The pupils and optical systems of gecko eyes

The nocturnal helmet gecko, Tarentola chazaliae, discriminates colors in dim moonlight when humans are color blind. The
sensitivity of the helmet gecko eye has been calculated to be 350 times higher than human cone vision at the color vision
threshold. The optics and the large cones of the gecko are important reasons why they can use color vision at low light
intensities. Using photorefractometry and an adapted laboratory Hartmann–Shack wavefront sensor of high resolution,
we also show that the optical system of the helmet gecko has distinct concentric zones of different refractive powers, a
so-called multifocal optical system. The intraspecific variation is large but in most of the individuals studied the zones
differed by 15 diopters. This is of the same magnitude as needed to focus light of the wavelength range to which gecko
photoreceptors are most sensitive. We compare the optical system of the helmet gecko to that of the diurnal day gecko,
Phelsuma madagascariensis grandis. The optical system of the day gecko shows no signs of distinct concentric zones and
is thereby monofocal.