Activity 3A: Measuring Visual Acuity

Visual acuity is a measure of the resolving power of the eye, particularly with its ability to distinguish letters and numbers at a given distance. This is the measure that is made at an optometrist's office when a person is having an eye examination.

Other important aspects of vision include:

1. Visual Field - the scope of what you see.
2. Contrast Sensitivity - the ability to discern between two objects at the same distance. For example, the ability to see the edge of a carpet under low light.
3. Depth Perception - the ability to judge the relative distance of objects in space and to orient one's position to them. For example, the ability to see the edge of stair or the edge of a curb. Depth perception depends on good binocular vision.

Measuring Visual Acuity:

Use the handout "MEASURING VISUAL ACUITY." Visual acuity is a function of the height of the letter to be seen and the distance from which it is viewed. The measure of visual acuity is the angle formed between the eye and the letter. The angle is recorded as 'minutes of arc' of a circle. So when an optometrist tells you have 20/40 vision, this means that at a distance of 20 feet, the smallest letter you could see had an angle of 40 minutes of arc. The angle will change as the size of the letters change or as the distance changes. The visual acuity is reported as a ratio with the top number being the distance and the bottom number the angle formed by the letter and the eye.

Try some problems with the students by changing the distance or the height of the letters:

1. Suppose the distance was cut in half (from 20 feet to 10 feet). What would happen to the angle formed with the letter? It would double to 80 minutes of arc and the acuity would be recorded as 10/80.
2. Suppose the distance was doubled from 20 feet to 40 feet. What would happen to the angle formed with the letter? It would be cut in half to 10 minutes of arc and the acuity would be recorded as 40/10.