses provide brilliant image quality edge-to-edge with minimal light fallout, enabling new possibilities for enhanced optics and compact designs.

And, also, while they were at it, it seems they took the time to redesign the electronic communication mechanism (which seems like a good idea, because once you've got the basics of that in place, you have to keep compatibility basically forever or else people will complain on the internet that you made the change just to force people to buy new lenses rather than for any actual good reason).

It may be worth considering that when Pentax dipped their toes in the mirrorless waters with the K-01, they decided to use the same classic K mount (well, K-AF3, for those keeping careful track). The camera has some questionable design choices, but that's not really why it flopped. It flopped because without a lens lineup designed to take advantage of the format, there wasn't much reason for it.

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  • They also increased the data transfer rate which was (to my knowledge) in order to allow the Lens IS to work together with the IBIS. Even though Canon launched the RF mount before they had officially announced IBIS, they were preparing for this. – Pete Nov 23 '20 at 8:12
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This may not fully answer the question, but there are a few videos on youtube that compare the RF equivalent against the EF to see the difference.

Canon RF vs EF Lenses - Which are the Best? | Comparative Review by BorrowLenses

In the video he talks about the flange distance and that in RF-lenses the sensor is closer to the glass, which means eliminating the need for a retro focal element group, which in other terms means less extreme image correction, fewer lens elements and in many cases a sharper image. (The latter I can personally attest to, when comparing my old 24-105 to its RF equivalent) In the video he specifically compares autofocus, sharpness, color rendition, weight and price on the lenses 50mm and 24-105mm to their EF equivalents. (Spoiler: The RF wins in both cases - especially in sharpness - even though the 50mm is quite a bit more expensive)

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