Ocular axial length and straylight

Abstract Purpose Straylight refers to an optical phenomenon that takes places in the eye and leads to a deterioration of the retinal image. Past clinical findings suggest an increase of straylight with the eye's axial length, but the aetiology of the phenomenon was unclear. The purpose of this work is to demonstrate, through raytracing, simple geometrical optics, and the well‐established inverse‐angle square law for the angular distribution of straylight, why straylight increases when a myopic eye is corrected with spectacles. Methods The angular dependence of straylight is investigated using geometrical optics. An expression relating the eye's 2 nd nodal point, the ocular axial length and the eye's straylight parameter S is found. Subsequently, using a model of the human eye, the location of the 2 nd nodal point is computed using ray tracing for different axial lengths and refractive corrections. Finally, the results are compared against psychophysical data for the straylight parameter, corrected for the subject's age. Results When correcting axial myopia using spectacles, the eye's 2 nd nodal point shifts towards the retina and away from the scattering plane, leading to an increase in straylight. Meanwhile, straylight should theoretically decrease in hyperopic eyes. Contact lenses keep the 2 nd nodal point relative stable, leading to a very minor change in straylight with axial length. Our model has shown good agreement with previously taken straylight measurements in real eyes, explaining the observed change of straylight with ocular axial length. Conclusion We proposed an explanation for the underlying optical mechanism for the clinically observed increase of straylight with axial myopia, when corrected with glasses. Our model predicts that the increase can be as high as 0.12 log units for a myopic eye with 10 dioptres, which agrees with prior observations.