Hey folks,

Got a quick questions here. When I was working in a lab a few years ago I remember that that base curve of a lens is always on the convex side of a meniscus lens. And I have only worked with minus cylinder form lenses (do any labs even do plus cylinder form lens anymore?) so that was easy enough for me to remember since the convex side was always the spherical side the base curve is situated on.

But now I am reading through Systems for Ophthalmic Dispensing and Optical Formula Tutorial book and they seem to have contradictory definition of where the Base Curve is situated on a plus cylinder lens. Systems states that the base curve is always on the convex side of a lens so that in a plus cylinder lens, the base curve is the flatter curve on the front toric surface. The Optical Formula Tutorial book also defines the base curve of a toric surface as the flatter curve BUT states that in a plus cylinder lens, the base curve is on the concave side (where the spherical surface is situated). Incidentally Systems did mention that a lot of manufactures/sources define the backside spherical surface of a plus cylinder form as the back base curve.

So for a plus cylinder lens, is the base curve situated on the convex or concave side?

Thanks in advance!