Lesson 10 - Effective Power and Compensated Power Calculations

Question 1

Vertex

Answer 1

The distance between the front of the eye and the back of the lens in a pair of glasses

Question 2

Effective power

Answer 2

The perceived change in power of a lens when the finished lens is not worn at the distance that the eyes were examined

Question 3

Compensated power

Answer 3

The new power that the new lens will need to have in order to have the same effective power as the original lens when it is placed in the new vertex distance

Question 4

Phoropter

Answer 4

A device used to test eyes that uses a vertex of 12 mm

Question 5

If the minus lenses are too close to the eye, their effective power is more what? If the lenses are too far away from the eye, their effective power is what?

Answer 5

If the minus lenses are too close to the eye, their effective power is more minus. If the lenses are too far away from the eye, their effective power is less minus or more plus.

Question 6

Distometer

Answer 6

A device used to measure vertex distance. It resembles a small protractor with a prong that can be expanded to measure the distance between the eyelid and the glasses.

Question 7

If the plus lenses are too close to the eye, their effective power is more what? If the lenses are too far away from the eye, their effective power is what?

Answer 7

If the plus lenses are too close to the eye, their effective power is less plus. If the lenses are too far away from the eye, their effective power is more plus.

Question 8

If the plus lenses are too close to the eye, their compensated power is more what? If the lenses are too far away from the eye, their compensated power is what?

Answer 8

If the plus lenses are too close to the eye, their compensated power is more plus. If the lenses are too far away from the eye, their compensated power is less plus.

Question 9

If the minus lenses are too close to the eye, their compensated power is more what? If the lenses are too far away from the eye, their compensated power is what?

Answer 9

If the minus lenses are too close to the eye, their compensated power is less minus or more plus. If the lenses are too far away from the eye, their compensated power is more minus.

Question 10

Focal length is calculated how?

Answer 10

Focal length is calculated by the inverse of the power of the lens in meters. The inverse is 1 divided by the power of the lens.

Question 11

How to calculate compensated or effective power

Answer 11

Where D1 is the original lens power.

(D1 x D1)/1000 = A
A multiplied by the change in vertex in millimeters = B
D1 add/subtracted by b = compensated or effective power

adding or subtracting b depends on whether or not the vertex change would result in a stronger or weaker compensated/effective power of the lens.