The measurement protocols followed the tenets of the Declaration of Helsinki,
30 and the study was approved by the Bioethics Committee of the University Research Center (Sede de Investigación Universitaria) from the University of Antioquia, Medellín, Colombia. To mimic presbyopia, cycloplegia was induced by applying 2 drops of 1% tropicamide ophthalmic solution at the beginning of the experimental session and 1 extra drop after each hour of measurements; this is a standard procedure in this kind of study.
31–33 We found the BCDVA using the BLS after induction of cycloplegia. The effectiveness of tropicamide was verified by obtaining the typical VA through-focus curve for a cycloplegic eye for all subjects (see
Fig. 6a) and finding little variation in the subjective measurement of the refraction. A bite bar was used during all tests to stabilize and align the subject. In addition, the alignment of the subject was controlled using the HSS, which was initially aligned with the LSL and the artificial pupil planes, and the diode laser, which had a maximum power of 30 μW. The reference point for the alignment of the eye was the center of the entrance pupil of the subject's eye. Refraction was corrected with the BLS after proper alignment to obtain a target vergence of 0 D. With the BLS properly positioned, the HSS was used to obtain a video to measure the ocular aberrations of the subject and thus determine the astigmatism. Then the BLS generated target vergence from −4 to +1 D in steps of 1 D (i.e., −4, −3, −2, −1, 0, and +1 D). A forced-choice test using FrACT version 3.8.0e software (Michael Bach, Sektion Funktionelle Sehforschung, Universitäts-Augenklinik, Freiburg, Germany) was performed to find the VA of the subject in each target vergence. The test was based on the Snellen-E optotype recognition test in which four different orientations and the best parameter estimation by sequential testing (PEST) adaptive method were used to determine the threshold size of optotype recognized after 48 trials.
34 The complete set of measurements was repeated to estimate the experimental error. Visual acuity of the naked eye, that is, without correction, was measured under high luminance condition of HL1, which is equal to 100 cd/m
2, and then with the LSL, with a power range of 0 to +3 D, located in the pupil conjugated plane (
Fig. 2). Finally, the measurements were repeated with another high luminance value of HL2 equals to 30 cd/m
2, where neutral density filters reduced the luminance to 30% of the initial level, preserving the photopic condition. The measurements took 2 h with a 10-min break after the first hour.