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Schematic diagram of the vertebrate eye.


Compound eye of Antarctic krill



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Simple eyes
Pit eyes

Pinhole eye

Visual pigments appear to have a common ancestor and were probably involved in circadian rhythms or reproductive timing in simple organisms. Complex vision - associated with dedicated visual organs, or eyes - evolved many times in different lineages. Nautiluses bear a pinhole eye The pinhole eye is an "advanced" form of pit eye incorporating several improvements, most notably a small aperture (which may be adjustable) and deep pit. It is only found in the nautiloids.[1] Without a lens to focus the image, it produces a blurry image, and will blur out a point to the size of the aperture. Consequently, nautiloids can’t discriminate between objects with an

Types of eye
Nature has produced ten different eye layouts — indeed every way of capturing an image has evolved at least once in nature, with the exception of zoom and Fresnel lenses. Eye types can be categorized into "simple eyes",


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Spherical lensed 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.[1] The most basic form, still 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.[1] 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.[1] Such an inhomogeneous lens is necessary in order for the focal length to drop from about 4 times the lens radius, to 2.5 radii.[1] Heterogeneous eyes have evolved at least eight times — four or more times in gastropods, once in the copepods, once in the annelids and once in the cephalopods.[1] No aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens is great enough for this stage to be quickly "outgrown".[1] This eye creates an image that is sharp enough that motion of the eye can cause significant blurring. To minimize the effect of eye motion while the animal moves, most such eyes have stabilizing eye muscles.[1] The ocelli of insects bear a simple lens, but their focal point always lies behind the retina; consequently they can never form a sharp image. This capitulates the function of the eye. 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.[10] 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.[10] 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).[10] Weaknesses One weakness of this eye construction is that chromatic aberration is still quite high[1] — although for organisms without color vision, this is a very minor concern. A weakness of the vertebrate eye is the blind spot at the optic disc where the optic nerve is formed at the

Multiple lenses
Some marine organisms bear more than one lens; for instance the copeopod Pontella has three. The outer has a parabolic surface, countering the effects of spherical aberration while allowing a sharp image to be formed. Copilla’s eyes have two lenses, which move in and out like a telescope.[1] Such arrangements are rare and poorly understood, but represent an interesting alternative construction. An interesting use of multiple lenses is seen in some hunters such as eagles and jumping spiders, which have a refractive cornea (discussed next): these have a negative lens, enlarging the observed image by up to 50% over the receptor cells, thus increasing their optical resolution.[1]

Refractive cornea

Reflector eyes


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Superposition eyes

Compound eyes
A compound eye may consist of thousands of individual photoreception units. The image perceived is a combination of inputs from the numerous ommatidia (individual "eye units"), which are located on a convex surface, thus pointing in slightly different directions. Compared with simple eyes, compound eyes possess a very large view angle, and can detect fast movement and, in some cases, the polarization of light.[12] Because the individual lenses are so small, the effects of diffraction impose a limit on the possible resolution that can be obtained. This can only be countered by increasing lens size and number — to see with a resolution comparable to our simple eyes, humans would require compound eyes which would each reach the size of their head. Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form a single erect image.[13] Compound eyes are common in arthropods, and are also present in annelids and some bivalved molluscs.[14] Compound eyes, in arthropods at least, grow at their margins by the addition of new ommatidia.[15]

Parabolic superposition


Apposition eyes

The compound eye of a dragonfly Good fliers like flies or honey bees, or prey-catching insects like praying mantis or dragonflies, have specialized zones of ommatidia organized into a fovea area which gives acute vision. In the acute zone the eye are flattened and the facets larger. The flattening allows more ommatidia to receive light from a spot and therefore higher resolution. There are some exceptions from the types mentioned above. Some insects have a so-called single lens compound eye, a transitional type which is something between a superposition type of the multi-lens compound eye and the single lens eye found in animals with simple eyes. Then there is the mysid shrimp Dioptromysis


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Acuity is higher among male organisms that mate in mid-air, as they need to be able to spot and assess potential mates against a very large backdrop.[16] On the other hand, the eyes of organisms which operate in low light levels, such as around dawn and dusk or in deep water, tend to be larger to increase the amount of light that can be captured.[16] It is not only the shape of the eye that may be affected by lifestyle. Eyes can be the most visible parts of organisms, and this can act as a pressure on organisms to have more transparent eyes at the cost of function.[16] Eyes may be mounted on stalks to provide better allround vision, by lifting them above an organism’s carapace; this also allows them to track predators or prey without moving the head.[7]

Relationship to lifestyle


A hawk’s eye Visual acuity is often measured in cycles per degree (CPD), which measures an angular resolution, or how much an eye can differentiate one object from another in terms of visual angles. Resolution in CPD can be measured by bar charts of different numbers of white — black stripe cycles. For example, if each pattern is 1.75 cm wide and is placed at 1 m distance from the eye, it will subtend an angle of 1 degree, so the number of white — black bar pairs on the pattern will be a measure of the cycles per degree of that pattern. The highest such number that the eye can resolve as stripes, or distinguish from a gray block, is then the measurement of visual acuity of the eye. For a human eye with excellent acuity, the maximum theoretical resolution would be 50 CPD[18] (1.2 arcminute per line pair, or a 0.35 mm line pair, at 1 m). A rat can resolve only about 1 to 2 CPD.[19] A horse has higher acuity through most of the visual field of its eyes


