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Michael Simpson; Parameters affecting both Far Peripheral Vision in phakic eyes and Negative Dysphotopsia with Intraocular Lenses. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3118.
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© ARVO (1962-2015); The Authors (2016-present)
Negative dysphotopsia with intraocular lenses (IOLs) has raised questions about vision in general at very large angles. The traditional data for the extent of the human visual field are from 1915 (Roenne, Traquair), and there is little published information about variations with age or other factors. Parameters that affect this visual region are explored.
Raytrace software was used to model both the phakic and the pseudophakic eye. The physical limit of the sensitive retina is not known accurately, but as the visual angle increases, rays are eventually too oblique to enter the pupil (Fig 1), which gives an upper limit for visual angles. The maximum angle of the chief ray, and the ray intersections with the cornea, were calculated for a range of iris locations. When an IOL replaces the crystalline lens, the limiting visual angle for light focused by the IOL is reduced due to vignetting at the IOL edge. This angle is calculated, along with angles for rays bypassing the IOL, which vary strongly with pupil diameter.
The variation of the limiting input angle with iris location for an average cornea is plotted in Fig 2. The crystalline lens pushes the iris forward with age, which increases the maximum potential visual angle. Similarly, the radial location of ray intersections with the cornea move more centrally, reducing the likelihood that the sclera might limit the max visual angle. These values can be compared to half the white-to-white value for a typical eye (about 6mm). The situation changes for an IOL, where vignetting can start at 60 degrees, and the last focused ray is at about 97 deg for an average IOL. Light at lower visual angles can also bypass the IOL, however, starting at visual angles of about 75 deg or 60 deg for 3 or 4mm pupils. This light appears to come from larger angles if the retina is scaled using the main image.
The clear diameter of the cornea, and the axial location of the iris, are generally matched well to a limiting temporal visual field of about 105 deg for an average phakic eye, though these are more likely to become limiting factors for a large ACD. The limiting visual field focused by an IOL, however, is much smaller than that of the phakic eye, and is probably the cause of negative dysphotposia. Although light can also bypass the IOL, it comes from a smaller angle, and it varies strongly with pupil diameter
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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