The appearance of correct focus might reflect learning the average blur we are exposed to and associating that with the structure of the world. An alternative is that visual coding is adapted to compensate for retinal image blur, in the same way that color appearance is compensated for the spectral biases present in the scene (e.g., because of the illumination) or the eye (e.g., because of filtering by the lens or macular pigment). There is now substantial evidence that the visual system does adapt or adjust to changes in the level of blur. For example, adaptation to optically induced blur has an effect on acuity (George & Rosenfield,
2004; Mon-Williams, Tresilian, Strang, Kochhar, & Wann,
1998; Pesudovs & Brennan,
1993; Rajeev & Metha,
2010) and contrast sensitivity (Mon-Williams et al.,
1998; Rajeev & Metha,
2010); and adapting to images with varying levels of blur induces strong biases in the shape of the contrast sensitivity function measured both psychophysically (Webster & Miyahara,
1997; Webster, Mizokami, Svec, & Elliott,
2006) and in single cells in primary visual cortex (Sharpee et al.,
2006). Moreover, adaptation to blurred images has a dramatic effect on the appearance of blur (Battaglia, Jacobs, & Aslin,
2004; Elliott, Hardy, Webster, & Werner,
2007; Vera-Diaz, Woods, & Peli,
2010; Webster, Georgeson, & Webster,
2002; Webster et al.,
2006). Specifically, after adapting to images that are blurred (or sharpened) by “distorting” the ratio of low to high spatial frequency content, a physically focused image appears too sharp (or blurred), so that the point of best subjective focus is shifted toward the prevailing frequency content of the adapting images.