Letter identification is not scale invariant!

Using critical-band masking, we characterized the channels mediating identification of letters in several fonts and alphabets (Bookman, Künstler, Sloan, and Chinese) over a wide range of size (0.07 to 10 deg). Line frequency is the average number of lines crossed by a rule through a letter, divided by the letter size. We expected channel frequency to be proportional to line frequency, but instead we find that it grows as only the 2/3 power of line frequency. Furthermore, the efficiency of letter identification in noise rises and falls as a function of line frequency, consistent with a simple model of the effect of the mismatch between channel and line frequency.