American Board of Opticianry (ABO) Practice Test

The optical center of a lens is located at the midpoint where the lens has an equal amount of curvature on both sides. When looking at a right lens in a lensometer, you would place the lens in the holder with the concave side (the side that curves inward) facing up. The prescription calls for 1 BU, which means that the bottom of the lens should be turned away from the examiner. Similarly, the prescription also calls for 2 BI, which means that the right side of the lens should be turned towards the examiner. Therefore, the optical center is located at the upper right corner of the lens, making option B the correct answer. The other options are incorrect because they either refer to the incorrect side of the lens (option A and C) or the incorrect direction of the lens (option D).