A. Colenbrander

California Pacific Medical Center
Smith-Kettlewell Eye Research Institute
2340 Clay Street,
San Francisco, CA 94115,
U.S.A.

The widespread belief that distance acuity and reading acuity are poorly correlated is counter-intuitive, since both are based on visual angle. This paper will show that there is a correlation and explore factors that may obscure it.

For reading vision, Snellen’s formula: V = d / D or m / M (viewing distance / letter size) is awkward, since the viewing distance is a fraction-within-a-fraction. If the viewing distance is a fraction of a meter, its reciprocal (known as diopter) is a whole number. Recording the viewing distance in diopters (D) changes Snellen’s formula from V = m / M to 1 / V = M x D, which is much easier to calculate. Use of diopters has the additional advantage that it relates directly to the required reading add.

For 150 patients the letter chart acuity at 1 meter (V = 1 / M) was plotted against the reading acuity (1 / V = M x D). All values follow the diagonal of the plot. In half of the cases the discrepancy was not more than +/- 1 line. In most other cases the magnification need for reading (1 / V) was 2, 3 or sometimes 4 lines greater (visual acuity 4 lines worse). A few cases (3%) with greater discrepancies all had special conditions that explained the discrepancy.

The poor correlation that is often claimed can be mainly attributed to: (1) poor accuracy in near-vision calculations, and (2) differences in criterion (threshold for letter chart acuity vs. comfort for reading acuity).