I am using the Red Epic with Canon mount. With my Canon 70 – 200 1:2,8 L IS II lens I noticed the focus is changing with different focal lengths.
For me this means the flange focal distance is incorrect. Is this right?
And is this a problem of Canon lenses or of the Epic mount?
Could be a back focus issue or more simply that a Canon still lens is not optimized for motion picture cinematography and if you want to avoid this sort of issue you would want to spring for higher quality lenses (or just live with it).
Canon stills lenses don't generally show constant focus with zoom - they were not designed for that property, unlike traditional cinema zooms. It's not a lens mount or backfocus issue, just a lens design limitation.