Higher-order Monochromatic Aberrations Research Articles

Monochromatic higher order aberrations in highly myopic eyes with Staphyloma

BackgroundPrevalence of high myopia is continuously increasing, thus, patients affected with staphyloma are abundant worldwide. Assessment of the quality of vision in these patients is mandatory for a proper clinical counselling, specially when undergoing surgical procedures that require intraocular lenses implantation. Thus, the purpose of the study was to assess monochromatic higher order aberrations (HOAs) in highly myopic eyes with staphyloma with or without a dome-shaped macula.MethodsParticipants underwent spectral-domain optical coherence tomography, ocular axial biometry, dual Scheimpflug photography and integrated Placido disk topography, and Hartmann-Shack wavefront analysis. Five groups were evaluated: a low-moderate myopia control group (< 6.00 diopters, n = 31) and four high myopia (≥6.00 diopters) groups: eyes without staphyloma (n = 18), eyes with inferior staphyloma (n = 14), eyes with posterior staphyloma without dome-shaped macula (n = 15) and eyes with posterior staphyloma with dome-shaped macula (n = 17). Subsequently, two new groups (including all participants) were created to assess differences between myopia with and without staphyloma. One-way analysis of covariance was performed using age and lens densitometry as covariates.ResultsStatistically significant (p ≤ 0.05) differences in anterior corneal fourth-order HOAs were observed between the low-moderate myopia and no-dome-shaped macula (Mean: 0.16 μm) and dome-shaped macula posterior staphyloma groups (Mean: 0.12 μm) in younger patients (≤45 years old). The same groups also showed (p ≤ 0.05) significant differences for anterior corneal primary spherical aberration (Mean: 0.19 and 0.13 μm, respectively). In addition, anterior corneal tetrafoil was significantly higher (p = 0.04) in dome-shaped macula compared to no-dome-shaped macula (Mean: 0.18 vs 0.06 μm, respectively). When all participants were grouped together, significantly lower mean anterior corneal primary spherical aberration (0.15 μm vs. 0.27 μm, p = 0.004) and higher internal primary spherical aberration (0.04 μm vs. -0.06 μm, p = 0.04) was observed in staphyloma compared to no-staphyloma myopic patients.ConclusionsEyes with high myopia and staphyloma have less positive anterior corneal primary spherical aberration and less negative internal primary spherical aberration, suggesting that the anterior corneal surface tends to mimic in a specular fashion the posterior pole profile. This corneal behaviour appears to change in patients older than 45 years.

Lower sensitivity to peripheral hypermetropic defocus due to higher order ocular aberrations.

Many myopia control interventions are designed to induce myopic relative peripheral refraction. However, myopes tend to show asymmetries in their sensitivity to defocus, seeing better with hypermetropic rather than myopic defocus. This study aims to determine the influence of chromatic aberrations (CA) and higher-order monochromatic aberrations (HOA) in the peripheral asymmetry to defocus. Peripheral (20° nasal visual field) low-contrast (10%) resolution acuity of nine subjects (four myopes, four emmetropes, one hypermetrope) was evaluated under induced myopic and hypermetropic defocus between ±5D, under four conditions: (a) Peripheral Best Sphere and Cylinder (BSC) correction in white light; (b) Peripheral BSC correction+CA elimination (green light); (c) Peripheral BSC correction+HOA correction in white light; and (d) Peripheral BSC correction+CA elimination+HOA correction. No cycloplegia was used, and all measurements were repeated three times. The slopes of the peripheral acuity as a function of positive and negative defocus differed, especially when the natural HOA and CA were present. This asymmetry was quantified as the average of the absolute sum of positive and negative defocus slopes for all subjects (AVS). The AVS was 0.081 and 0.063 logMAR/D for white and green light respectively, when the ocular HOA were present. With adaptive optics correction for HOA, the asymmetry reduced to 0.021 logMAR/D for white and 0.031 logMAR/D for green light, mainly because the sensitivity to hypermetropic defocus increased when HOA were corrected. The asymmetry was only slightly affected by the elimination of the CA of the eye, whereas adaptive optics correction for HOA reduced the asymmetry. The HOA mainly affected the sensitivity to hypermetropic defocus.

Peripheral resolution and contrast sensitivity: effects of monochromatic and chromatic aberrations.

