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Y. Benard, H. Rouger, R. Legras; Tolerance to Blur and Image Quality Metrics. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1116.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate whether tolerance to dioptric spherical defocus could be derived from through-focus image quality metrics when testing in various conditions of monochromatic aberrations.
Subjective depth-of-focus (DOF) was defined as the range of defocus for which the target (three 20/50 high-contrast letters) was perceived acceptable. We used an adaptive optics system to dynamically control the observer’s wavefront aberration. The subject viewed the target through an artificial pupil conjugated to the pupil plane, and modified the mirror shape to change the defocus blur. The subject’s head was stabilized with a bite bar. In a pre-experiment, we measured the subjective DOF of 3 subjects in two conditions of dynamic aberrations correction (i.e. sphero-cylinder corrected aberrations with a 2.5mm and 5mm pupil diameter) with and without cyclopegia. The average difference was null and not significant (p=0.92). In fact, if the subject tried to accommodate, the mirror shape changed immediately (1 Hz) to maintain the desired defocus blur constant. We measured DOF of 11 subjects, aged between 20 and 37 years old, when dynamically (i) correcting the entire observer’s eye aberrations, and (ii) correcting the 2nd order aberrations. The pupil size was 5mm except for condition (ii), which was also assessed using a pupil diameter of 2.5mm. In a second experiment, we measured DOF of 12 subjects, aged between 20 and 37 years old, when dynamically (ii) correcting the 2nd order aberrations, (iii) correcting the 2nd order aberrations and simulated a center-near multifocal contact lens, and (iv) correcting the 2nd order aberrations and simulated a center-distance multifocal contact lens. The pupil size was 5mm. For each aberration condition, we calculated various through-focus MTF and OTF based metrics. DOF was defined as the width of the curve at half the peak value.
Inter-subject DOF measures showed large variability. In the first experiment, we obtained a DOF of 1.38 ± 0.47D (condition i), 1.47 ± 0.56D (condition ii with a 5mm pupil size) and 1.72 ± 0.54D (condition ii using a pupil diameter of 2.5mm). These results are comparable to the literature. In the second experiment, we obtained a DOF of 1.57 ± 0.43D (condition ii with a 5mm pupil size), 1.62 ± 0.61D (condition iii) and 1.92 ± 0.71D (condition iv). None of the thirty through-focus image quality metrics were able to predict the DOF (R=0.35).
It appears that retinal image quality alone is not enough to predict the tolerance to blur. Moreover, large inter-individual differences suggest that tolerance to spherical defocus could be affected by the adaptation to blur of each subject. However, subjective DOF might be influenced by the type of target presented to the subject.
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