If a +12.00 lens is prescribed at a vertex distance of 13mm and the optician fits glasses at 9mm, what is the new power of the lens at 9mm?

To determine the new power of a lens when the vertex distance changes, it's necessary to apply the lens formula that relates the power of the lens to its vertex distance. When a lens is moved closer to the eye (decreasing the vertex distance), the effective power of the lens will decrease.

In this scenario, a +12.00 diopter lens at a vertex distance of 13 mm is prescribed. However, the glasses are fitted at 9 mm, which is closer to the eye. The formula used to calculate the change in power based on the vertex distance is:

New Power (D) = Old Power (D) - (2.00 * (F - V))

Where:

• F is the original vertex distance (13 mm)
• V is the new vertex distance (9 mm)

Converting these values to meters gives us F = 0.013 m and V = 0.009 m, and when we plug in the numbers, we find that:

New Power = +12.00 - (2.00 * (0.013 - 0.009)) = +12.00 - (2.00 * 0.004) = +12.00 - 0.008 = +