Lecture 2: Effective Power

Question 1

Back Vertex Power: What is it?

Answer 1

Reciprocal of the distance from the back pole of the lens to the Secondary Focal Point (1/fv)

Question 2

Placement of Correcting Lenses

1. Where is the Correcting lens placed?
2. Power of the Spectacle Lens is universally understood to mean its what?
   a. What is it?
3. What is the main advantage of labeling Ophthalmic lenses by their BACK VERTEX POWER?

Answer 2

1. In the Spectacle Plane
2. its BACK VERTEX POWER
   a. Reciprocal of distance in Meters from Back Vertex of the lens to its 2ndary Principle Point.
3. Fitting the lens at the Same vertex distance from the cornea means we get the same back vertex power thus it has the SAME FFECT on the Pts Vision even if they were composed of different lens forms or different lens thickness.

Question 3

Effective Power: Define

Answer 3

Lens POWER NEEDED to replace the original lens at a NEW POSITION and still MAINTAIN the SAME FOCAL POINT

Question 4

Lens Power Effectivity: Plus Lenses

1. As VERTEX DISTANCE INCREASES, a PLUS Lens is what?
   a. What does this mean?

b. The EFFECTIVE (COMPENSATING) POWER Needed to Maintain the Same FOCAL POINT is what?

Answer 4

1. is MORE EFFECTIVE
   a. It brings about MORE Change in Vergence than needed

b. is DECREASED

(This is the OPPOSITE for MINUS LENSES)

Question 5

Lens Power Effectivity Eqn

Fn = ?

When is (d) Positive?

Answer 5

Fo/(1-dFo)

*When the Lens is MOVED CLOSER to the EYE

Question 6

Effective Power Problems

1. Hyperope: INCREASING the VD what will happen?
   a. Decreasing the VD?

Answer 6

1. Plus power of specs INCREASES, so you have to DECREASE Rx to compensate. (Pt that needs stronger add will move readers down her nose)
   a. Plus power of specs will decrease, so you have to INCREASE the Rx to compensate.

(all of this is OPPOSITE for a MYOPE!!)