Twenty presbyopic subjects (addition higher or equal to 1.25 D), aged between 48 to 62 years old, with good ocular and general health were enrolled. Fifteen subjects were progressive addition lens wearers and 5 subjects wore only near vision lenses. Refractive error (i.e., spherical equivalent) and absolute cylinder value respectively ranged from -5.50 to +2.50 D and from 0 to 1.50 D. All the subjects had monocular ETDRS visual acuity better than 0.0 logMAR with their best correction. Their higher order aberrations measured on a 4.5 mm pupil size were lower than 0.25 µm RMS. We measured their right eye's remaining accommodation level using the dynamic retinoscopy procedure which provides a precise and valid method for assessing the objective amplitude of accommodation.13-15 Their amplitude of accommodation ranged from 0.08 to 1.72 D (0.76 ± 0.43 D in average).

The main goal of this study was to test the ability of high contrast visual acuity to differentiate the quality of vision obtained with two progressive addition lenses.

We performed two different experiments in laboratory-controlled conditions: (i) subjective preference gradation and (ii) high contrast visual acuity measurement at controlled eccentricities, with two progressive addition lenses (plano addition 2.00D).

For the subjective preference gradation, the subject viewed a text displayed on a monitor covering the main part of the field of view seen through the aperture of the lenses (around 50°) at 50 cm. For the visual acuity tests, the subject viewed the target on a micro-display (OLED micro-display, Emagin, New-York, United States) through progressive addition lenses. The angular size of the micro-display was 114 × 86 arc minutes with a resolution of 800 × 600 pixels (angular pixel size of 0.143 arc minutes). Its luminance was 100 cd.m−2. During the measurement, the subject's head was stabilized thanks to a bite bar and forehead rest. The micro-display was moved along the horizontal axis placed 50 cm from the subject. The subject did not move his or her head but shifted his or her gaze to look at the image. The goal was that the subject viewed the image through the desired peripheral area of the progressive addition lens. All the experiments were carried out monocularly with the right eye. To achieve the distance refraction and the adjustment for comfortable viewing of the micro-display, subjects wore one or two lenses (i.e., subjects respectively without or with astigmatism) placed either side of the PAL front face (see figure 1). PAL A was used to adjust for comfortable viewing. The same adjustment was used for both PAL since PAL A & B differed by only 0.1 D in the central 6 mm in diameter. Subjects began either with experiment 1 or 2 (half began with experiment 1). The experiments were randomly conducted to limit the impact of the order in the experiments. In both experiments, PAL A and B were randomly tested. In experiment 2, visual acuity was measured at each eccentricity in a random order.

Figure 1.

Experimental set-up. For the procedure of calibration a mask with different eccentricities was used. During the experiment the mask was replaced by a PAL.

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The subject's pupil size and position were controlled by a camera system placed perpendicularly to the subject's axis of vision. To ensure an identical correspondence between the position of the eccentricities on the PAL and the eccentricity of the gaze direction for all subjects, the eye-lens vertex distance was also controlled with the same camera system and adjusted to be 12 mm for all subjects. The cornea-vertex distance (i.e., 12 mm) was measured between the corneal surface and the rear face of the PAL. The corrective spherical lens was placed at 10 mm from the eye and the corrective astigmatism lens was placed at 14 mm from the cornea (see figure 1). The PAL are optimized for a vertex distance of 12 mm and a pantoscopic tilt of 10°. In the experiments, the same vertex distance was used, however, null angles were used to keep an accurate control of the centering of the lens. Nevertheless, as mentioned in the caption of table 1, the exact values corresponding to the condition of the experiment have been recalculated for the lenses. To ensure a perfect positioning of the eccentricity of the point used on the PAL surfaces, whatever was the correction worn by the subjects, we calibrated the corresponding positions of the micro-display by using a mask with pinholes set at each eccentricity. The calibration mask was positioned at the place of the PAL (i.e., between the eye and subject's refraction (see figure 1). The calibration measurements were made 3 times for each subject. This calibration step was made with the subjects wearing the correction with an addition of 2D, thus allowing taking into account the prismatic effects of the lenses used during the experiments. After the calibration step, the pinhole mask was removed and replaced by the PAL, the field of view remained fully open in the limits of the aperture of the edged lens (36 mm diameter).

