## 2 Materials and Methods  
**Study Design:** A prospective, observational, longitudinal, double-masked study was carried out to evaluate test duration and characteristics of eye fixation when performing VA tests with 3 different PPL types. The factors analyzed were eye chart size, gaze direction, and lens design. The study followed the principles of the Declaration of Helsinki. Full study approval was obtained from the Complutense University of Madrid Committee Review Board (CE_20210715-3_SAL). All participants provided written informed consent before the start of the study, and at the end of the study, subjects were compensated with one pair of glasses. **Participants:** The study sample was made up of presbyopic participants of both genders who were older than 44 and had worn PPLs for at least six months before the start of the study. The inclusion criteria were: (1) Refractive error range of \(-6.00\) D to \(+5.00\) D with astigmatism less than or equal to 2.50 D. (2) Near addition power from \(+1.00\) D to \(+3.00\) D. (3) Best-corrected VA is better than 0.1 logMAR monocularly and 0.05 logMAR binocularly. (4) Anisometropia below 1.50 D. Subjects were rejected if they had any ocular diseases, non-compensated binocular vision anomalies, medical conditions that could affect vision, or if they were undergoing any pharmacological treatments that might have affected the subjects' visual function. The sample size was calculated based on data from a preliminary study with five participants who met the same inclusion criteria as above. The calculation was performed using the GRANMO sample size calculator, version 7.12 (Institut Municipal d'Investigacio Medica, Barcelona, Spain). Two-tailed testing with an alpha risk of 0.05, a beta risk of 0.1, and a dropout rate of 30% was set to estimate a sample size of 37 participants. **Procedure:** All participants underwent a full optometric assessment to check whether they met the inclusion criteria. The visual examination included VA measurement using the PVVAT test (Precision Vision, La Salle, III), subjective refraction at a distance and near vision, stereo acuity assessment by the [PERSON] test, the Worth test, the Cover test, and ocular motility examination. After the optometrists determined the participant met the inclusion criteria, the fitting parameters and position of wear for the eye-tracker glasses were measured: pupillary position, segment height, back vertex distance, frame wrap angle, and pantoscopic tilt. Once these data were collected, the PPL study lenses were ordered. VA measurements incorporating an eye-tracking system for three different PPL designs at far and near distances were recorded in two different day visits with a duration of two hours. During the first visit, far-distance VA recordings were collected for the three different PPLs at three different gaze directions, and a two-minute break was taken between each experimental condition to minimize the participant's fatigue. In a similar way, during the second visit, near-distance VA measurements were collected for the three different PPLs and three gaze directions, including two-minute breaks between each experimental condition to minimize visual fatigue. **Progressive Power Lenses:** Three different individualized free-form PPL designs were used for this study: (1) a balanced design, PPL-Balance (Endless Steady Balance, IOT, Madrid, Spain); (2) a lens with a wider field of view for near vision, PPL-Near (Endless Steady Near, IOT, Madrid, Spain); and (3) a lens with a wider field of view for distance vision, PPL-Distance (Endless Steady Distance, IOT, Madrid, Spain). The PPL's technical characteristics (cylinder and mean power distribution maps) for a plano prescription, addition 2D, using standard position-of-wear parameters are shown in Figure 1. The lenses were placed on a specific clip-on frame that was attached to the eye-tracker glasses. This configuration allows for direct pupil registration without any interference from the PPL.