Since light passes through a lens, the R side is always the "back" of the lens. This designation contrasts with the R side of a mirror, which is always the front of the mirror.
The determination of sign for lenses obeys the following conventions:
(1) o is positive when the object is on the V side (the side of the origin of the ray), negative when the object is on the R side (the side of the destination of the ray)
(2) i is positive when the image is on the R side, negative when the image is on the V side
(3) r is positive for lenses with the first surface convex (center of curvature on R side) , negative for lenses with the first surface concave (center of curvature on V side)
(4) f is positive for converging lenses, negative for diverging lenses
(5) m is positive when the image is rightside up, negative when the image is upside down
