[Brightcolours]

Focus breathing is a common feature in all non-IF lenses also, courtesy of the distance from the focal point to the sensor. Focus breathing is not equivalent to "change of focal length". In order to eliminate focus breathing, one will have to change the focal length.

 

So, if you move the whole camera for focus stacking, you have an issue because the perspective (view point) changes. But you have no focus breathing. If you keep the camera at the same spot but you move the lens, you have a FOV change due to "focus breathing". In theory, one can design an IF lens which has no focus breathing at close up distances. But I do not know of any information of such macro lenses being available?

 

So yes, just shooting a bunch of images and combine them into RAW at the sensor resolution is not doable.

 

The focus breathing at close up distances is easily seen/checked through the view finder or on the LCD in live view, so do not treat it as something elusive. This tutorial video of the focus stacking features of Magic Lantern shows it clearly.

https://www.youtube.com/watch?v=CcRVXuc6Yt4

 

You will always need to align the images, and crop the result.