Originally Posted by
BTDT
Maximum POI shift in any direction
O x (P-R) / 2P
Where:
O = Objective lens radius
P = Parallax focus range
R = Target range
Therefore, a 40mm objective focused at 35 yards will give a maximum POI shift at 15 yards of
20 x (35-15) / 70
Or 5.71mm.
You'd only get that sort of shift if you deliberately tried to look through the scope as far from its axis as possible; in practice, shift will be nearer a quarter, or ~1.4mm.
No big deal.