DCWatch has posted an internal Canon presentation about the advantages of the RF mount’s large diameter. Canon describes why they developed a new mount over using the existing EF-M mount. The large diameter of the RF mount allows for simpler and better-balanced lens design.
The RF mount allows for the larger lens elements to be near the rear of the lens and closer to the camera mount and the image sensor. This should provide better balance when larger aperture lenses are affixed to the EOS R. While this could technically have been done with the EF-M mount, it’s far easier for Canon to design lenses for the RF mount.
Canon News breaks this down further, showing the differences in philosophy between Sony and Canon. Sony puts the larger lenses at the front of the lens, while Canon’s RF lenses seem to put the larger ones at the rear of the lens.
Another apparent advantage of the larger diameter mount is less vignetting on the image sensor itself. Software generally handles vignetting for us now, but this should make sensor and software design a bit easier going forward.
From DCWatch (Google Translated):
According to the optical designer of Canon, it is said that the following three points are the main judgment criteria because “it is optically good” in what “beautiful” is said.
· The symmetry of the optical system is high before and after the diaphragm
· The refraction of the light is gentle
· the large ball and the small ball are lined up in good balance Read the full article