Correction and manipulation of peripheral refractive errors are indispensable for people with central vision loss and in optical interventions for myopia control. This study investigates further enhancements of peripheral vision by compensating for monochromatic higher-order aberrations (with an adaptive optics system) and chromatic aberrations (with a narrowband green filter, 550 nm) in the 20° nasal visual field. Both high-contrast detection cutoff and contrast sensitivity improved with optical correction. This improvement was most evident for gratings oriented perpendicular to the meridian due to asymmetric optical errors. When the natural monochromatic higher-order aberrations are large, resolution of 10% contrast oblique gratings can also be improved with correction of these errors. Though peripheral vision is mainly limited by refractive errors and neural factors, higher-order aberration correction beyond conventional refractive errors can still improve peripheral vision under certain circumstances.

Impact of higher-order aberrations on depth-of-field.

It is well known that depth-of-focus (DOF) is influenced by optical factors (such as pupil size and monochromatic aberrations). However, neural factors such as blur sensitivity and defocus adaptation may play an important role on the extent of DOF. A series of experiments were conducted to study if optical or neural factors are most pertinent in explaining the variability of DOF across subjects. An adaptive optics system with a black and white target, a 3.8-mm artificial pupil, and a subjective criterion (based on objectionable blur) were used to measure depth of field ([DOFi]; DOF computed in the object space) in 11 participants, after at least 6 min of adaptation. This was done under three conditions: (a) with their own higher order aberrations (HOA); (b) after correction of their monochromatic HOA; and (c) after altering the HOA pattern for some participants to reflect the HOA pattern measured for a different participant. Natural DOFi and DOFi after HOA correction were positively correlated (R2 = 0.461), but a significant decrease in DOFi (21% on average) was found after HOA correction (p = 0.042). Effect of HOA on the intersubject variability of DOFi was 3.9 times smaller than the effect of the image neural processing. This study shows that DOFi depends on both optical and neural factors, but the latter seems to play a more important role than the former.

Two-dimensional simulation of eccentric photorefraction images for ametropes: factors influencing the measurement.

Eccentric photorefraction and Purkinje image tracking are used to estimate refractive state and eye position simultaneously. Beyond vision screening, they provide insight into typical and atypical visual development. Systematic analysis of the effect of refractive error and spectacles on photorefraction data is needed to gauge the accuracy and precision of the technique. Simulation of two-dimensional, double-pass eccentric photorefraction was performed (Zemax). The inward pass included appropriate light sources, lenses and a single surface pupil plane eye model to create an extended retinal image that served as the source for the outward pass. Refractive state, as computed from the luminance gradient in the image of the pupil captured by the model's camera, was evaluated for a range of refractive errors (-15D to +15D), pupil sizes (3mm to 7mm) and two sets of higher-order monochromatic aberrations. Instrument calibration was simulated using -8D to +8D trial lenses at the spectacle plane for: (1) vertex distances from 3mm to 23mm, (2) uncorrected and corrected hyperopic refractive errors of +4D and +7D, and (3) uncorrected and corrected astigmatism of 4D at four different axes. Empirical calibration of a commercial photorefractor was also compared with a wavefront aberrometer for human eyes. The pupil luminance gradient varied linearly with refractive state for defocus less than approximately 4D (5mm pupil). For larger errors, the gradient magnitude saturated and then reduced, leading to under-estimation of refractive state. Additional inaccuracy (up to 1D for 8D of defocus) resulted from spectacle magnification in the pupil image, which would reduce precision insituations where vertex distance is variable. The empirical calibration revealed a constant offset between the two clinical instruments. Computational modelling demonstrates the principles and limitations of photorefraction to help users avoid potential measurement errors. Factors that could cause clinically significant errors in photorefraction estimates include high refractive error, vertex distance and magnification effects of a spectacle lens, increased higher-order monochromatic aberrations, and changes in primary spherical aberration with accommodation. The impact of these errors increases with increasing defocus.

