Sign convention is a set of rules to set signs for image distance, object distance, focal length, etc for mathematical analysis of image formation. According to it: Object is always placed to the left of lens. All distances are measured from the optical centre of the mirror.

1. SING
CONVENTIONS
FOR THE RADII OF
THE LENSES

2. RADIUS OF CURVATURE
O The radius of the circle formed with the curved part of the lens
is known as radius of curvature. We draw a circle with the help
of the curved part of the lens, and locate its center, by
measuring the radius of the circle from the center we get radius
of curvature
CONVEX LENSE :- CONCAVE
LENSE:-

3. SING CONVENTIONS
Radius of curvature has specific meaning and sign convention in optical
design. A spherical lens or mirror surface has a center of curvature located
either along or decentered from the system local optical axis. The vertex of
the lens surface is located on the local optical axis. The distance from the
vertex to the center of curvature is the radius of curvature of the surface.The
sign convention for the optical radius of curvature is as follows:
O If the vertex lies to the left of the center of curvature, the radius of
curvature is positive.
O If the vertex lies to the right of the center of curvature, the radius of
curvature is negative.
O Thus when viewing a biconvex lens from the side, the left surface radius
of curvature is positive, and the right radius of curvature is negative.
O Note however that in areas of optics other than design, other sign
conventions are sometimes used. In particular, many undergraduate
physics textbooks use the Gaussian sign convention in which convex
surfaces of lenses are always positive.Care should be taken when using
formulas taken from different sources.