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Alexander Leube, Arne Ohlendorf, Juan Tabernero, Siegfried Wahl; Relationship between the subjectively and objectively determined depth of focus of the human eye using defocus curves. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):6016.
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© ARVO (1962-2015); The Authors (2016-present)
The study compared the depth of focus (DoF) of the human eye, calculated from objective image quality metrics (IQM) and subjectively measured defocus curves.
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.Profilerplus, 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.
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).
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.
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