Refractive plasticity of the developing chick eye: a summary and update

To summarize the OPO 1992 Classic Paper: Refractive plasticity of the developing chick eye (12: 448-452) and discuss recent findings in refractive development. The classic paper shows that when lightweight plastic goggles with rigid contact lens inserts are applied to the eyes of newly hatched chicks, the eye responds accurately to defocus between -10 and +20 D, although hyperopia develops more rapidly. While the changes largely are due to change in axial length, high levels of hyperopia are associated with corneal flattening. Also, newly hatched chicks are better able to compensate for the induced defocus than chicks that are 9 days old. In addition, astigmatism of 2-6 D can be produced by applying 9 D toric inducing lenses on the day of hatching, and the most myopic meridian coincides with the power meridian of the inducing lens. This astigmatism appears to be primarily due to corneal toricity. Furthermore, the greatest magnitude was produced when the plano meridian of the inducing lens was placed 45° from the line of the palpebral fissure. Since our publication in 1992, it has been shown that similar results can be produced in a variety of species, including; tree shrews, marmosets, monkeys and fish. Considerable effort has been spent in trying to determine what the eye uses, if not the brain, as the signal to the sign of the defocus. Accommodation, chromatic aberration, diurnal variation, astigmatism and higher order monochromatic aberrations have all been considered. Choroidal thinning and thickening play a role in myopia and hyperopia development, respectively, in chicks. High light levels (15,000 lux) increase the rate at which chicks compensate for positive lenses and decrease the compensation rate for negative lenses. However these light levels do not prevent the eye from fully compensating for either type of lens. It has also been shown that brief periods of normal vision prevent the development of form deprivation myopia. Finally, the importance of the peripheral retina in refractive development has been explored and lenses designed to reduce relative peripheral hyperopia have resulted in variable effects as far as myopia control is concerned. A growing body of evidence, from both animal models and human clinical trials indicates that the development of myopia is related both to genetics and environment / lifestyle. Nevertheless, we are far from understanding how this interaction takes place.

Simultaneously measuring ocular aberration and anterior segment biometry during accommodation

In the human eye, accommodation is essential for functional vision. However, the mechanisms regulating accommodation and the ocular parameters affecting aberrations remain to be explored. In order to measure the alterations of ocular aberration and crystalline lens biometry during dynamic accommodative stimuli, we designed an optical coherence tomography with ultra-long penetration depth (UL-OCT) combined with a Shack–Hartmann wavefront sensor (SHWFS). This integrated set up measures human eye's anterior segment as well as monochromatic high-order aberrations (HOAs) with 6 μm resolution and (1/20) λ accuracy. A total of 10 healthy volunteers without ocular diseases were examined. Upon exposure to accommodative stimuli, the wavefront aberrations became larger. Among the anterior segment biometry, the anterior crystalline lens demonstrated significant curvature during accommodation and was the major cause of high-order aberration. These findings suggest that the front surface of the crystalline lens can significantly affect variation among aberrations, which is a key factor underlying the quality of human vision.

Effect of ocular transverse chromatic aberration on detection acuity for peripheral vision.

We examined the effect of transverse chromatic aberration (TCA) on detection acuity for white-light interference fringes seen in Maxwellian view at various orientations and locations in the visual field. A circular patch (3.5° diameter, 3.2 log Trolands) of nominally high-contrast fringes was produced on the retina by a commercial instrument (the Lotmar Visometer, Haag Streit) mounted on a gimbal for controlled positioning of the stimulus in the visual field from 0° to 35° eccentricity. Detection acuity for white light fringes for all meridians and eccentricities ≥15° was maximum when fringes were oriented parallel to the visual meridian line. This meridional effect disappeared when a narrow-band filter was used to eliminate TCA. The meridional effect also disappeared when the interferometric stimulator was displaced laterally to align the instrument with the eye's local achromatic axis. Modelling confirmed that TCA is the major factor responsible for white-light meridional bias, with minor contribution arising from higher-order monochromatic aberrations and neural factors.

Chromatic aberration and polychromatic image quality with diffractive multifocal intraocular lenses

To evaluate the impact of target distance on polychromatic image quality in a virtual model eye implanted with hybrid refractive-diffractive intraocular lenses (IOLs). School of Optometry, Indiana University, Bloomington, Indiana, USA. Experimental study. A pseudophakic model eye was constructed by incorporating a phase-delay map for a diffractive optical element into a reduced eye model incorporating ocular chromatic aberration, pupil apodization, and higher-order monochromatic aberrations. The diffractive element was a monofocal IOL with a +3.2 diopter (D) diffractive power or 2 types of bifocal IOLs (nonapodized or apodized) with a +2.92 D addition (add) power. Polychromatic point-spread functions and image quality for white and monochromatic light were quantified for a series of target vergences, wavelengths, and pupil diameters using modulation transfer functions and image-quality metrics. Ocular longitudinal chromatic aberration was largely corrected by the monofocal design and by both bifocal designs for near targets. In the bifocal design, add power and the ratio of distance:near image quality changed significantly with wavelength and pupil size. Also, image quality for distance was better with the apodized design. Achromatization by the diffractive IOL provided significant improvement in polychromatic retinal image quality. Along with apodization and higher-order aberrations, it can significantly affect the near-distance balance provided by a diffractive multifocal IOL. No author has a financial or proprietary interest in any material or method mentioned.

