Autorefraction is a cornerstone of optometric and ophthalmologic practice, providing an objective assessment of refractive error. A key limitation, however, especially in children and young adults, is its vulnerability to inaccuracies induced by accommodation. This physiological process allows the eye to change the shape of the crystalline lens to focus on objects at varying distances and is mediated by the ciliary muscles under neural control. When active during autorefraction, accommodation can produce an artificial myopic shift.1 This problem is particularly evident with closed-view autorefractors, which present subjects with an internal fixation target located only a few centimeters inside the instrument. The artificial nature of the target and the enclosed viewing environment tend to stimulate, rather than relax, accommodation2 (instrument-induced myopia). To mitigate this effect, and the resulting over-minusing of spherical refraction, closed-view devices employ blurred fixation targets, dim illumination, or fogging lenses within the optical path. While these strategies reduce accommodative demand, they are only partially effective and frequently insufficient to eliminate the need for cycloplegia in some subjects.3-5

To address these limitations, open-view autorefractors have been developed to allow subjects to look through the device and fixate on a distant external target, thereby better mimicking natural viewing conditions. This open-view design promotes a more relaxed accommodative state compared to closed-view systems. Studies have shown that open-view systems yield measurements closer to subjective refraction and to those obtained under cycloplegia, particularly in non-presbyopic individuals.6-8 However, even with this design, residual accommodation may persist in subjects with high accommodative tone, most notably children, young adults and individuals with latent or uncorrected hyperopia, potentially requiring additional cycloplegic measurements for an accurate assessment. To further control accommodation, fogging lenses are often used in open-view systems. Fogging involves introducing a low-powered plus lens (typically between +1.00 D and +2.00 D)9–11 in the viewing path to blur the image on the retina, thereby encouraging the eye to relax accommodation. This technique has been shown to reduce the accommodative response and improve the agreement between objective and cycloplegic refraction.9,10,12 Studies using open-view wavefront aberrometry based autorefractors have confirmed that fogging with low-powered plus lenses can induce measurable reductions in accommodative response within seconds. This response is dose- and time-dependent wherein mild fogging for short durations (e.g., +1.00 D for 5 - 30 s) can provide significant accommodation relaxation, whereas excessive fogging (+3.00 D or more) or long application durations may result in paradoxical re-accommodation due to the eye reverting to its resting or “dark focus” state.1,13,14

This study aims to evaluate the impact of fogging lenses on accommodative behavior when using an open-view handheld wavefront autorefractor, the QuickSee Free Pro Keratometry (QSF Pro; PlenOptika Inc., USA), which allows fogging lenses to be magnetically attached to the viewing path. In addition, we assess how the effect of fogging varies according to the subject’s refractive status. Previous studies have shown that myopes exhibit minimal changes due to their lower accommodative demand, whereas emmetropes, and particularly hyperopes, demonstrate stronger accommodative responses. Notably, hyperopes appear to benefit the most from fogging, underscoring the importance of tailoring fogging strategies to refractive status, especially in non-cycloplegic conditions.10 Finally, we evaluated the influence of fogging lens application time by comparing measurements obtained without fogging to those acquired with a shortened acquisition of 5-seconds versus the device default setting of 10-second application.

This work investigated the influence of a + 2.00 D fogging lens on refractive measurements obtained with a portable open-view wavefront autorefractor, assessing its effect across different refractive groups and application times.

Our results indicate that the effect of the +2.00 D fogging lens differs according to refractive status. In myopic and emmetropic subjects, accommodative relaxation was observed in only a small proportion of participants, and the magnitude of the relaxation was generally comparable to or smaller than the device's intrinsic repeatability limits (95% LOA). Conversely, many subjects exhibited either no measurable effect or accommodative induction, suggesting that the clinical benefit of routine fogging may be limited in these groups. Statistical comparison of spherical refraction with and without the fogging lens showed no significant differences in myopes (mean diff = 0.05 D, p = 0.12) or emmetropes (mean diff = 0.07 D, p = 0.05), whereas hyperopes exhibited a small but statistically significant difference (mean diff = −0.25 D, p = 0.02 for the 10-second application; mean diff = −0.25 D, p = 0.01 for the 5-second application), indicating that the lens produced its greatest measurable effect in this refractive group. Accordingly, our findings suggest that routine application of the fogging lens in myopic and emmetropic subjects is unlikely to provide substantial additional benefit and, in some individuals, may introduce additional measurement variability (Figs. 4 and 5).

