Why is the RF flange distance 18mm?

Antono Refa

EOS R
Mar 26, 2014
1,075
259
If it was 19mm, Canon's 25mm extension tube could have served as an EF to RF adapter. Has Canon chosen a slightly shorter flange distance just so people couldn't reuse their extension tubes?
 

Codebunny

EOS R1
Sep 5, 2018
570
536
The extension tubes would still be the wrong size and only a very small amount of people have them. It is really daft to think Canon even considered the extension tubes when designing a mount.
 
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cornieleous

5D4 + R5
Jul 13, 2020
208
733
If it was 19mm, Canon's 25mm extension tube could have served as an EF to RF adapter. Has Canon chosen a slightly shorter flange distance just so people couldn't reuse their extension tubes?
The RF mount has different electronics/pins, and also has different physical layout of the three retaining tabs. So no, extension tubes would not work. The new mount is much more than just a distance change. Also, regarding the distance change, getting the sensor closer to the lens allows for smaller lenses and more controlled / better optical designs. The EF RF adapter converts the electrical and mechanicals, and preserves the distance from the sensor, but native RF lenses will be able to have better performance in many ways. The first goal was to make better lenses and more capabilities, not think about backward compatibility with inferior technology or whatever extension tubes people might have lying around. Engineers must design for the future capability, not the past. Also, those tubes reduce image quality to change MFD, so few people have them.
 

Mt Spokane Photography

I post too Much on Here!!
CR Pro
Mar 25, 2011
16,280
1,315
There was a lot of discussion about it when the R came out. The answer is that no one knows why the specific number was chosen. We can speculate forever, but unless Canon supplies the answer, we won't know.
Using a 25mm tube with a 20mm flange distance would require optics, as well as a way to adapt the different mounts. A adapter is cheap and easy, you can get 3rd party ones for a few dollars.