Peripheral astigmatic asymmetry and angle alpha

Dunne, Mark; Misson, Gary P.; White, E. K.; Barnes, D. A.

doi:10.1111/j.1475-1313.1993.tb00475.x

Cited by 42 publications

(39 citation statements)

References 9 publications

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“…In the literature it is reported that peripheral astigmatism is minimum about one point of the nasal retina. 18,19 Our results seem to agree with this finding, since we find a double minimum in the astigmatism. Nevertheless, it has also been reported that in primates the fovea appears at the point where the best image is formed.…”

Section: Discussionsupporting

confidence: 81%

Corneal primary aberrations compensation by oblique light incidence

2009

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Abstract. The eye is not a centered system. The line of sight connects the fovea with the center of the pupil and is usually tilted in the temporal direction. Thus, off-axis optical aberrations, mainly coma and oblique astigmatism, are introduced at the fovea. Tabernero et al. ͓J. Opt. Soc. Am. A 24͑10͒, 3274-3283 ͑2007͔͒ showed that a horizontal tilt of the crystalline lens generates a horizontal coma aberration that is compensated by the oblique light incidence on the eye. Here we suggest that corneal astigmatism may also play a role in compensation of oblique aberrations, and we propose a simple model to analyze such a possibility. A theoretical Kooijman eye model with a slight ͑ϳ0.6 D͒ with-the-rule astigmatism is analyzed. Light rays at different incidence angles to the optical axis are considered, and the corresponding point spread functions ͑PSFs͒ at the retina are calculated. A quality criterion is used to determine the incidence angle that provides the narrowest and highest PSF energy peak. We show that the best image is obtained for a tilted incidence angle compatible with mean values of the angle kappa. This suggests that angle kappa, lens tilt, and corneal astigmatism may combine to provide a passive compensation mechanism to minimize aberrations on the fovea.

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Section: Discussionsupporting

confidence: 81%

Corneal primary aberrations compensation by oblique light incidence

2009

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“…It is possible that the flatter peripheral cornea has relatively more influence on refraction in patients with large eyes and extreme myopia; for instance, if the corneal apex is relatively farther from the optical axis in myopic eyes with a greater angle alpha. 9 This would lead to a more hyperopic outcome than predicted if biometry corneal curvature readings relate to stronger, more central areas of the cornea. 10 Another study looking specifically at errors in corneal curvature readings in high myopia, however, still found that corneal curvature readings were less significant than axial length measurements in producing biometry errors.…”

Section: Discussionmentioning

confidence: 98%

Biometry accuracy using zero- and negative-powered intraocular lenses

MacLaren

Sagoo²,

Restori³

et al. 2005

Journal of Cataract and Refractive Surgery

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“…It is important to note that though selective individual ocular component modeling yielded the required levels of myopia and their respective models, all the parameters were found to be within the range of normal values of the population. [53][54][55][56][57][58][59][60] In this experiment, we considered the retinal surface to be a rotationally symmetric aspheric surface for all the myopic models and assumed it to be invariable over the range and types of myopic prescription chosen. The rationale was to limit the degrees of freedom so as to keep the experiment simpler.…”

Section: Discussionmentioning

confidence: 99%

Do Peripheral Refraction and Aberration Profiles Vary with the Type of Myopia? - An Illustration Using a Ray-Tracing Approach

Bakaraju

Ehrmann

Papas

et al. 2009

Journal of Optometry

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Myopia is considered to be the most common refractive error occurring in children and young adults, around the world. Motivated to elucidate how the process of emmetropization is disrupted, potentially causing myopia and its progression, researchers have shown great interest in peripheral refraction. This study assessed the effect of the myopia type, either refractive or axial, on peripheral refraction and aberration profiles. METHODS: Using customized schematic eye models for myopia in a ray tracing algorithm, peripheral aberrations, including the refractive error, were calculated as a function of myopia type. RESULTS: In all the selected models, hyperopic shifts in the mean spherical equivalent (MSE) component were found whose magnitude seemed to be largely dependent on the field angle. The MSE profiles showed larger hyperopic shifts for the axial type of myopic models than the refractive ones and were evident in -4 and -6 D prescriptions. Additionally, greater levels of astigmatic component (J180) were also seen in axial-length-dependent models, while refractive models showed higher levels of spherical aberration and coma. RESUMENSe considera que la miopía es el error refractivo más frecuente a nivel mundial en niños y en adultos jóvenes. Con el objetivo de esclarecer de qué modo se ve perturbado el proceso de emetropización, potencialmente dando lugar a la aparición de miopía y a su progresión, los investigadores se han mostrado muy interesados en estudiar la refracción periférica. Este estudio evalúa en qué medida la refracción periférica y los perfiles de aberración dependen del tipo de miopía predominante, ya sea refractiva o axial. MÉTODOS:Partiendo de modelos de ojo esquemáticos adaptados al caso de la miopía y utilizando un algoritmo de trazado de rayos, se calcularon las aberraciones periféricas, incluyendo el error refractivo, en función del tipo de miopía. RESULTADOS: En todos los modelos estudiados, se observó un cambio o desplazamiento hipermetrópico en el equivalente esférico medio (EEM), cuyo valor parecía depender mayormente del ángulo analizado (excentricidad). En los perfiles de EEM se aprecia que dicho desplazamiento hipermetrópico es mayor para los modelos de miopía de tipo axial que para los de tipo refractivo, diferencia que se hace evidente en los casos correspondientes a -4 D y a -6 D. Además, se observó que el coeficiente asociado al término de astigmatismo (J180) presentaba un valor más alto en los modelos con un mayor peso de la miopía axial, mientras que en los modelos predominantemente refractivos se obtuvieron mayores niveles de aberración esférica y de coma. CONCLUSIONES: Este estudio ha concluido que aquellos ojos miopes donde prima la componente axial pueden presentar un mayor riesgo de progresión que ojos miopes que tengan errores refractivos similares pero donde prime la componente refractiva. Esta predicción surge de los resultados teóricos obtenidos en modelos de ojos utilizando un algoritmo de trazado de rayos, por lo que necesitan ser confirmados experimentalment...

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Peripheral astigmatic asymmetry and angle alpha

Cited by 42 publications

References 9 publications

Corneal primary aberrations compensation by oblique light incidence

Corneal primary aberrations compensation by oblique light incidence

Biometry accuracy using zero- and negative-powered intraocular lenses

Do Peripheral Refraction and Aberration Profiles Vary with the Type of Myopia? - An Illustration Using a Ray-Tracing Approach

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