What is the true power of a CR-39 lens with a marked power of +4.00D front curve and -6.00D inside curve?

To determine the true power of a CR-39 lens marked with a front curve of +4.00D and an inside curve of -6.00D, you need to consider how the two curves interact to produce the overall lens power.

The power of a lens is calculated using the formula for the combined power of two spherical surfaces:

[ P = P_1 + P_2 ]

Where ( P_1 ) is the power of one surface and ( P_2 ) is the power of the other surface. The starting point is that each surface curve can be expressed in diopters based on its radius of curvature, with the power of a lens being positive for convex (plus) and negative for concave (minus) surfaces.

In this case:

• The front curve contributes +4.00D (a convex surface).

• The inside curve contributes -6.00D (a concave surface).

Therefore, we combine these values:

[ P = +4.00D + (-6.00D) ]

[ P = 4.00D - 6.00D ]

[ P = -2.00D ]

This calculation results in a net power of -2.00