Activity 3B: Working with Eye Charts

Hand out the copy of the eye chart to each student. These particular charts are called ETDRS charts because they were developed for a research program called “Early Treatment of Diabetic Retinopathy Study.” This study was important because it showed that laser treatment of the retina in persons with diabetes could prevent the development of blindness due to diabetic retinopathy. The charts included here are for measuring near visual acuity, not distance vision.

The ETDRS charts have three unique features:

1. Each line has five letters, so there is an equal opportunity to make an error on each line.
2. The letters have been selected so that there is an equal probability that an error may be made in reading them. For example, the letter ‘I' does not appear because it is rarely misread.
3. The letter sizes change from row to row in a standard geometric progression.

On the left hand side of the chart is the height of the letters in millimeters. On the right hand side of the chart is the ‘Snellen Equivalent,' that is, what the visual acuity would be if vision were being tested at 20 feet. Note that for halving the distance from 40 cm (16 inches) to 20 cm (8 inches), the angle of acuity doubles.

Have the students pair up together and take turns testing and recording their visual acuity. Have the recorder hold the chart at 16 inches from the face of the person being tested. Have the student being tested cover their left eye and read with their right eye. Then have the student cover their right eye and read with their left eye. Use the recording sheet for students to place their responses. Then have the students switch roles and repeat the activity. Students should record the visual acuity as the last line on which their partner correctly read at least 4 of the letters correctly.

Students should also measure visual acuity under other conditions as well. They should vary the distance and see what happens to their visual acuity. Graphs may be plotted that show different curves of acuity by distance for the students. This may also be done by keeping the distance fixed and measuring acuity with and without glasses.

The ETDRS charts reproduced here are blurred by the copying process on the smallest rows. Teachers have two options to overcome this problem. First, you can double or quadruple the distance of testing. Doubling the distance will change a 20/100 line to 20/50, quadrupling the distance will change a 20/100 line to 20/25. The second option is to obtain other hand held visual acuity charts from optometrists or ophthalmologists.