APSC LENS ON FULL FRAME CAMERA REGISTRATION

To increase the image circle of an EF-S lens larger than the size it is projected onto the sensor of an EF mount camera, the lens would either need to be moved even further forward than the 24mm the EF→RF adapter provides or magnifying optical elements would need to be placed between the lens and the EF→RF adapter. Of course, in such a case if the lens were focused at infinity the sensor would be too close to the lens and the entire image would be too blurry. If it were possible to mount the EF-S lens closer than 44mm from the sensor of an RF mount camera, the image circle would be smaller than it would be at 44mm behind the lens' flange ring. Your intuition is partly correct, though, in a reverse sort of way. The light projected by the EF-S lens will then come into focus 44mm behind the lens flange, just as it would when the lens is mounted on an EF mount camera. When placed on an RF mount camera it provides a flange 44mm in front of the sensor on which an EF or EF-S lens can be mounted. RF lenses are designed to focus the light they project 20mm behind the lens flange ring.

This is the distance from the sensor to the flange on all RF mount cameras.

APSC LENS ON FULL FRAME CAMERA REGISTRATION

The design registration distance for RF cameras and lenses is 20mm.

EF and EF-S lenses are designed to focus the light they project 44mm behind the lens flange ring.

This is the distance from the sensor to the flange on EF mount cameras, including all FF, APS-H, and APS-C models.

The design registration distance (sometimes colloquially referred to as the flange focal distance) for EF-S lenses is 44mm.

The EF-S lens will always converge focused light 44mm behind the flange ring. The image circle at the sensor is the same size whether an EF-S lens is used with an APS-C EF mount camera or with an RF mount camera + EF→RF adapter. The adapter moves the lens away from the camera so that the lens is the same distance away from the sensor as it would be when mounted on a camera for which it is designed. The entire point of the EF→RF adapter is to place an EF or EF-S lens at exactly the same distance from the sensor when used with an RF mount camera as the lens is placed when used with an EF mount camera. Instead, the EF→RF adapter places the lens the exact same distance from the sensor of an RF mount camera that the lens is from the sensor when directly connected to an EF/EF-S camera. No, because the adapter doesn't move the lenses further from the sensor in an RF mount camera than the lens is when attached directly to an EF mount camera. Since the adapter moves the lenses further from the sensor, I’d imagine that the coverage of the lens would be larger, and that the adapter or body alters focus to compensate.

Apsc lens on full frame camera