"But what wasn't mentioned was that horses can change their focus by raising and lowering their heads. I can't remember which way round it was though - would raising or lowering the head help bring an approaching jump into better focus?"
With the demise of the "ramp-retina" theory, I think it is more accurate to suppose that in this situation horses raise their heads in order to bring the fence
into view rather than
into focus. That is, it's more to do with where the blind-spots are than how near vs far objects are accommodated. In any case, if a horse has to raise its head and finds the view obscured by tack, it will suffer a perceptual disadvantage.
There's a chapter in Paul McGreevy's EQUINE BEHAVIOR which goes into this and includes some figures from Alison Harman that illustrate the issue rather well:
http://www.elsevier-international.com/e-books/viewbook.cfm?ID=932