Quote Originally Posted by BTDT View Post
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.
Thanks for that Jim.