Question

Aperture and optical centre are only in lenses​

Answer 1

the position of the optical centre depends upon the relative values of the radii of curvature of the two Refracting surfaces of the lens. if the radii of curvature are exactly equal then the optical centre is exactly at the centre of the lens .however , if the radii of the curvature have a large difference , it is possible that optical centre is outside the lens