American Board of Opticianry (ABO) Practice Test

If a lens has a front curve of +4.50, which combination of back curves would be required to create a lens with a power of -2.00 -1.00 x 180?

1. -1.00; -2.00

2. -6.50; -7.50

3. -2.00; -2.00

4. -1.00; -1.00

The correct answer is: -6.50; -7.50

To understand the correct answer, we must first understand the concept of lens powers and the two types of curves on a lens. A lens power refers to the strength of a lens, in this case, -2.00 -1.00 x 180 is a prescription requiring a lens with a power of -2.00 at the center and an additional power of -1.00 in the axis of 180 degrees. The two curves on a lens are the front curve and back curve, with the front curve being the surface facing the eye and the back curve being the surface facing away from the eye. Option A is incorrect because it does not provide enough power in the back curve to counteract the +4.50 front curve, resulting in a final lens power that is much stronger than the prescribed -2.00. Option C is also incorrect because it provides the same power in the back curve as the front curve, resulting in a lens with a power of +2.50 at the center and an additional power of -1.00 in the axis of 180 degrees. Option D is incorrect because it only provides -1.00 power in the back curve, which is not enough to balance out the +4.