Abstract
Angles subtended at the second nodal point of the eye (NP2) are approximately the same as input visual angles over a very large angular range, despite the nodal point being a paraxial lens property. Raytracing using an average pseudophakic eye showed that the angular nodal point criterion was only valid up to about 10°, and that the linear relationship was due instead to the cornea and lens initially creating chief ray angles at the exit pupil that are about 0.83 times input values for this particular eye, and then by the retina curving around to meet the rays in a manner that compensates for increasing angle. This linear relationship is then also maintained when retinal intersections are calculated relative to other axial points, with angles rescaled approximately using the equation R/(R + delta), where delta is the axial distance from the center of a spherical retina of radius R. Angles at NP2 approximately match the input angles, but the terminology is misleading because this is not a paraxial property of the eye. Chief rays are used with finite raytracing to determine the actual behavior.
Highlights
An evaluation of why a small number of patients with intraocular lenses (IOLs) see bothersome dark shadows in the far periphery involved modeling the eye at very large visual angles [1,2,3,4]
The dark shadow evaluations indicate that the limit of the visual field may be reduced for the pseudophakic eye due to vignetting, which is very unexpected after 50 years of IOL usage with nobody mentioning it, though “far peripheral vision” seems to have never been explored in detail, even for the phakic eye [1]
These are the distances in mm from the anterior corneal apex, and for small input angles the axial ray locations match the traditional nodal point values, so that P1 = NP1, and P2 = NP2, with the curves in Figure 2b being flat to about 10 degrees of input visual angle
Summary
An evaluation of why a small number of patients with intraocular lenses (IOLs) see bothersome dark shadows in the far periphery (negative dysphotopsia) involved modeling the eye at very large visual angles [1,2,3,4]. It was found that the intersection angle of the chief ray with the retina was substantially the same as the input visual angle over a very large range when it was calculated relative to the optical axis from the second nodal point (NP2) [2,5] This would be expected for small angles, but it was not clear why that would be the case for angles as large as 70–90◦, and that is evaluated here. Light might bypass the IOL and illuminate the peripheral retina directly, and with small pupils there can be a gap between the two illuminated regions This can cause a shadow-like region that goes away as the pupil opens up. The dark shadow evaluations indicate that the limit of the visual field may be reduced for the pseudophakic eye due to vignetting, which is very unexpected after 50 years of IOL usage with nobody mentioning it, though “far peripheral vision” seems to have never been explored in detail, even for the phakic eye [1]
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