Prentice's rule: prism (Δ) = decentration (cm) × lens power (D). Minus lens, horizontal. Example values shown — enter your own.
Right (OD)
Left (OS)
True PD (measured)
Ordered PD (used)
Lens power (D)
Enter monocular PDs (nose-to-pupil, each eye). Power = the horizontal-meridian power of each lens (≈ your sphere for a with/against-rule Rx).
Right eye · OD
0.09 Δ
Left eye · OS
0.00 Δ
Top view. Dashed = relaxed aim · solid = where each eye must point to keep the target single · curved arrow + shaded muscle = the constant pull you feel as strain. Rotation is exaggerated to be visible.