Regular FoV headsets use Parallel Magnification. The screen is parallel to the lens. As you can see in the pic above the lens distance is equal on both sides. The lens magnification/focus is relatively even; like a Camera etc..
Wide FoV headsets use Angled displays that are not Parallel to the lens. As you can see in the pic above. Because of this a standard lens will not work. Tge magnification/focus needs to increase as A moves towards B and to C.
This if course makes lens design more difficult.
So @sjefdeklerk removing the lens & placing it over a smartphone screen with even spacers on each side to raise lens to view screen will not work as the lens needs to see the screen on the correct angle. If this is not done with the proper amount of angle (pic above A =3 & C=4 for height). One will come to incorrect conclusions to lens quality & how much of the lens uses the screen; the magnification will be horrible with focus etc.
@cdaked this will be of interest.