Tolerance to visual defocus

Abstract

Low-resolution optical systems are more tolerant to defocus than are high-resolution systems. We wished to determine whether this principle applies to human vision. We used psychophysical methods to measure the effects of defocus in normal eyes under low-resolution conditions. Modulation transfer of sine-wave gratings was measured as a function of dioptric defocus at low and medium spatial frequencies. We defined the depth of focus at a given spatial frequency to be the dioptric range for which the modulation transfer exceeds 50% of its peak value. For dilated pupils, depth of focus increased from about 2.5 diopters (D) at 3.5 cycles/deg to about 17 D at 0.25 cycles/deg. From our results we predicted that tasks requiring only low spatial frequencies will be more tolerant to defocus than tasks requiring higher spatial frequencies. This prediction was confirmed in a letter-recognition experiment. The increasing tolerance to defocus at low spatial frequencies also implies that individuals with low acuity will be more tolerant to defocus than people with normal vision. We confirmed this prediction by measuring tolerance to defocus in 30 low-vision eyes.

Minimum change in spherical aberration that can be perceived

“…By increasing the amplitude of the movement a threshold was reached at which the subject perceived changes in blur at the moving frequency. From this, they estimated that the minimum value of perceptible focus difference is around 0.2 D. Subsequently, further work [2][3][4][5][6] was carried out to study the effect of different factors such as pupil size, luminance, and object's spatial frequency content.…”

Phenomenological model of visual acuity

“…For healthy eyes without abnormal amounts of high order aberrations (HOA), and for defocus and astigmatism larger than about 0.75 D, blur and pupil size will mainly determine the visual acuity. [3][4][5] However, for small values of second-order aberrations (defocus and astigmatism), visual acuity will depend on the complex interactions among all the factors affecting it.…”

“…Legge et al 4 proposed an inverse relationship between V and defocus after measuring the Snellen visual acuity of four subjects. This model was further refined by Smith,11 after analyzing a number of classical clinical studies and models on refractive error and visual acuity.…”

“…Smith also proposes a functional form that sets V equal to one when the geometrical blur tends to zero. Neither Legge et al 4 nor Smith 11 considered the effect of astigmatism in their models. Raasch 12 proposed the quantification of secondorder blur (including spherical defocus and astigmatism) as the modulus of the optometric vector defined by Thibos.…”

Computational aspects of the through-focus characteristics of the human eye

“…Also, the procedures are lengthy, tedious, and require a certain degree of cooperation from the measured subject, as it is the case in the assessment of just detectable image blur. On the other hand, an objectively measured wavefront aberration contains information from which one can derive through-focus characteristics of the eye's optical system through the so-called virtual refraction paradigm [12,13] and derive the dioptric estimate of the depth-of-field using an appropriately selected threshold to a through-focus characteristic function of the retinal image quality [4,[14][15][16].…”