In the DSLR days, a camera with a smaller sensor also had a smaller mirror. So EF–S lenses could have their rear elements closer to the sensor. Canon had to make them physically incompatible with full frame DSLR‘s, otherwise the mirror would hit the back of the lens when it flipped up. That’s not an issue on mirrorless.I thought that if they are EF-S or RF-S lenses that there isn't the equivalent factor. Isn't that why they don't work correctly on a full frame camera?