By use of a blur-pseudostimulus technique and high-speed infrared optometric measurements, the singularity of blur as a stimulus to human accommodation was studied. Blur is not the sole stimulus; it is a necessary cue, but not a sufficient one. The accommodative system makes use of one or more available odd-error (error sign) cues, which are believed to supplement blur with requisite focusing information.
Leonard M. Smithline, "Accommodative response to blur," J. Opt. Soc. Am. 64, 1512-1516 (1974)