All the measurements were repeated three times. The measurements lasted approximately 2 hours per subject. In this study, the subjects tested had a wide range of refractive errors, which raises the possibility that the level of refractive error affects the results. However, the calibration step, with the pinhole mask significantly reduces this risk. Indeed, the calibration step is made with the subjects wearing the correction and the addition used during the experiment, thus allowing taking into account the prismatic effects of the lenses in the settings of the eccentricities. Moreover, the experiment was set so that for each subject, the gaze directions and the point of the PAL that were tested was always the same (the eye-lens vertex distance was adjusted to be 12 mm for all subjects), irrespective of their initial correction needs, reducing even more the variation between subjects and the impact of their refractive error.

In this study, we have decided not to use a gaze contingent method to measure the visual acuity at desired gaze eccentricities. Instead, we use a more basic method with a pinhole calibration. Though gaze contingent presentation is seen as the most advanced method today to control the gaze direction,16-17 we believe that it has some inconvenient and that it was not adapted to our experiment: to avoid any bias in the visual acuity, it would have requested a perfect synchronization with very low latency that would have need expensive material and add a high complexity in the experiment. Moreover, the system for eye tracking may have disturbed or limited the field of view, something that we try to avoid as much as possible in our experiment. Finally, eye tracking systems are poorly adapted to work through ophthalmic lenses, especially progressive ones. On the contrary, our calibration system with pinholes to place correctly the micro-display and the displayed target was simple, robust and very efficient. The accuracy of the micro-display positioning was better than +-5 mm at 50 cm, corresponding to an error less than +-0.12mm in the positioning of the point used on the PAL surfaces, without the need of any complex system potentially disturbing the field of view during the experiment.

To remove the impact of varying the addition, all the subjects viewed the target (either the text or the visual acuity target), positioned at 50 cm, through a progressive addition (plano, add +2.00D) and their best distance correction, adjusted to be comfortable at 50 cm, irrespective of their natural addition needs in natural condition, that may take into account various near vision distance. The main reason for doing this is to avoid the natural variation in subjective judgment, due to the change in peripheral aberration according to the addition level. The second reason is to better control the conditions (distance) in which the lenses are evaluated. By doing this, the lenses were not used in their optimal conditions, since they were optimized for a near vision distance of 40 cm. Moreover, subjects were not with their optimal natural correction for near vision (prescribed addition). However, both quality of vision and visual acuity were measured in the same condition. Moreover, since the control of gaze direction and conditions implied the use of a bit bar with fixed head, the objective of this study was not to reproduce the natural conditions of use of the lenses but rather, by controlling the conditions, to ensure a comparison in similar conditions as much as possible. This implies that the aberrations perceived in any of the conditions were different from the one seen in natural conditions of use. The goal of this experiment was not to compare lenses in real conditions of life but to test the ability of high contrast visual acuity measurement to differentiate two progressive addition lenses. However, the differences are small compared to the level of aberration typically observed in the lenses (see table 1).

Two progressive addition lenses (Varilux Ipseo Crizal Stylis Essilor© available in different designs associated with different eye-head coordination) were tested. They differ in the level of peripheral astigmatism along horizontal eccentricities at the height of the near vision point. They were edged (i.e., 36mm full aperture) such that the near vision was in the center of the lens.

The design of the first lens (Lens A), dedicated to patients who mainly turn their eyes to view an object in their eccentric field of view, is called “hard” design since the level of astigmatism increases rapidly in the periphery of the lens. The area of the lens devoted to near vision and free of astigmatism is large. The design of the second lens (Lens B), called “soft” design, is worn by patients who preferentially turn their head when viewing an eccentric object. The level of astigmatism increases slowly from the near vision point of the lens. The area free of astigmatism is only present close to the near vision point. A quantitative description of the variation of astigmatism is given in table 1 for the two designs. Hard and soft designs are more largely discussed by Meister et al.4.

Nine eccentricities were tested in the experiment (see Table 1). They corresponded to horizontal eccentric points ranging from -12 mm (i.e., nasal direction) to +12 mm (i.e., temporal direction) every 3 mm from the near vision point, with levels and axis of astigmatism and defocus that can be calculated from the progressive addition lens design and are detailed in table 1. When viewing at the edge of the page or when measuring acuities at the highest eccentricities, the distance between the eye and the target increased by around 10% involving a less accommodative demand of around 0.2 D.

Two experiments were carried out in random order.

Subjects viewed a page of a newspaper with different font sizes and different contrast (see figure 2). It was displayed on a monitor at 50 cm. The total angular size of the page was around 50° (+-25°) when rotating the eye. To view the edges of the page, the subject had to turn his/her eyes so that his/her visual axis passed at ± 12 mm of eccentricity from the near vision point of the progressive addition lens.

