Lecture 3 - Lens Power Specification and Measurement

Question 1

When the back side of a lens is being referenced with lensometry, what is being measure?

Answer 1

BVP. Back Vertex Power

Question 2

If nothing comes into focus, with a lensometer, what should you do as your next step?

Answer 2

Turn both the power drum and cylinder axis

Question 3

When using the lensometer, minus cylinder format power will be the _______ reading

Answer 3

First.

Note: It will be the second reading

Question 4

Why are glasses flipped around when a bifocal is being tested?

Answer 4

You will have less error due to the sphere addition

Question 5

If the BVP is 060 degrees, what will be the FVP?

Answer 5

120 degrees.

Question 6

What is the formula for target movement?

Answer 6

X=fs^2 (Fv)

X= lensometer target mvt. in meters
fs = std. lens focal length in meters
Fv = lens power in diopters

Question 7

1) A lensometer has a +20D std lens. How far does the target move in measuring a +1.00D lens?
2) If the same lensometer power drum moves from +20 D to -20D reading, how far would the target move?

Answer 7

x=(0.05)^2 (1)
x = 2.5 mm

x= (0.05)^2 (40)
x=.1m = 100 mm

Question 8

Given an Rx +3.00 - 6.00 x 180. How far does the target move between the 2 meridians if the std. lens is +25 D.?

Answer 8

x = fs2 (Fv)
= (0.04)2(?)
= (0.04)2(6)
= 0.0096 m = 9.6 mm

Question 9

• A lensometer with a +20 D std. lens.
• The single line focuses 5 mm behind the plano position.
• The triple line focuses 6.25 mm in front of the plano position.
• The axis indicator reads 45° What is the Rx?

Answer 9

x = fs2 (Fv)

• .005 = (0.05)2 (Fv)
• 2.00 = Fv

x = fs2 (Fv)
.00625 = (0.05)2 (Fv)
+2.50 = Fv

+2.50 -2.00 x 135
Note: The axis was rotated from its original position 45 to 135