<< Abstract

Refraction Training Manual

3. Vision and Visual Acuity

We need to show the difference between visual acuity obtained with and without glasses. The measurement of how well the eye can see detail without glasses is called Vision (V) whilst with correction is called Visual Acuity (VA).

There are several ways that vision and visual acuity can be recorded. The system that seems to be most commonly used is the Snellen Fraction notation. This shows two numbers, the first (numerator) tells us the distance at which the test is done and the second (denominator) is a measure of the eye's standard of vision. This number represents the distance at which a 'normal' eye would be able to see the letter.

Examples

You should decide on which notation to use, based on that used by your colleagues, and use this one exclusively. In this manual we will use Snellen measure in metres and not in feet.

Table of different systems of visual acuity notation

Snellen Letters 6m

Snellen Letters 20ft

Decimal Notation

Percentage Notation

6/6

20/20

1.0

100

6/9

20/30

0.67

67

6/12

20/40

0.5

50

6/18

20/60

0.33

33

6/24

20/80

0.25

25

6/36

20/120

0.17

17

6/60

20/200

0.1

10

Snellen Charts

This is the normal letter chart used for vision testing. They can have normal lettering, reverse lettering (for use with a mirror), 'illiterate E' or numbers. Some charts have pictures, but these are not so successful. The chart is normally designed for use at 6 metres, either directly or in a mirror.

Each eye is tested separately, with the other eye occluded. The patient reads the letters (or indicates which way the 'E' is pointing) until he can read no further. Do not assume that this is his visual acuity, however, and you must encourage him to try to read further! Guessing must be encouraged, as you will often find that the patient can read further. Note that you need to watch the patient to make sure they do not look around the occluder.

Near Vision Testing

There are several charts available for near vision assessment. They are mainly Times Roman' styles, with the letter sizes marked in N point size (eg N5). The Jaegar scale may also be found in use. We normally use the 'N' scale.

To test near vision the patient needs to hold the book at their normal reading distance. Do not tell the patient where this is, let them decide! This will frequently explain their visual problems as they read too close. The normal reading distance is between 33 and 40 cm. This will be covered in more detail later.

We always attempt to obtain N5 vision with each patient. However, N8 is the equivalent of normal newsprint, so we are trying to obtain better vision than necessary. If this is possible, then the normal near vision tasks are made easy and comfortable.

Asking patients, who need to sew, to thread a needle is also a near vision test!

Other Vision Tests

With children, it is often difficult to determine vision with our normal tests. There is a test called the Sheridan-Gardiner test which may overcome these problems. It requires the patient to be able to match letters, without them actually having to know what the letters are.

Problems

In all tests, vision will be affected if:

The Pinhole Disc

If poor vision is found, then use of the pinhole will show us whether the vision may be improved with glasses. This works by reducing the blur circle caused by refractive errors.

The Pinhole Disc

It is important, when checking vision with a pinhole, that the patient is able to see through the pinhole to the chart!

There are a few occasions when you will find improved vision with a pinhole that will NOT improve with glasses. They are all medical conditions:

It is possible to make a pinhole from stiff card or thin plywood (see picture).

Making a pinhole from stiff card or thin plywood.

Making a pinhole from stiff card or thin plywood.