Focimetry - Intro

Question 1

What does the Focimeter do?

Answer 1

• measures the FVP & BVP of lenses (NOT principal powers)
• measures the power for meridians in the cylinder and toric lenses = cross cyl powers
• if the power of meridians is the same, gives spherical power (a circle of dots)
• works out the sphere/cylinder axis in the two meridians
• measures the prismatic effect, at the optical centre

Question 2

What are the two types of Focimeters?

Answer 2

1. Fixed Target

2. Rotating Target (this has a rotating wheel)

Question 3

Which type of Focimeter is more accurate?

Answer 3

• Rotating Target is more accurate when finding cylinder axis

Question 4

What is the eyepiece used for?

Answer 4

• used to observe/ look through

Question 5

What is the power wheel used for?

Answer 5

• moves the target

- can twist to see the power of the lens

Question 6

What is the lens table used for?

Answer 6

• moves up/ down

- used to rest the glasses/ lenses

Question 7

What is the lens lamp used for?

Answer 7

• holds the glasses being observed in place

Question 8

What does the Focimeter consist of?

Answer 8

1. Observation system

2. Foccussing system

Question 9

What are the features of the Foccussing system?

Answer 9

• collimator
• produces an image of the target
• projected at infinity
• light rays will leave the focussing system as parallel to the axis
• power wheel moves the target

Question 10

What are the features of the Observation system?

Answer 10

• graticule

- eyepiece

Question 11

Define ‘ZERO position’

Answer 11

• you adjust the lens wheel so that the reading is zero
• the target is in front of the collimator so that the parallel rays of light enter the completely relaxed eye
• both graticule and target are in focus
• no lens in the way

Question 12

What does the graticule show?

Answer 12

• consists of a scale

- in 1 prism dioptre steps

Question 13

Define ‘optical centre’

Answer 13

• the target is at the centre of the graticule

Question 14

How do we know if the target is NOT at the optical centre?-

Answer 14

• lens behaves like a prism = prismatic effect

Question 15

How can we measure the prismatic effect?

Answer 15

• measured using the scale going up in 1 PRISM DIOPTERS

Question 16

What are the benefits of the prismatic effect?

Answer 16

• helps people to see comfortably in single vision

Question 17

How do we determine vertex power?

Answer 17

• rotate the power wheel
• moves the target from its ‘zero position’
• if POSITIVE LENS (target can only be imaged if the light enters is parallel) so MOVE TARGET TOWARDS the eyepiece
• if NEGATIVE LENS (target can only be imaged if the light enters is parallel) so MOVE TARGET AWAY from the eyepiece

*twist power wheel

Question 18

What does a Toric lens do to a Fixed Target?

Answer 18

• both toric & cyl lens produces 2 line foci
• light is focused on either the power/ axis meridian (maximally defocused) for each foci
• meridians are always 90 degrees apart
• in between these 2 foci, an elliptical image is produced (this is not focused on any meridian)
• we focus the focimeter target to find the tangential & sagittal foci.

Question 19

What does a Cylinder lens do to a Focimeter target?

Answer 19

• we find the cylinder axes by looking at the orientation of the lines
• each dot of the focimeter target will form two lines in two major meridians
• neutralizing the power in the meridians, will allow these lines to come into focus 90 degrees from each other

Question 20

What does the Focimeter Power tell us?

Answer 20

• We don’t measure sph or cyl power, but power meridians & axes.
• We convert to sph/cyl form, either + or - (very easy to convert)
• Tells us nothing about lens form

Question 21

What is important to note when reading the cyl axis off the external wheel?

Answer 21

• The 2 sets of lines on a rotating target from the centre are different lengths.
• When reading the cyl axis from the external wheel, it gives the axis of the longest line.