It is possible to estimate the depth of focus (DOF) of the eye directly from wavefront measurements using various retinal image quality metrics (IQMs). In such methods, DOF is defined as the range of defocus error that degrades the retinal image quality calculated from IQMs to a certain level of the maximum value. Although different retinal image quality metrics are used, currently there have been two arbitrary threshold levels adopted, 50% and 80%. There has been limited study of the relationship between these threshold levels and the actual measured DOF. We measured the subjective DOF in a group of 17 normal subjects and used through-focus augmented visual Strehl ratio based on optical transfer function (VSOTF) derived from their wavefront aberrations as the IQM. For each subject, a VSOTF threshold level was derived that would match the subjectively measured DOF. Significant correlation was found between the subject's estimated threshold level and the HOA RMS (Pearson's *r* = 0.88, *p* < 0.001). The linear correlation can be used to estimate the threshold level for each individual subject, subsequently leading to a method for estimating individual's DOF from a single measurement of their wavefront aberrations.

^{2}. During the test, the subject is asked to focus on the letter in the middle of the first line of the letter chart. Through the optics, the letter size produces a visual angle of approximately 20 min of arc (0.60 logMAR detail, similar to reading print of 12 point font size at a distance of 40 cm away).

*OTF*

_{DL}(

*f*

_{x},

*f*

_{y}) denotes the diffraction limited optical transfer function,

*CSF*

_{N}(

*f*

_{x},

*f*

_{y}) is the neural contrast sensitivity function, and (

*f*

_{x},

*f*

_{y}) are the spatial frequency coordinates. Here the VSOTF was based on calculated optical transfer function across all spatial frequencies up to 60 cycles per degree (Iskander, 2006).

*F*(

*r, θ*), is calculated from the resampled wavefront

*W*(

*r, θ*) using the method of the refractive Zernike power polynomials (Iskander, Davis, Collins, & Franklin, 2007):

*Z*{·} denotes the wavefront to refractive power transformation.

*F*

_{Zer}and

*F*

_{SC}is the refractive power calculated from the subject's original wavefront and the estimated best S/C, respectively. To simulate through focus, in the through-focus loop, a desired level of defocus is added to the refractive power from step 4. In step 5, an inverse transformation from the refractive power domain to the wavefront domain is performed (Iskander, Davis, & Collins, 2007):

*W*

_{out}(

*r, θ*) with a new defocus value, the corresponding point spread function and the optical transfer function (OTF) is calculated using fast Fourier transforms (Artal, 1990; Iskander, Collins, Davis, & Carney, 2001). The through-focus VSOTF is obtained in step 7. The calculation was repeated in a total of 49 steps corresponding to a defocus level ranging from −3 D to +3 D in 0.125-D intervals.

*D*

_{2}−

*D*

_{1}gives the closest match to the subjectively measured DOF. This threshold value was taken as the matching threshold to estimate the DOF for this subject. The same procedure was performed for measurements of each individual subject.

*SD*(standard deviation). Collected data including subjective DOF, individual matching thresholds, and HOA RMS in both 5-mm and 3.5-mm pupils were tested for normal distribution. For correlating the estimated VSOTF threshold values with other measures of retinal image quality, Pearson's correlation coefficient was calculated.

Pupil size | Subjective DOF (D) | Matching threshold (%) | HOA RMS (μm) | Z(4,0)
(μm) | DOF (D) for a 50% threshold | DOF (D) for an 80% threshold |
---|---|---|---|---|---|---|

5 mm | 0.79 ± 0.15 | 65.6 ± 10.1 | 0.30 ± 0.08 | 0.075 ± 0.062 | 1.12 ± 0.34 | 0.58 ± 0.17 |

3.5 mm | 1.30 ± 0.21 | 36.9 ± 18.4 | 0.12 ± 0.05 | 0.020 ± 0.015 | 1.07 ± 0.54 | 0.52 ± 0.24 |

*r*= 0.36,

*p*> 0.05) and between subjective DOF and SA (

*r*= 0.24,

*p*> 0.05). The matching threshold showed significant correlation with the total HOA RMS (Pearson's

*r*= 0.88,

*p*< 0.001). Moderate correlation was shown between the estimated threshold and the spherical aberration value in the eye (

*r*= 0.52,

*p*= 0.05). For a 3.5-mm pupil diameter, there was no significant correlation observed between the DOF and HOA RMS. There was weak correlation between DOF and SA (

*r*= 0.49,

*p*> 0.05). No correlation was found between the estimated threshold and the spherical aberration value (Pearson's

*r*= 0.36,

*p*> 0.05). However, significant correlation was found between the estimated threshold and the HOA RMS (Pearson's

*r*= 0.62,

*p*< 0.05).

*r*= 0.88,

*p*< 0.001) in a 5-mm pupil. By fitting a linear function to the data, we obtained

*r*= 0.88,

*p*< 0.001 to

*r*= 0.77,

*p*< 0.003.

*r*= 0.68,

*p*= 0.025).