No EF-M lens can be mounted on an RF or EF/EF-S mount camera and be able to focus to infinity. This is because the EF-M mount is has the shortest registration distance (a/k/a flange focal distance) of the three mounts. The EF-M mount also has a smaller throat diameter, which is the diameter of the hole in the camera's flange ring.
No, You still have it wrong….. An EF 50mm and an EF-S/EF-M 85mm will NOT produce similar field of view. Not even close. Neither will an EF 85mm and an EF-S/EF-M 50mm. If she buys an EF 50mm and an ef-s 50mm, they will both look the same when used on the same camera.
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