Editing Eye (section)

====Spherical lens eye====
The resolution of pit eyes can be greatly improved by incorporating a material with a higher [[refractive index]] to form a lens, which may greatly reduce the blur radius encountered—hence increasing the resolution obtainable. The most basic form, seen in some gastropods and annelids, consists of a lens of one refractive index. A far sharper image can be obtained using materials with a high refractive index, decreasing to the edges; this decreases the focal length and thus allows a sharp image to form on the retina. This also allows a larger aperture for a given sharpness of image, allowing more light to enter the lens; and a flatter lens, reducing [[spherical aberration]]. Such a non-homogeneous lens is necessary for the focal length to drop from about 4 times the lens radius, to 2.5 radii.<ref name=Land1992/>

So-called under-focused lens eyes, found in gastropods and polychaete worms, have eyes that are intermediate between lens-less cup eyes and real camera eyes. Also [[box jellyfish]] have eyes with a spherical lens, cornea and retina, but the vision is blurry.<ref>[https://books.google.com/books?id=vQgWDAAAQBAJ&dq=Under-focused+lens+eyes+intermediate+cup+box+jellyfish&pg=PA76 Animal Eyes]</ref><ref>[https://books.google.com/books?id=A26JAgAAQBAJ&dq=Box+jellyfish+rhopalium+cornea+lens+pigment+retina&pg=PA306 Perceiving in Depth, Volume 1: Basic Mechanisms]</ref>

Heterogeneous eyes have evolved at least nine times: four or more times in [[Sensory organs of gastropods|gastropods]], once in the [[copepod]]s, once in the [[annelid]]s, once in the [[cephalopod]]s,<ref name=Land1992/> and once in the [[chiton]]s, which have [[aragonite]] lenses.<ref name="Speiser2011">{{Cite journal | last1=Speiser | first1=D.I. | last2=Eernisse | first2=D.J. | last3=Johnsen | first3=S.N. | doi=10.1016/j.cub.2011.03.033 | title=A Chiton Uses Aragonite Lenses to Form Images | journal=Current Biology | volume=21 | issue=8 | pages=665–670 | year=2011 | pmid= 21497091| s2cid=10261602 | doi-access=free | bibcode=2011CBio...21..665S }}</ref> No extant aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens is great enough for this stage to be quickly "outgrown".<ref name=Land1992/>

This eye creates an image that is sharp enough that motion of the eye can cause significant blurring. To minimise the effect of eye motion while the animal moves, most such eyes have stabilising eye muscles.<ref name=Land1992/>

The [[ocellus|ocelli]] of insects bear a simple lens, but their focal point usually lies behind the retina; consequently, those can not form a sharp image. Ocelli (pit-type eyes of arthropods) blur the image across the whole retina, and are consequently excellent at responding to rapid changes in light intensity across the whole visual field; this fast response is further accelerated by the large nerve bundles which rush the information to the brain. Focusing the image would also cause the sun's image to be focused on a few receptors, with the possibility of damage under the intense light; shielding the receptors would block out some light and thus reduce their sensitivity. This fast response has led to suggestions that the ocelli of insects are used mainly in flight, because they can be used to detect sudden changes in which way is up (because light, especially UV light which is absorbed by vegetation, usually comes from above).<ref name="Wilson1978">{{Cite journal |last=Wilson |first=M. |year=1978 |title=The functional organisation of locust ocelli |journal=Journal of Comparative Physiology |volume=124 |issue=4 |pages=297–316 |doi=10.1007/BF00661380 |s2cid=572458}}</ref>