The Snellen Chart is constructed on the principle that the standard limit for distance visual acuity under good lighting conditions, is a separation of 1 minute of arc: that is, that the normal eye is just able to distinguish as separate, two objects separated by an angle of 1 minute (1/60th of a degree) of arc. (This is angle separating the straight lines drawn from each object to the pupil of the eye - or better, to a point on the retina).

Letters are constructed as shown in the diagram, so that each block subtends 1' (minute) of arc at the eye, when viewed at a stated distance. The entire letter will thus subtend 5' at the prescribed distance. The chart is viewed in good lighting, at 20 ft or 6 m, at which distance the eye should be quite relaxed for distance vision. At that distance, the boxes in letters in the top line (200 ft) should subtend much more than 1', and should be easily discernible. Letters in the 20 ft (6 m) line should be just discernible, as their boxes should subtend just 1' of arc. Letters in the lower line, meant to be discerned at a closer distance, should fall beyond the normal resolution, and should not be discernible. Persons with normal vision therefore should just discern the 20 ft line, when standing at 20 ft (hence 20/20 vision). If they can read the 15 ft line, their vision will be better than normal (20/15). And if they can read the 30 ft line but not the 20 ft line, their vision will be impaired (20/30). The ratio then has the viewing distance at the top, and the normal resolving distance for the last discernible line at the bottom. If at 20 ft, the subjects cannot resolve the top line (200 ft), then they should approach the chart until the letter(s) in this line can be clearly read, and the viewing distance noted. If this distance is 10 ft, then what will be the visual acuity?

[Answer: 10/200]. [Return to Main Document] [See Jaeger Charts]

The Jaeger Charts are a series of passages written in typescripts of different sizes - from very small (J1) to very large (J14). They are designed to test near visual acuity. Persons with normal vision should be able to read the smallest prints in good lighting, at comfortable reading distance. Failure to read below a J6 chart with optimally corrected vision is taken to represent a 50% loss of visual acuity. [Return to Start] [Return to Main Document]