W10: Focimeter

Question 1

What is a focimeter?

Answer 1

An instrument used to:
- Measure the prescription of a lens
- Find the optical centre of a lens
- Prepare a lens for edging.

Question 2

Focusing the eyepiece

Answer 2

For correct measurements to be taken, the eyepiece must first be focused.
1. Turn the eyepiece fully anticlockwise such that the graticule is out of focus.
2. Looking down the eyepiece, turn the eyepiece clockwise until the graticule first comes into a clear focus. If you focus it any further than this point your eye will be over working.
3. Focus the focimeter target with no lens in place – it should read zero power

o If the reading is not zero, the focimeter needs recalibration.
o In the mean-time you will need to account for the misalignment in the power reading in your measurements.

Question 3

Finding the optical centre of the lens

Answer 3

The 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.
o Other clue: If you have the crossed line target, then each target will be aligned with the
graticule axis. If only one line target is aligned and the other is off-centre, this means that
you are not at the exact optic centre

Question 4

MEASURING SPHERICAL LENSES

Answer 4

The simple procedure is as follows;
1. Focus the eyepiece and set a comfortable viewing position.
2. Place the concave/back surface of the lens against the lens rest, supporting the bottom of the lens against the frame table. Lock the lens into place.
3. Move the lens and frame table in order to place the target in the centre of the graticule.
4. Rotate the power drum until the target is clear and focused.
5. Read off the power. E.g. + 3.50 DS

Question 5

Emetropia

Answer 5

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

Question 6

Myopia

Answer 6

o Far point (farthest point of clear vision
without accommodation) is real and located
closer than optical infinity, in front of the
cornea.
o Near point located between far point and eye
(depending on accommodation)
o Correction with minus (concave) lenses

Question 7

Hyperopia

Answer 7

Far point – virtual and behind the eye

Two types:
1) Manifest
o Component of absolute hyperopia that cannot be corrected by accommodation
2) Latent
o Amount of hyperopia that is overcome by accommodation

Correction with plus (convex) lenses

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

Question 8

Astigmatism

Answer 8

o Refractive power of the eye is not equal in all meridians
o Usually due to the shape of the cornea
o Football ball (astigmatism) versus soccer ball
(myopia/hyperopia)
o Light does not focus at a single point on the retina
o Corrected with a cylinder lens or toric lens

Question 9

Types of astigmatism:

Answer 9

Two meridians in astigmatism
o Meridians of greatest and least power are called principal meridians

Regular astigmatism : Principal meridians are 90o 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

Irregular astigmatism: two principal meridians are not 90o apart
o Curvature of any one meridian is not uniform
o Associated with trauma, disease or degeneration
o Vision is often not correctable to 6/6 (0.00 logMAR) using spectacles

Question 10

Interval of Sturm

Answer 10

The distance between the anterior and posterior focal lines in a spherocylindrical
lens combination.

Question 11

circle of least confusion

Answer 11

The smallest circle that encloses the beam at its narrowest point