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The merits and examples of large RF mount described at a Canon meeting
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The merits and examples of large RF mount described at a Canon meeting

DC Watch posted an article describing some internal documents that Canon has used to suggest that using the large mount diameter for the RF system, basically saved them a few problems that would have transpired if they used the smaller EF-M mount.

According to the article, Canon initially attempted to design the system around the EF-M mount;

Initially, Canon also considered to realize a 35 mm full size with the EF-M mount of the APS - C mirrorless "EOS M" series, but satisfactory results were not obtained, such as no performance aimed at . And since the system becomes large when unnecessarily increasing the mount diameter, we actually designed multiple lenses, decided to 54 mm while looking at the balance of optical merit and size, the strength of the camera and so on.

Canon suggests that the main feature the larger mount allows them to do, is move the elements around to where the larger elements can move to the back of the lens, closest to the sensor.  They don't suggest that it's impossible to do with a smaller mount, just easier, and also a better overall balance to the lens and camera system.

Sony of course refuted some of the claims, however, if you do compare the Sony FE 35mm 1.4 with the Canon RF 35mm 1.8 element designs you can certainly see what Canon is talking about. 

Above is the element design of the Sony FE 35mm 1.4 lens, and when we compare it to the Canon RF 35mm 1.8 shown below, the Sony FE lens has the biggest element at the front, whereas it's reversed on the RF lens, as Canon suggested they can do because of the larger diameter mount.

So while Sony's claims may be technically correct, it certainly seems like Canon has a valid point here.

Canon also compared it to their own example, the EF 35mm versus the RF 35mm;

In this diagram, we can see that the Sony FE 35mm mimics the more conventional element layout of the EF 35mm.

In this diagram, Canon further illustrates the difference, and also seems to be implying that the element design is reversed, and also, simpler.  Also interesting they demonstrate that the angle of incidence of light that hits the sensor in the corners is a smaller angle, making the sensor design even easier with a larger mount.

So what advantages can we glean that Canon suggests the RF mount provides?  We can see that Canon is suggesting that the lenses will have a better balance with the larger elements at the rear of the lens, versus at the front, and it will be easier for Canon to design the lenses themselves.  Also, the angle of light which causes vignetting on the sensor is lessened making sensor design a more simple process.

All of these reasons, probably made Canon make the hard choice to not use the EF-M mount, and to design a new mount from scratch for full frame mirrorless.

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