Changes in Ocular Monochromatic Higher-Order Aberrations in the Aging Eye

To characterize corneal, internal, and total ocular monochromatic higher-order aberration (MHOA) changes that occur in the aging eye. Prospective observational case series including 300 eyes of 167 patients (mean age = 63.8 years) attending the ophthalmology service at University Hospital Ayr, Scotland. Corneal, internal, and total ocular aberrations were measured over a 6-mm dilated pupil. Zernike coefficients were obtained to the sixth order. Changes in MHOA between age groups and inter-eye correlations between right and left eyes were analyzed. A significant inter-eye correlation was found for refractive mean spherical equivalent and cylinder. A significant inter-eye correlation for the whole eye, corneal, and internal MHOA was found (p < 0.001). Right eye analysis found a significant positive correlation between age and the root mean square of whole eye MHOA (p = 0.012), with an increase from 0.517 μm in the fifth decade to 0.824 μm in the ninth. Total internal MHOA increased from 0.411 to 0.704 μm. A significant positive correlation was found between age and internal fourth- (p = 0.007), fifth- (p = 0.029), and sixth-order (p = 0.025) root mean square aberrations. There were no significant age-related changes in corneal MHOA or corneal spherical aberration. Overall mean (SD) corneal SA was 0.203 (0.082) μm. A strong correlation between the right and left eyes exists for MHOA. Whole eye MHOA increases with age. Such changes can be attributed to age-related changes in the internal optical quality of the eye. Such normative data are useful to the cataract surgeon when considering the use of an aspherical IOL to counteract corneal-induced SA during cataract surgery.

Measurement of ocular anterior segment dimension and wavefront aberration simultaneously during accommodation

In order to understand the relationship between accommodation and vision quality, a custom-built ultra-long scan depth spectral domain optical coherence tomography (UL-SDOCT) and a Shack-Hartmann wavefront sensor (HSWFS) were combined. The resolution and scan depth of UL-SDOCT are 6 μm and 15.6 mm, respectively, which allow for high-resolution imaging of the whole anterior segment. The HSWFS consists of a 32 × 32 microlens array, and is able to measure first 35th-order Zernike aberrations with (1/20)λ measurement accuracy. The integrated system succeeded in measuring the ocular anterior segment dimension parameters and the ocular monochromatic high-order aberrations simultaneously under the conditions of nonaccommodative and accommodative stimuli. This may help understand the regulatory mechanism of image quality control in the human eye.

Contrast sensitivity benefit of adaptive optics correction of ocular aberrations

While correcting the aberrations of the eye produces large increases in retinal image contrast, the corresponding improvement factors in the contrast sensitivity function have been little explored and results are controversial. We measured the CSF of 4 subjects with and without correcting monochromatic aberrations. Monochromatic CSF measurements were performed at four orientations (0, 45, 90, and 135 deg) and at six spatial frequencies (2-30 c/deg). In two subjects, the CSF was also measured in polychromatic light. The MTF increased on average by 8 times and meridional changes in improvement were associated to individual meridional changes in the natural MTF. CSF increased on average by 1.35 times (only for the mid- and high spatial frequencies) and was lower (0.93 times) for polychromatic light. Under natural aberrations, the horizontal and vertical CSFs tended to be higher than the oblique CSFs, but the meridional differences in the CSF were partially reduced when the aberrations were corrected. The consistently lower benefit in the CSF than in the MTF of correcting aberrations suggests a significant role for the neural transfer function in the limit of contrast perception. Polychromatic aberrations play an additional role in degrading contrast, particularly in the absence of monochromatic high-order aberrations.

