The goal of the EDoF design was to enlarge the binocular DoF so that accommodative responses caused by vergence responses to the displayed disparity stimulus could be maximized. The resulting DoF allowed accommodation to follow the vergence response via cross-link interactions described as convergence accommodation (
Fincham & Walton, 1957). The monocular DoF was extended by combining first- and second-order spherical aberrations (
\(Z_4^0,\;Z_6^0\)), which are proven to effectively extend the DoF (
Benard, Lopez-Gil, & Legras, 2010). The monocular image quality varied across a range of defocus, and a peak in the quality was observed at one polarity of the DoF. By creating opposite signs of spherical aberrations in the two eyes, two peaks of image quality were located at opposite polarities of the DoF. The superior eye at either side improved the perceived binocular image. This method could optimize binocular image quality with the least sacrifice in stereoacuity. Spherical aberrations contain equivalent dioptric power (
Salmon, West, Gasser, & Kenmore, 2003); thus, different defocus power was added to compensate for the offset caused by the different signs in spherical aberrations.
Table 1 shows the coefficient magnitudes of the profiles for each eye. In all experiments, the subject's native aberrations (including the change in spherical aberrations with accommodation) were corrected before the EDoF profile was induced.
Figure 2A shows the simulated images of a Maltese cross perceived by the subjects with respect to accommodation change. As shown in
Figures 2B and
2C, image quality at various accommodation levels was examined by calculating the Pearson's correlation coefficient (
Zheleznyak, Sabesan, Oh, MacRae, & Yoon, 2013) between a 20/40 size letter “E” convolved with the EDoF profile and the reference image (perfect 20/40 letter “E”), where 1 corresponded to a perfect image (
Zheleznyak et al., 2013). Compared with the monovision profile with 1.5 D negative power added to the left eye (
Figure 2B), for EDoF both eyes had balanced retinal image qualities through the accommodation range from 0 D to 3 D (
Figure 2C). The peaks of image quality were observed at 0 D (right eye) and 2.5 D (left eye), as shown in
Figure 2C.