Different aberrations raise contrast thresholds for single-letter identification in line with their effect on cross-correlation-based confusability.

We previously showed that different types of aberration defocus, coma, and secondary astigmatism affect reading performance via different mechanisms. In this paper, we show the contrary result that, for identification of isolated letters, the effects of rendering different types of aberration can be described by a single cross-correlation-based metric. Aberrations reduce the effective resolution of an optical system, quantified by the high-frequency fall-off of the modulation transfer function. They additionally cause spatial-frequency-dependent phase and contrast changes, which have a size-dependent effect on letter forms. We used contrast threshold as our performance measure, instead of distance acuity, to separate the effects of form alterations from those of resolution limits. This measure is additionally appropriate in comparing single-letter-based performance to reading at a fixed distance. The relationship between a cross-correlation-based measure of letter confusability and performance was the same for all three types of aberration. For reading, we had found a different relationship for coma than for defocus and secondary astigmatism. We conclude that even when two tasks--letter identification and reading--use the same component stimulus set, the combination of multiple letters in a reading task produces functional differences between the effects of these aberrations that are not present for isolated letters.