Abstract
Crowding distance (the critical centre-to-centre distance between optotypes to escape crowding) in adult central vision is ~0.05 deg. This is smaller than a standard 20/20 letter (size=0.08 deg) so it needs to be measured with a skinny optotype, like the Pelli font, that allows very close spacing. Compared to visual acuity, crowding distance is larger in strabismic amblyopia and better correlates with critical spacing for reading, giving it high clinical value. Relative to adults, crowding distance is 3-5x larger in 3-year old children, which might be big enough to measure without need for a skinny font. Crowding distance was estimated in 201 children (3-11 years) and in 16 adults using a 9AFC paradigm with 1) Pelli optotypes (1:2 stroke:letter width) in trigram and full-screen repeated arrangements, and 2) standard Sloan letters (1:5 stroke:letter width) where the target had 4 surrounding flankers. The spacing to width ratio was 1.4x, so size covaried with spacing. The QUEST procedure estimated required centre-to-centre letter spacing. Results with Sloan letters are consistent with those of previous researchers who used other standard optotypes and varied spacing in central vision of children and adults. Threshold centre-to-centre spacing for 3-year olds was not significantly different across optotypes (p > 0.10) with a mean of 0.25±0.03 deg (n=11). For adults, crowding distance was 0.05±0.02 deg with Pelli optotypes, significantly different (p < 0.01) from the 0.09±0.01 deg estimate for Sloan letters, which are artifactually limited by letter width. Differences were significant (p<0.01) between Pelli and Sloan optotypes for ages 5 and up. It is well known that measuring central crowding distance instead of just acuity increases sensitivity to amblyopia. We find that for ages 5 and up this cannot be done with standard letters, but can be done with a skinny optotype, like the Pelli font.