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than a human has, but does not match the high acuity of the human eye’s central fovea region. Spherical aberration limits the resolution of a 7 mm pupil to about 3 arcminutes per line pair. At a pupil diameter of 3 mm, the spherical aberration is greatly reduced, resulting in an improved resolution of approximately 1.7 arcminutes per line pair.[20] A resolution of 2 arcminutes per line pair, equivalent to a 1 arcminute gap in an optotype, corresponds to 20/20 (normal vision) in humans.


All organisms are restricted to a small range of the electromagnetic spectrum; this varies from creature to creature, but is mainly between 400 and 700 nm[21]. This is a rather small section of the electromagnetic spectrum, probably reflecting the submarine evolution of the organ: water blocks out all but two small windows of the EM spectrum, and there has been no evolutionary pressure among land animals to broaden this range.[22] The most sensitive pigment, rhodopsin, has a peak response at 500 nm.[23] Small changes to the genes coding for this protein can tweak the peak response by a few nm;[2] pigments in the lens can also "filter" incoming light, changing the peak response.[2] Many organisms are unable to discriminate between colors, seeing instead in shades of "grey"; color vision necessitates a range of pigment cells which are primarily sensitive to smaller ranges of the spectrum. In primates, geckos, and other organisms, these take the form of cone cells, from which the more sensitive rod cells evolved.[23] Even if organisms are physically capable of discriminating different colors, this does not necessarily mean that they can perceive the different colors; only with behavioral tests can this be deduced.[2] Most organisms with color vision are able to detect ultraviolet light. This high energy light can be damaging to receptor cells. With a few exceptions (snakes, placental mammals), most organisms avoid these effects by having absorbent oil droplets around their cone cells. The alternative, developed by organisms that had lost these oil droplets in the course of evolution, is to make the lens impervious to UV light — this precludes the possibility of any UV light being detected, as it does not even reach the retina.[23]:309

The pigment molecules used in the eye are various, but can be used to define the evolutionary distance between different groups, and can also be an aid in determining which are closely related – although problems of convergence do exist.[23] Opsins are the pigments involved in photoreception. Other pigments, such as melanin, are used to shield the photoreceptor cells from light leaking in from the sides. The opsin protein group evolved long before the last common ancestor of animals, and has continued to diversify since.[2] There are two types of opsin involved in vision; copsins, which are associated with ciliary-type photoreceptor cells, and r-opsins, associated with rhabdomeric photoreceptor cells.[24] The eyes of vertebrates usually contain cilliary cells with c-opsins, and (bilaterian) invertebrates have rhabdomeric cells in the eye with ropsins. However, some ganglion cells of vertebrates express r-opsins, suggesting that their ancestors used this pigment in vision, and that remnants survive in the eyes.[24] Likewise, c-opsins have been found to be expressed in the brain of some invertebrates. They may have been expressed in ciliary cells of larval eyes, which were subsequently resorbed into the brain on metamorphosis to the adult form.[24] C-opsins are also found in some derived bilaterian-invertebrate eyes, such as the pallial eyes of the bivalve molluscs; however, the lateral eyes (which were presumably the ancestral type for this group, if eyes evolved once there) always use ropsins.[24] Cnidaria, which are an outgroup to the taxa mentioned above, express c-opsins - but r-opsins are yet to be found in this group.[24] Incidentally, the melanin

Rods and cones
The retina contains two major types of light-sensitive photoreceptor cells used for vision: the rods and the cones. Rods cannot distinguish colors, but are responsible for low-light (scotopic) monochrome (black-and-white) vision; they work well in dim light as they contain a pigment, rhodopsin (visual purple), which is sensitive at