In contrast, hyperopic subjects exhibited the highest proportion of accommodative relaxation, with 43% of eyes showing relaxation and an average spherical refractive change of approximately 0.75 D, well above the device's intrinsic repeatability limits (95% LOA). These findings suggest that hyperopic individuals are more likely to benefit from the use of fogging lenses. Regarding application time, the distributions of accommodative responses observed after 5 and 10 s were broadly comparable. Although the 5-second protocol showed a slightly higher proportion of accommodation relaxation and a lower proportion of accommodation induction, these differences were modest, and no significant differences were observed in the overall population-level bias between the two application times (overall mean diff = −0.01 D, p = 0.72 for 10 s; mean diff = −0.02 D, p = 0.61 for 5 s). Together, these findings suggest that extending the fogging period beyond 5 s provides limited additional clinical benefit under the conditions evaluated in this study.”

Our findings are partially consistent with previous reports on the effect of fogging lenses in open-field autorefractors. Queirós et al.10 evaluated the impact of a + 2.00 D fogging lens in a cohort of 142 young adults (22.3 ± 3.7 years) using a well-established binocular open-view desktop system (WAM-5500, Grand Seiko Co. Ltd). In their study, the average accommodation relaxation induced by the lens was comparable to that achieved under cycloplegia in terms of spherical equivalent refraction. Notably, they analyzed these differences by refractive group, reporting the spherical equivalent (M) of average accommodation relaxation of 0.4 D for myopes (n = 38), 0.3 D for emmetropes (n = 42), and 0.28 D for hyperopes (n = 62). These results contrast with our findings, where we observed mean differences of only 0.06 D for myopes (n = 21), 0.07 D for emmetropes (n = 26), and 0.24 D for hyperopes (n = 14). A plausible explanation for this discrepancy is the difference in viewing conditions between the two studies. In the present study, measurements were intentionally performed under monocular viewing conditions, consistent with the intended clinical use of the QuickSee Free Pro, with the contralateral eye occluded. This eliminates the influence of convergence and may reduce both the amplitude and stability of accommodation, as the accommodative-convergence mechanism is not fully engaged.19 In contrast, Queirós et al.10 performed binocular measurements, preserving the natural coupling between accommodation and convergence, which typically yields more stable and physiologically representative accommodative responses. Therefore, differences in viewing conditions should be considered when comparing the results of both studies. Nevertheless, viewing conditions alone are unlikely to fully explain the discrepancies observed, particularly in the myopic subgroup. As suggested by Queirós et al. and other authors,20,21 accommodative relaxation in myopic participants may be influenced by changes in pupil diameter under fogging conditions, as larger pupils can increase optical aberrations and affect the refractive endpoint measured by the autorefractor. Although the QSF Pro acquires dynamic wavefront data, including pupil diameter and higher-order aberrations such as spherical aberration, these parameters were beyond the scope of the present study and were therefore not analyzed. Future studies incorporating these measurements could help clarify the optical mechanisms underlying the inter-subject variability observed during fogging. Finally, differences in the duration of fogging lens application and the variability observed within each refractive group were not reported in the Queirós study, limiting a more precise comparison with our findings.

Bui et al. investigated the effect of a + 2.00 D fogging lens in 53 children aged 4 to 12 years with varying degrees of hyperopia.22 In that study, the authors employed an earlier model of a portable open-view autorefractor (QuickSee, PlenOptika Inc, USA),23,24 which relied on wavefront sensing technology similar to that of the monocular QSF Pro used in the present work but differed in several important design aspects. These design differences included a binocular measurement configuration, a reduced field of view, and the placement of the fogging lens within the measurement path and requiring post hoc subtraction of its optical power from the refractive results. In contrast to the findings of the present study and those reported by Queirós et al., Bui et al. found no statistically significant difference in spherical refraction with and without the fogging lens (mean difference = −0.085 D; p = 0.51), which they primarily attributed to effective accommodative control provided by a distant, non-accommodative, colour-changing fixation target. Beyond these considerations, discrepancies between studies may also reflect differences in subject populations and fixation behaviour (children versus adults), as well as methodological factors such as monocular versus binocular measurements and the use of static versus dynamic fixation stimuli (visual acuity chart in the present study versus colour-changing lights in Bui et al.).

