Relationship between the subjectively and objectively determined depth of focus of the human eye using defocus curves

Abstract

Purpose: The study compared the depth of focus (DoF) of the human eye, calculated from objective image quality metrics (IQM) and subjectively measured defocus curves.

Methods: 15 subjects with a mean age of 25.5±3.3 years and a mean spherical equivalent refractive error of M=-0.45D±2.46D participated and mydrias was assessed using three drops of 1% cyclopentolat (assessed three times with 10 min between applications). Monocular subjective defocus curves (range: ±1.5D in 0.5D steps) were measured in a distance of 5m in the fully corrected dominate eye using a 4mm artificial pupil. The DoF was calculated as the dioptric range under the defocus curve at the threshold “maximum visual acuity [logMAR] + 0.1”. A commercial aberrometer (i.Profiler^plus, ZEISS, Germany) was used to assess the ocular wavefront. The point spread function (PSF) and the optical transfer function (OTF) were analyzed of the single wavefront aberrations for a 4mm pupil, using Matlab (MathWorks, Natick, USA). The DoF was calculated using the augmented visual Strehl-Ratio of the OTF (VSOTFa) at the thresholds 80% and 50% of the maximum value as well as the visual Strehl-Ratio of the PSF (VSPSF) at a threshold of 50% of the maximum value. A two-tailed Student’s t-test was used for statistical analysis.

Results: Using the VSOTFa, the DoF was 0.41±0.08D for the 80% and 0.76±0.11D for the 50% threshold, while the DoF was 0.71±0.12D for the VSPSF at the 50% threshold. Subjective assessment of the DoF gave a mean value of 0.70±0.23D and showed a correlation to the individual RMS of the higher order aberrations (RMS HOA) (r=0.677, p=0.006). DoF was significantly different for 80% VSOTFa (p<0.001) and 80% VSPSF (p<0.001) compared to the subjective DoF, while the DoF at 50% VSOTFa (p=0.349) and 50% VSPSF (p=0.730) was not. Nevertheless, there was no significant relationship between the metrics and subjective measurements of DoF (50% VSOTFa r=0.20; 50% VSPSF r= 0.24). Individual thresholds for VSOTFa showed a significant correlation with the RMS-value of the HOA (r=-0.621, p=0.013).

Conclusions: The estimation of the DoF using the VSOTFa and the VSPSF at a 50% threshold showed no significant difference to the subjectively measured DoF, but lacked a significant correlation. To predict the DoF from objective wavefront measurements, we propose the use of the VSOTFa at an individual threshold that is estimated from the RMS HOA.