Figure 2.

page of a newspaper used in experiment 1. The total angular size of the page was around 50°.

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Subjects were asked to look at the whole document, to look at peripheral areas of the document, to read some pieces of text and to appreciate the overall view. Both progressive addition lenses were randomly tested. Subjects did not know which lens they wore. Subjects had to indicate which of the progressive addition lenses gives the overall best quality of vision and they had to score their preference (i.e., low, moderate or high preference).

We used the software FrACT (Freiburg Acuity and Contrast Test) developed by the University of Freiburg18-19 to measure the visual acuity with a forced-choice 8 orientations Landolt's C following a best-PEST psychometric procedure. Visual acuity was measured three times for each eccentricity from -12 mm to +12 mm (corresponding to +/-24° eccentricity for a 12 mm vertex distance when rotating the eye) in a 3 mm step and in a random order. The order in which lenses A and B were tested varied randomly between subjects.

A Friedman and Wilcoxon signed rank test were used to test the significance of the difference between all the visual acuity measurements. A Mann-Withney U test was used to check the potential link between age, addition or amplitude accommodation with the preferred lens.

Subjects had to choose which lens A or B gave the better quality of vision when viewing a page of a newspaper and to grade this difference (i.e., low, moderate or high). Figure 3 represents the number of subjects according to their preference. 15 subjects preferred the quality of vision with lens B and 5 preferred Lens A and in most cases, the preference was slight or moderate. For 8 of them (40%) the difference was strong, for 6 of them (30%), it was moderate and for the remaining 6 (30%), the difference was low. However, Lens B was often highly preferred. No relations were found between the appreciations and the age (p=0.29), nor with the addition (p=0.46) and the amplitude of accommodation (p=0.87) of the subjects (Mann-Withney U test).

Figure 3.

Distribution of the lens preference gradation, ++ means a strong preference, + means a moderate preference and no mention means a low preference.

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The averaged intra-individual standard deviation was 0.03 logMAR in the lenses A and B conditions, which is close to the step of measurement (i.e., 0.02 logMAR). The average inter-individual standard deviations were 0.09 and 0.08 respectively for lens A and lens B. Figure 4 represents the average visual acuity for each eccentricity (-12 to +12 mm each 3 mm) and each progressive addition lens. The difference between the two PALs was statistically significant at -12, -9, 0 and +12 mm of eccentricity (Wilcoxon signed rank test, P < 0.01). Close to the near vision point of the progressive addition lenses (between -6 to +6 mm), visual acuity with lens A was better than with lens B, but the difference was only statistically significant in the near vision point (i.e., 0 mm). In the periphery (i.e., -12 to -9 mm and +9 to +12 mm), visual acuity with lens B was better than with lens A. The average difference was always lower than one line of acuity (0.1 logMAR). The mean visual acuity difference between both progressive addition lenses was 0.02 logMAR, with a 95% limit of agreement of 0.18 logMAR. The Friedman test shows that the eccentricity had an effect on visual acuity (p<0.001). However, the lens did not show any significant effect (p=0.76).

Figure 4.

Visual acuity (logMAR) obtained with PAL A (green) and PAL B (grey) for each eccentricity (mm). Vertical bars represent standard deviations. Positive values are for nasal eccentricities.

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The level of preference (low, moderate, high measured in experiment 1) was analyzed against the mean difference of visual acuity between both PALs considering either the total eccentricities, or only the center or the periphery. Figure 5(A) shows the mean difference in visual acuity between progressive addition lenses A and B as a function of the subjective preference. Each point corresponds to one subject. To consider visual acuity as a discriminant factor, all the dots should either be on the top-left or bottom-right part of the graph. The higher visual acuity difference (X-axis) should involve a higher preference gradation (Y-axis). Figure 5 (B) and (C) considered respectively only the center of the progressive addition lens (from -6 to +6 mm) or only the periphery (from -12 to -9 mm and +9 to +12 mm). Visual acuity is not in accordance with the preference gradation for 7 subjects (35%) when considering all the eccentricities, for 13 subjects (65%) when only considering the central area of the lens close to the near vision point and for 4 subjects (20%) when only considering the more peripheral area from the near vision point of the lens.

Figure 5.

Preference gradation as a function of the difference in visual acuity when considering all the eccentricities (A), when only considering the more central eccentricities (-6 to +6 mm) (B) and when only considering the more peripheral eccentricities (-12 to -9 mm and +9 to +12 mm) (C).

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