The Effect of Contact Lens Correction and Accommodation upon Monochromatic Higher Order Aberrations in Myopia and Hyperopia

Purpose:To measure the effects of contact lens correction and accommodation upon SA in myopia and hyperopia.Methods:Eleven myopes (Average MSE −3.22±2.21D; mean age 20.7±1.1 years) and eleven hyperopes (Average MSE 4.20±2.20; mean age 20.1±1.5) participated. Subjects were corrected with daily disposable contact lenses and pupil dilation was induced with 2.5% phenylephrine hydrochloride. SA was measured at accommodation stimulus levels of 0D and 4D, using the Zywave Aberrometer while subjects viewed a high contrast letter target array in a +5D Badal system, aligned perpendicularly to the measurement axis of the instrument via a semi-silvered mirror.Results:For 0D of accommodation, the hyperopic group had significantly (p < 0.01) greater positive SA. Contact lens correction produced a positive shift in SA in the hyperopic group (p < 0.01), whereas there was a negative shift in SA in the myopic group (p < 0.01). Negative shifts in SA were found with accommodation in both myopic (p < 0.01) and hyperopic (p < 0.01) groups, with the shift being greater in the hyperopic group (p < 0.01).Conclusions:The net effect of contact lens correction and accommodation result in SA for near visual tasks, which is predominantly negative in myopia and positive in hyperopia.

Vision improvement when correcting monochromatic higher-order aberrations with phase plates in normal and abnormal human eyes

Vision improvement when correcting monochromatic higher-order aberrations with phase plates in normal and abnormal human eyes

Effects of age on peripheral ocular aberrations

On-axis monochromatic higher-order aberrations increase with age. Few studies have been made of peripheral refraction along the horizontal meridian of older eyes, and none of their off-axis higher-order aberrations. We measured wave aberrations over the central 42 degrees x32 degrees visual field for a 5mm pupil in 10 young and 7 older emmetropes. Patterns of peripheral refraction were similar in the two groups. Coma increased linearly with field angle at a significantly higher rate in older than in young emmetropes (-0.018+/-0.007 versus -0.006+/-0.002 microm/deg). Spherical aberrationC40was almost constant over the measured field in both age groups and mean values across the field were significantly higher in older than in young emmetropes (+0.08+/-0.05 versus +0.02+/-0.04 microm). Total root-mean-square and higher-order aberrations increased more rapidly with field angle in the older emmetropes. However, the limits to monochromatic peripheral retinal image quality are largely determined by the second-order aberrations, which do not change markedly with age, and under normal conditions the relative importance of the increased higher-order aberrations in older eyes is lessened by the reduction in pupil diameter with age. Therefore it is unlikely that peripheral visual performance deficits observed in normal older individuals are primarily attributable to the increased impact of higher-order aberration.

Blur on the retina due to higher-order aberrations: Comparison of eye growth models to experimental data

In the simplest model of eye growth, the ocular optics uniformly scale upwards, as do monochromatic higher-order aberrations (HOA) and linear blur on the retina. However, measured HOA remain constant or decrease with growth in some species. A new model, which holds HOA and the associated linear blur on the retina constant, was used to predict changes in HOA and resulting image quality on the retina during growth, in each of chick, monkey, and human. Models used rates of growth in each of the three species. Angular optical quality on the retina due to HOA, and its metrics improved, in contrast to the constancy predicted by uniform scaling. The model with constant linear HOA blur predicts well the improvement in human optical quality between infant and adult. Overall, in chick and monkey, angular blur improves at a rate faster than that predicted by the constant linear blur model, implying that linear retinal blur due to HOA decreases with age. On the other hand, in chick, angular blur due to third-order aberrations decreased at a rate predicted by the constant linear blur model. Growth changes in retinal blur due to HOA are species dependent but can be better understood by comparison with the new model predictions.

Role of high-order aberrations in senescent changes in spatial vision

The contributions of optical and neural factors to age-related losses in spatial vision are not fully understood. We used closed-loop adaptive optics to test the visual benefit of correcting monochromatic high-order aberrations (HOAs) on spatial vision for observers ranging in age from 18 to 81 years. Contrast sensitivity was measured monocularly using a two-alternative forced-choice (2AFC) procedure for sinusoidal gratings over 6 mm and 3 mm pupil diameters. Visual acuity was measured using a spatial 4AFC procedure. Over a 6 mm pupil, young observers showed a large benefit of AO at high spatial frequencies, whereas older observers exhibited the greatest benefit at middle spatial frequencies, plus a significantly larger increase in visual acuity. When age-related miosis is controlled, young and old observers exhibited a similar benefit of AO for spatial vision. An increase in HOAs cannot account for the complete senescent decline in spatial vision. These results may indicate a larger role of additional optical factors when the impact of HOAs is removed, but also lend support for the importance of neural factors in age-related changes in spatial vision.