In another similar study, Turull-Mallofré et al.13 also examined the influence of fogging lens power and application duration on accommodation in young adults using a wavefront aberrometer and open-view measurements. They found that lower-power lenses (+1.00 D and +2.00 D) produced the greatest accommodation relaxation, with 59.3% and 46.9% of participants, respectively, showing a median relaxation ≥ 0.125 D Regarding duration of fogging lens application, no consistent trend was observed for all lens powers; however, maximum relaxation with all lenses generally occurred within the first ≈5 s of measurement, which is consistent with our results. Furthermore, the authors also reported substantial inter-individual variability with all lenses, which increased for longer fogging lens application durations. They suggested that this variability, also seen in our study (Table 3), might stem from the wide inter-subject differences in stimulus–response function and tonic accommodation previously reported in the literature.25 Unfortunately, since the results were not stratified by refractive group, it is unclear whether refractive status contributed to the observed variability. Furthermore, although some findings were consistent with ours, methodological differences, particularly their use of a desktop prototype aberrometer where one eye viewed the target through the fogging lens while the contralateral eye was measured in a closed-view configuration, limit direct comparison with our results and with other studies using fully open-view monocular or binocular systems.

Finally, several limitations of the present study should also be acknowledged. First, measurements were always performed in the same sequence, with the non-fogging condition preceding the fogging condition. Although this protocol was selected to reflect the intended clinical workflow, a randomized crossover design would have further minimized the possibility of measurement-order effects. However, the refractive-group specific response observed, with significant effects primarily in hyperopic subjects and minimal changes in myopic and emmetropic eyes, suggests that the findings cannot be explained solely by a measurement-order effect. Nevertheless, future randomized crossover studies would be valuable to further confirm these findings. Second, participants were classified based on non-cycloplegic objective refraction, which may not fully represent true refractive status, particularly in young or hyperopic subjects with active accommodation. Although the more positive average of repeated measurements was used to reduce the influence of residual accommodation, some misclassification cannot be excluded. In particular, latent hyperopic subjects may have been classified as emmetropic, reducing the separation between these groups and potentially attenuating, rather than exaggerating, the apparent effect of the fogging lens. A classification based on cycloplegic or subjective refraction would therefore improve group separation and strengthen the interpretation of accommodative responses. Furthermore, the relatively small hyperopic subgroup also limited the statistical precision of the estimated effect size and reduced the ability to detect more subtle accommodative responses or variability patterns. Although the findings in this subgroup were internally consistent across the different analyses performed, they should be interpreted with appropriate caution until confirmed in larger cohorts. Future studies should therefore include larger and more balanced samples and incorporate cycloplegic measurements alongside fogging protocols to better validate accommodative relaxation and individual control. Additionally, assessing accommodation response under different fogging conditions such as monocular, binocular, or applied to the contralateral eye, would allow direct comparison of measurement paradigms and help clarify the influence of convergence and device design. Such work would further contextualize the present findings within the broader literature.

This study demonstrates that the effect of a + 2.00 D fogging lens used with the open-view wavefront autorefractor is strongly dependent on refractive status. In myopic and emmetropic subjects, the lens did not reliably induce accommodative relaxation. Most eyes showed no measurable change, and a substantial proportion exhibited accommodation induction, with mean differences small and not statistically significant. These findings suggest that routine use of the fogging lens in these groups is unlikely to provide substantial additional benefit and may potentially increase measurement variability.

By contrast, hyperopic subjects exhibited larger mean differences and a substantial proportion of eyes showing accommodative relaxation beyond the device's intrinsic repeatability, suggesting that this population is more likely to benefit from the use of a + 2.00 D fogging lens. Moreover, similar accommodative response distributions were observed after both 5- and 10-second applications, indicating that extending the fogging period beyond 5 s may offer little additional practical benefit. Overall, these findings support a more selective use of fogging lenses in monocular open-view autorefractors according to the subject's refractive status, rather than their routine application in all patients.

P.B performed the measurements and analyzed the data presented in the figures and tables. G.G., P.E., A.H. and M.R. designed, implemented, and supervised the device software. A.W., I.C. and A.L. built the device and supervised its proper functionality. J.O., F.F., E.L., S.D. and D.L. prepared and supervised the regulatory documentation and clinical trial protocol. P.B., E.L. and J.O. wrote the main manuscript text. All authors reviewed the manuscript. All authors listed on the title page consent to the publication of the text, data and images contained within this paper.

Research related to the autorefractor technology used in this work was partially supported by the Spanish Ministry of Science and Innovation (TED2021-131541A-I00), the US National Eye Institute of the National Institutes of Health (R44EY025452), and Translational Research Institute for Space Health (NASA NNX16AO69A, USA). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health or the Spanish Ministry of Science and Innovation.

The study was approved by the Autonomous University of Madrid Research Ethics Committee. The study adhered to the tenets of the Helsinki Declaration at all stages. We obtained written informed consent from all participants.