O object distance The distance from the vertex to the object. From the thin-lens equation, the image distance s for a thin-lens, whose focal length is f , can be calculated from object distance s′ as

1 s

= 1 f +

1 s′ ,

1 f

= (n′ − n)

n

( 1 R

+ 1 R′

) where the radius surface of the lens isR andR′. The object distance of a thick lens should be measured from the principal planes of the lens. See image distance.