Week 10: Focimetry

Question 1

What does a Focimeter/Vertometer/Lensmeter do?

Answer 1

• Measure the prescription of a lens
• Find the optical centre of a lens
• Prepare a lens for edging

Question 2

In an optometric practice, a focimeter is likely to be found in?

Answer 2

• Optometrist’s consulting room
• Dispensing/back room

Question 3

Situations Focimeter Would be Used? List 5

Answer 3

1. Find optical centre
2. To prepare a lens for edging
3. To check prescription
4. Px brings in glasses not prescribed at practice
5. To check px’s specs are correct

Question 4

What are the Target Types for focimeter and describe briefly?

Answer 4

1. Crossed-line target
   - 2 sets of lines at 90o to each other
   - In a lens with cylinder, none of the lines will become completely clear unless they are parallel to one of the principal meridians of the lens
2. Ring-of-dots target
   - Target consists of a circle of dots
   - When testing a lens with cylinder, the dots become drawn out into focal lines at right angles to the principal meridians of the lens

Question 5

How do you find the Optical Centre of Lens?

Answer 5

• Optical centre (OC) of a lens is the point at which light is not refracted
• Typically want the patient to be looking through the optical centre of the lens, as there is no distortion here
• Optical centre of the lens is found by placing the lens in the focimeter, and moving the lens until the target is in the centre of the graticule

Question 6

Describe meridians

Answer 6

• Any line through the centre of the lens a meridian
• All meridians in a spherical lenses will have the same power

Question 7

Describe emmetropia

Answer 7

• Far point located at optical infinity (when eye is unaccommodated)
• Near point located between far point and eye (depending on accommodation)
• Axial length correct for the light to focus on retina

Question 8

Describe ametropia

Answer 8

• Far point is the point at which an object must be placed along the optical axis of the eye for its image to be focused on the retina when the eye is unaccommodated
• Optical axis of the eye is an imaginary line passing through the centre of the lens to the retina

Question 9

Describe myopia

Answer 9

• Far point is real & located closer than optical infinity, in front of the cornea
• Near point located between far point & eye (depending on accommodation)
• Correction with minus (concave) lenses

Question 10

Describe hyperopia

Answer 10

• Far point – virtual and behind the cornea
• Correction with plus (convex) lenses

Question 11

What are the types of hyperopia and describe briefly?

Answer 11

1. Manifest
   - Component of absolute hyperopia that cannot be corrected by accommodation
2. Latent
   - Amount of hyperopia that is overcome by accommodation

Question 12

Describe the relationship between accommodation and hyperopia

Answer 12

• Young hyperopes are able to move far point of the eye onto the retina via accommodation
• To determine actual amount of hyperopia, need to relax accommodation (pharmacologically)

Question 13

Describe what astigmatism is?

Answer 13

• Parallel rays of light do not form a single point focus at near or distance
• Two points of focus
• Refractive power of the eye is not equal in all meridians (usually due to the shape of the cornea)
• Light does not focus at a single point on the retina
• Corrected with a cylinder lens or toric lens

Question 14

What is principal meridians?

Answer 14

Meridians of greatest & least power

Question 15

Describe regular astigmatism

Answer 15

Principle meridians are 90 degrees apart
- With-the-rule: vertical meridian is steeper
o Correcting minus cylinder axis is 180o ± 20o)

-Against-the-rule: horizontal meridian is steeper
o Correcting minus cylinder axis is 90o ± 20o)

• Oblique: in between horizontal and vertical meridians

Question 16

Describe irregular astigmatism: two principle meridians are not 90o apart

Answer 16

Two principle meridians are not 90o apart
- Curvature of any one meridian is not uniform
- Associated with trauma, disease or degeneration

Question 17

Describe Interval of Sturm

Answer 17

Distance between the anterior & posterior focal lines in a sphero-cylindrical lens combination

Question 18

Describe circle of least confusion

Answer 18

Smallest circle that encloses the beam at its narrowest point

Question 19

Briefly describe Optics of Astigmatism

Answer 19

• POWER in vertical meridian projects a horizontal focal line
• POWER in the horizontal meridian projects a vertical focal line
• AXIS is perpendicular to the power of the lens/eye and therefore in the same direction as the focal line

Question 20

How do you assess astigmatism Clinically

Answer 20

• Optometry tool to refine astigmatic power and axis during refraction
• Cross cyls we use clinically +/- 0.50 cross-cyl is commonly used

Question 21

What is Simple Myopic Astigmatism?

Answer 21

• When one of the principal meridians is focused in front of retina and the other is focused on the retina (with accommodation relaxed)
• With-the-rule (correcting minus cylinder axis is 180o ± 20o)

Question 22

What is Compound Myopic Astigmatism

Answer 22

• When both principal meridians are focused in front of retina (with accommodation relaxed)

Question 23

What is Simple Hyperopic Astigmatism?

Answer 23

• When one of the principal meridians is focused behind retina and the other is focused on the retina (with accommodation relaxed)
• With-the-rule (correcting plus cylinder axis is 180o ±20o)

Question 24

What is Compound Hyperopic Astigmatism?

Answer 24

• When both principal meridians are focused behind the retina (with accommodation relaxed)

Question 25

What is Mixed Astigmatism?

Answer 25

• When one of the principal meridians is focused in front of retina and the other is focused behind the retina (with accommodation relaxed)