Changes in monochromatic higher-order aberrations in different pupil sizes with accommodation in young emmetropes

To study the effect of accommodation on monochromatic higher-order aberrations in different pupil sizes in the young emmetropes. Intervention trial design was used in the study. The monochromatic aberrations were measured in 12 eyes from 12 undulated young emmetropes in 4.0 mm, 3.0 mm, and 2.0 mm pupil sizes with a ray tracing wavefront aberrometer under accommodative stimuli from 0 to 4.00 D. RMS values of total high-order aberrations (HOAs), total coma, total higher-order astigmatism, total spherical, and total trefoil aberrations significantly decreased as pupil size decreased in the relax state. In 4.0 mm, 3.0 mm, and 2.0 mm pupil sizes, the average root mean square (RMS) value of total coma was the highest item of HOAs in the relaxed state. With accommodation up to 3.00 D, the RMS of HOAs remained constant but changed significantly at 4.00 D stimulation (P <0.05) in all these three pupil sizes. The 4th spherical aberration decreased and changed from positive to negative with increasing accommodation over 4.0 mm size, whereas it remained positive under all accommodative levels over the 3.0 mm and 2.0 mm zones, and no trend was found. The 3rd order trefoil 0 changed from negative to positive with increasing accommodation, with 4.0 mm, 3.0 mm, and 2.0 mm pupil sizes. In 4.0 mm, 3.0 mm, and 2.0 mm pupil sizes, under accommodative stimuli from 0.00 to 4.00 D, dramatical trends amongst the HOAs, spherical, coma, and trefoil aberrations were demonstrated. Most of the higher-order aberrations induced by accommodation deserves further investigation. Simple elimination of the higher-order aberrations will be replaced by the optimization of aberrations.

Einfluss von Phenylephrin und Tropicamid auf Aberrationen höherer Ordnung

For wave-front guided corneal surgery, measuring higher order monochromatic aberrations in mydriasis is needed. However, a potential influence of mydriatic drugs on such aberrations could distort the ablation profile. Wave-front analysis was carried out on 20 (tropicamide) and 19 (phenylephrine) eyes after dark adaptation, followed by measurement after the instillation of the mydriatics one after another. Phenylephrine had no significant influence on the wave-front; neither sphere nor RMS data differed from those taken after dark adaptation. After instilling tropicamide, significant changes in Z(2) (0) and, in parallel, also of the sphere were found. The RMS showed no significant difference, only the spherical aberration Z(4) (0) was reduced by an average of 0.035 microm. The wave-front changes individually through the mydriasis due to phenylephrine and tropicamide. In the case of tropicamide, the deviation is statistically significant. Therefore, abandonment of these mydriatics before refractive surgery can be recommended, as can the use physiological pupil dilatation. Because of its lower influence, phenylephrine should be the first choice if dimout effects no adequate mydriasis.

Accommodation with higher-order monochromatic aberrations corrected with adaptive optics

Higher-order monochromatic aberrations in the human eye cause a difference in the appearance of stimuli at distances nearer and farther from best focus that could serve as a signed error signal for accommodation. We explored whether higher-order monochromatic aberrations affect the accommodative response to 0.5 D step changes in vergence in experiments in which these aberrations were either present as they normally are or removed with adaptive optics. Of six subjects, one could not accommodate at all for steps in either condition. One subject clearly required higher-order aberrations to accommodate at all. The remaining four subjects could accommodate in the correct direction even when higher-order aberrations were removed. No subjects improved their accommodation when higher-order aberrations were corrected, indicating that the corresponding decrease in the depth of field of the eye did not improve the accommodative response. These results are consistent with previous findings of large individual differences in the ability to accommodate in impoverished conditions. These results suggest that at least some subjects can use monochromatic higher-order aberrations to guide accommodation. They also show that some subjects can accommodate correctly when higher-order monochromatic aberrations as well as established cues to accommodation are greatly reduced.