"Bothersome blur": a functional unit of blur perception.

Knowledge regarding the amount of blur perceived to be "bothersome" to an individual, namely that which is assumed to be annoying and to adversely affect task performance, remains limited. A Badal optical system was used to measure the blur detection, bothersome blur, and non-resolvable blur dioptric thresholds monocularly either to an isolated 20/50 or 20/200 Snellen E, or to three 20/50 lines of text. Subjects were comprised of 13 visually normal young adults and 3 absolute presbyopes. Cycloplegia was used to paralyze accommodation in the young adults. Within each target type for the young adults, the mean bothersome blur threshold was always significantly larger than that found for blur detection and significantly smaller than that found for non-resolvable blur. Across target types and blur criteria, the bothersome blur thresholds for the isolated 20/50 E (1.02 D) and the 20/50 text (1.34 D) were not significantly different, although in 12 of the 13 subjects the latter were larger (p<0.002, sign test). However, both were significantly smaller than for the isolated 20/200 E (1.80 D). In a subset of young adult subjects, bothersome blur was found to be repeatable over time. The results were similar in the absolute presbyopes. The bothersome blur threshold was primarily influenced by target detail and secondarily by target extent. These findings have important implications with respect to tolerances for optical lens design and refractive surgery outcomes, as well as provide insight into basic aspects of human blur perception.