Canon EOS R

The benefits of the large diameter of the EOS R’s RF mount explained

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.

Comparing EF and RF lens design
Comparing EF and RF lens design

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

Chaitanya

EOS 6D MK II
Jun 27, 2013
1,005
114
33
Pune
Very interesting compromises and good to read how engineers made those compromises with explanation.
 
Looking at the photo sample of the 35mm lenses, it is shocking how small the front glass is on the RF version. Traditionally I always thought aperture was a formula of front glass size in relation to lens barrel length. Or maybe I am wrong and it is barrel size not front element size?
 
Reactions: sanj

photonius

EOS RP
Jul 13, 2013
226
12
Looking at the photo sample of the 35mm lenses, it is shocking how small the front glass is on the RF version. Traditionally I always thought aperture was a formula of front glass size in relation to lens barrel length. Or maybe I am wrong and it is barrel size not front element size?
from the single lens formula just calculate it: f/2: 35mm/2 = 17.5mm diameter. f/1 lens: 35mm diameter...
 

LDS

EOR R
Sep 14, 2012
1,508
118
was a formula of front glass size in relation to lens barrel length
AFAIK, it's the entrance pupil - not the front glass size. For some kind of lens designs they can be quite close, but for others they're not. Retrofocus designs have large frontal lenses, but the entrance pupil is smaller.
If you look at the two lenses, the pupils are quite alike, despite the different size of the front lens.
Anyway, keeping larger lenses close to the camera should also make handling the camera more comfortable, especially with smaller and lighter mirrorless.
 
Reactions: Kharan

degos

EOS 80D
Mar 20, 2015
175
102
It's not the *diameter* of the R-mount that is significant, since it's the same as the EF mount. They weren't going to use an APS-C-sized mount in any event. It's the combination of maintained-diameter and shorter flange distance.
 

highdesertmesa

EOS T7i
Apr 17, 2017
55
19
It's not the *diameter* of the R-mount that is significant, since it's the same as the EF mount. They weren't going to use an APS-C-sized mount in any event. It's the combination of maintained-diameter and shorter flange distance.
Exactly. Please admin: change this article to stop the spread of this wider RF mount misinformation.
 

venusFivePhotoStudio

I'm New Here
Aug 4, 2018
9
3
Same Mount size, RIGHT! But because is closer to the sensor, you can make bigger back lens elements...

So the article is not so wrong after all.
ex-monture.jpg
 

canonnews

EOS RP
Dec 27, 2017
227
111
Canada
www.canonnews.com
It's not the *diameter* of the R-mount that is significant, since it's the same as the EF mount. They weren't going to use an APS-C-sized mount in any event. It's the combination of maintained-diameter and shorter flange distance.
actually, the diameter *IS* very important. it's a combination of lens registration distance and mount diameter that gives the flexibility.

which is why they didn't use the EF-M mount. they could use the EF-M mount, and the article states that Canon even considered it.

the larger mount diameter allows them to "flip" the elements around, and not use an element to "spread" the light across the entire sensor.

This is why when I wrote CN's article, I compared the RF lens to a similar Sony FE lens, and the differences are as Canon discussed in their presentation.
 

4fun

picture? perfect!
Nov 19, 2018
181
53
it is and was rather obvious all along. fairly simple geometry. i stated many times that well chosen mount parameters relative to image circle cannot be understated.

of course Canon could have "pulled a Sony" and used APS-C optimized M-mount also for FF. But it would have resulted in the same design restrictions that Sony FE lenses suffer from: lenses that are too big, too heavy, too complex and too expensive.

It was clear (to me) that Canon would not go that route. as with SLRs (EF/EF-S) they continue with 2 mounts for mirrorfree: one optimally chosen for APS-C, 1 for FF image circle.

now all we have to wait for is RF lens prices dropping to EF levels.
 

melgross

EOS RP
Nov 2, 2016
309
108
This explains why the RF 50mm f1.2 does not have as large a diameter front element as I would have expected.
F stop has to do with the maximum diameter of the diaphragm. Divide the focal length of the lens by the diaphragm diameter, and you get the f stop.

The size of the front element group mostly determines how much light is gathered at the edges and corners. Bigger front elements give less vignetting. It’s not entirely that simple, but that’s the gist of it.

A very short backfical distance allows smaller lens elements overall, because the rear element group still is smaller than what the front would have been. What I’m seeing with all the illustrations here, and elsewhere, is an exaggeration of the size of elements for lens designs for mirrorless. The reality isn’t so exaggerated.
 

dak723

EOS 6D MK II
Oct 26, 2013
1,141
434
To me, it shows that Canon is considering the "long game." They know that in the short term they may be at a disadvantage to Sony with far fewer native lenses, but in the long term, their choice of wider diameter mount will be an advantage. Of course, the constant Sony bias in reviews and Youtube videos usually fails to mention this, even though most pro photographers know that it is the lenses that make the system.
 
Reactions: gkamadi

Ah-Keong

EOS 80D
Dec 1, 2016
157
9
The new mount would unlock the development potential especially towards ultra-wide angle end. Would Sony "upsize" the mount? Would Canon scrap the EF-M mount?
 
Dec 17, 2018
5
2
I think this lens design could be quite interesting in long telephoto lenses. Would they benefit from the same architecture?

So maybe instead of huge, tapering, front heavy lenses, we get huge 'rectangular' lens bodies with all the heavy glass near the mount instead of in the front.
 
Reactions: Ah-Keong

4fun

picture? perfect!
Nov 19, 2018
181
53
The new mount would unlock the development potential especially towards ultra-wide angle end.
yes, it does.
no, it will not allow for miracles in design of longer focal lengths/tele lenses

Would Sony "upsize" the mount?
no, they won't. Way too late. Sony and their customers now have to live with the consequences of Sony's bad decision to use their crop-sensor E-mount also for FF image circle.

Nikon however may well come up with a second, smaller mount for a mirrorfree APS-C system - if they ever make one.

Would Canon scrap the EF-M mount?
no, they will not. Why should they? EOS M/EF-M is the market leading APS-C ecosystem (probably ahead now of EF-S and certainly ahead of Fujifilm C and Sony E), because it offers good capability at affordable prices in very compact gear - thanks to optimally chosen EF-M lens mount parameters. Not possible to the same degree with R mount.
 
Reactions: Ah-Keong