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I tried searching this on the web, but all that turned up says C70 can mount RF lenses, with no mention of RF-S.

So Q: Can Canon EOS C70 mount APS-C RF-S lenses?

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According to https://www.canon-europe.com/pro/infobank/rf-mount/

The RF mount on an RF-S lens is identical to the mount on all RF lenses, and RF-S lenses are compatible with all EOS R System cameras. However, full-frame EOS R System cameras, when fitted with an RF-S lens, will automatically crop the image area to match the APS-C coverage of the lens.

So it looks like that RF-S lenses can be mounted on any camera that supports RF lenses.

For EF lenses, the mirror on EF-S cameras was smaller (due to reduced sensor size), which meant that the rear element of an EF-S lens could extend further back without touching the mirror (called "short back focus"). This meant that if EF-S lens could be mounted on an EF-only camera, it would touch the mirror, causing damage. Thus, Canon had to prevent mounting of EF-S lenses on EF-only cameras.

However, on the mirrorless world things are different. There is no mirror. So RF and RF-S lenses allow the rear element to be very close to the sensor, with no difference whatsoever.

RF-S lenses have a smaller image circle. I understand that Canon RF cameras automatically detect whether a crop lens is installed (which might not work properly for third-party lenses) and switch to a crop mode. Unless the camera has huge megapixel count like that in EOS R5, you won't be happy with crop lenses on a full frame camera, though, because the resolution will be poor. But this automatic detection might not work with video cameras.

Googling around, it seems that C70 has a 26.2mm x 13.8 mm sensor. The 26.2mm side has a crop factor of 1.37, not suitable for full image circle on RF-S lenses, whereas the 13.8mm side has crop factor of 1.74. The diagonal has crop factor of 1.46, not suitable for full image circle.

So if you mount an RF-S lens, it is possible that the corners of the image will be dark. It is also possible that it being a video camera and not a still camera, that the software doesn't automatically detect that RF-S lens is installed and crop accordingly, but rather produces video with dark corners.

It is also possible that the diagonal crop factor being 1.46, very close to 1.6, that the slightly-larger-than-crop image circle would be still fully within the image circle of typical RF-S lenses, allowing perfect operation with no dark corners.

So, you can mount it, but whether it is useful, it's up to you to test. I recommend testing with an existing RF-S lens if you have such a lens, if not I recommend testing with a lens you can return with full refund. If you can't find a lens you can return with full refund, test with the cheapest lens you can find or rent a lens if you live in an area where a lens rental service is available. Also remember that lenses can vary, some RF-S lens can have slightly larger image circle than some other, so it's possible not all lenses work in a useful manner even though you can mount them.

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  • \$\begingroup\$ Thanks for the URL to the European market. Also, did you mean the "firmware" doesn't automatically detect in the 3rd-to-the-last paragraph? I know the difference is minor though. \$\endgroup\$
    – DannyNiu
    Commented Jun 22 at 12:50
  • \$\begingroup\$ Yes, I meant the software in the camera, often called "firmware". \$\endgroup\$
    – juhist
    Commented Jun 22 at 13:10
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    \$\begingroup\$ Very few EF-S lenses actually exploited to possibility of extending further into the light box. Offhand, I can only think of two that did. No third party APS-C only lenses for Canon EF mount did, either. So in most cases the extra tab on the EF-S lens flange was unnecessary for the clearance issue and only put there by Canon so users wouldn't complain when their EF-S lenses mounted on FF bodies didn't have full sensor coverage, like they complained when using third party APS-C EF lenses on FF bodies. \$\endgroup\$
    – Michael C
    Commented Jun 22 at 19:41
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    \$\begingroup\$ FWIW, only the sensor diagonal is typically relevant to crop factor... it is the longest dimension and correlates to (spans) the diameter of the image circle. \$\endgroup\$ Commented Jun 23 at 13:59
  • \$\begingroup\$ @StevenKersting However, the aspact ratio of the sensor on EOS C70 is different from DSLRs and mirrorlesses - C70 is Super 35mm, whereas others are 3:2. This needs to be taken into account. \$\endgroup\$
    – DannyNiu
    Commented Jun 24 at 1:19

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