Sign Convention for Lenses

Following sign convention is used for measuring various distances during the formation of images by lenses:

The distances measured in the direction of incident rays are positive and all the distances measured in the direction opposite to that of the incident rays are negative.

All distances measured above the principal axis are positive. Thus, height of an object and that of an erect image are positive and all distances measured below the principal axis are negative.

The following table gives the sign convention for lenses:

Type of lens	u	v		f		Height of the image (h_I)		Height of the object (h_O)
		Real	Virtual	Real	Virtual	Real	Virtual
Convex	-	+	-	+	No virtual focus	-	+	+
Concave	-	No real image is formed	-	No real focus	-	No real image is formed	+	+