Nocturnal geckos must see well to move around and hunt at night. Large eyes and pupils, highly light-sensitive cells (photoreceptors), and a short focal length (the distance from the center of the eye’s lens to the point where light converges) make not only viewing and then capturing prey possible at night, but also help geckos to see in color in low-light conditions.
These same nighttime visual adaptations pose a problem during the day because they would normally lead to a defocusing of the image and blurring of colors. The combination of a large pupil and short focal length creates an optics problem where the eye bends some wavelengths (or colors) of light more than others. Nocturnal geckos, however, appear to have strategies to counteract this. First, their eyes have multifocal lenses. This means that different parts of the lens each focus a different range of wavelengths onto the eye’s light-sensitive cells. The result is increased focus for all the colors of light the eye can perceive. Second, when a nocturnal gecko’s pupil is fully constricted in high-light conditions, it forms two sets of pinholes along a vertical slit. Scientists hypothesize that when the pupil is fully constricted the four small stacked pupils that form reduce the total amount of light entering the eye, while still enabling the different parts of the multifocal lens to receive light.
The unique nocturnal gecko eye might have two additional functions alongside preventing visual disturbances during the day. The multiple pupils generate several images on the retina with varying levels of focus, which the gecko can interpret to judge distance. The iris is also a similar color to the gecko’s scales, and so the thin slit with pinholes may facilitate daytime vision while increasing the nocturnal gecko’s ability to blend into its environment.
“At night,…[the tokay] gecko has a large, nearly round, pupil when fully dilated. But when these geckos must be active during the day…their pupils constrict to a vertical ellipse with four diamond-shaped pinholes…With the pupil constricted, the vertical [pinholes] combined with the…slit would produce an image of considerable clarity…and would significantly limit the light flux to the retina…The multiple apertures and the slit…provide a relatively larger visual field than…a circular pupil of equal surface area.” (Schwabb 2000:1215)
From eye spot to eye shineBritish Journal of OphthalmologyNovember 1, 2000
“In dim light conditions the large pupil of the gekko provides it with a small depth of field, allowing it to accurately focus on its prey, while in high light conditions, the four pupillary apertures provide the same depth of field…” (Murphy and Howland 1986:816)
On the gekko pupil and Scheiner's discVision ResearchJanuary 1, 1986
“Chromatic defocus [blurry vision in bright light] is a particularly severe problem in eyes with high light-gathering ability [e.g. nocturnal gecko eyes], since depth of field is small due to a pupillary opening that is large in relation to the focal length of the eye.” (Kröger et al. 1999:361)
Multifocal lenses compensate for chromatic defocus in vertebrate eyesJournal of Comparative Physiology AMay 1, 1999
“Multifocal optical systems with distinct concentric zones of different refractive powers have been suggested to correct for some of the defocus on the retina caused by chromatic aberration.” (Roth et al. 2011:1)
“The light-adapted pupil could also, in addition to the ability to effectively shut out light and protect the light-sensitive retina, function as camouflage…A round pupil is more conspicuous and attracts possible predators more than the irregular shape of a multiple-pinhole pupil. During the day…the light-adapted pupil and irregular pattern of the iris, being color-matched to the body might help them [the helmet gekko] stay hidden from birds and other predators.” (Roth et al. 2011:9)