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[12] Völkel, R.; Eisner, M.; Weible, K. J. (June 2003). "Miniaturized imaging systems" (PDF). Microelectronic Engineering 67-68 (1): 461–472. doi:10.1016/ S0167-9317(03)00102-3. Miniaturized_Imaging_Systems.pdf. • Arthropod eye [13] Gaten, Edward (1998). "Optics and phylogeny: is there an • Human eye insight? The evolution of superposition eyes in the • Mammalian eye Decapoda (Crustacea)". Contributions to Zoology 67 (4): 223–236. art01#FIGURE1. [14] Ritchie, Alexander (1985). "Ainiktozoon loganense [1] ^ Land, M F; Fernald, R D (1992). "The Evolution of Eyes". Scourfield, a protochordate? from the Silurian of Annual Review of Neuroscience 15: 1–29. doi:10.1146/ Scotland". Alcheringa 9: 137. [15] Mayer, G. (2006), "Structure and development of [2] ^ Frentiu, Francesca D.; Adriana D. Briscoe (2008), "A onychophoran eyes: What is the ancestral visual organ in butterfly eye’s view of birds", BioEssays 30: 1151, arthropods?", Arthropod Structure and Development 35 doi:10.1002/bies.20828 (4): 231–245, doi:10.1016/j.asd.2006.06.003 [3] Nairne, James (2005). Psychology. Belmont: Wadsworth [16] ^ Land, M. F. (1989), "The eyes of hyperiid amphipods: Publishing. ISBN 049503150x. OCLC 61361417. relations of optical structure to depth" (PDF), Journal of Physiology A: Sensory, Neural, and Q0oUC&pg=PA146&dq=iris+intitle:psychology+inauthor:Nairne&lr=&as_brr=0&ei=jD_dR4jcDorysgP6yMXqAQ&sig=225aBFAHEbzLuL9jBuNMC Behavioral Physiology 164 (6): 751–762, doi:10.1007/ [4] Vicki Bruce, Patrick R. Green, and Mark A. Georgeson BF00616747, (1996). Visual Perception: Physiology, Psychology and P0P467K474307K3N.pdf Ecology. Psychology Press. pp. 20. ISBN 0863774504. [17] Zeil, J. (1996), "The variation of resolution and of ommatidial dimensions in the compound eyes of the books?id=ukvei0wge_8C&pg=PA20&dq=iris+aberrations+intitle:psychology&lr=&as_brr=0&ei=EEDdR-2gAZOMtAPWpJX4AQ&sig=lTs0jcmjOgc fiddler crab Uca lactea annulipes (Ocypodidae, [5] BioMedia Associates Educational Biology Site: What Brachyura, Decapoda)" (PDF), Journal of Experimental animal has a more sophisticated eye, Octopus or Biology 199 (7): 1569–1577, Insect? reprint/199/7/1569.pdf [6] Who You Callin’ "Shrimp"? – National Wildlife [18] John C. Russ (2006). The Image Processing Handbook. Magazine CRC Press. ISBN 0849372542. OCLC 156223054. [7] ^ Cronin, Thomas W. (2008). "Exceptional Variation on a Common Theme: the Evolution of Crustacean Compound books?id=Vs2AM2cWl1AC&pg=PT110&dq=%2250+cycles+per+degree%22+a Eyes". Evolution Education and Outreach 1: 463. "The upper limit (finest detail) visible with the human doi:10.1007/s12052-008-0085-0. edit eye is about 50 cycles per degree,… (Fifth Edition, 2007, [8] Kozmik, Zbynek; Ruzickova, Jana; Jonasova, Kristyna; Page 94)" Matsumoto, Yoshifumi; Vopalensky, Pavel; Kozmikova, [19] Curtis D. Klaassen (2001). Casarett and Doull’s Iryna; Strnad, Hynek; Kawamura, Shoji; Piatigorsky, Toxicology: The Basic Science of Poisons. McGraw-Hill Joram; Paces, Vaclav; Vlcek, Cestmir (2008), "Assembly of Professional. ISBN 0071347216. OCLC 47965382. the cnidarian camera-type eye from vertebrate-like components" (PDF), Proceedings of the National books?id=G16riRjvmykC&pg=PA574&dq=cycles-perAcademy of Sciences 105 (26): 8989–8993, doi:10.1073/ degree+acuity+rat&as_brr=3&ei=i_8zR_iPKZu8swPhpty3AQ&sig=sI3lzlLAN pnas.0800388105, PMID 18577593, [20] Robert E. Fischer; Biljana Tadic-Galeb. With cgi/reprint/0800388105v1.pdf contributions by Rick Plympton… (2000). Optical System [9] Fernald, Russell D. (September 2006), "Casting a Genetic Design. McGraw-Hill Professional. ISBN 0071349162. OCLC Light on the Evolution of Eyes", Science 313 (5795): 247851267. 1914–1918, doi:10.1126/science.1127889, PMID 17008522 books?id=byx2Ne9cD1IC&pg=PA164&ots=bPLQRko6xA&dq=eye+resolution [10] ^ Wilson, M. (1978), "The functional organisation of pairs+1.7&sig=eA2LwmFzvZ9mvlocust ocelli", Journal of Comparative Physiology (4): nzZaEINXiWKJE#PPA164,M1. 297–316 [21] Barlow, Horace Basil; Mollon, J. D. (1982), The Senses, [11] Wagner, H.J., Douglas, R.H., Frank, T.M., Roberts, N.W., pp. 98, ISBN 0521244749, and Partridge, J.C. (Jan. 27, 2009). "A Novel Vertebrate books?id=kno6AAAAIAAJ&pg=PA98&vq=human+spectral+sensitivity&dq= Eye Using Both Refractive and Reflective Optics". Current [22] Fernald, Russell D. (1997), "The Evolution of Eyes" (Free Biology 19: 108–114. doi:10.1016/j.cub.2008.11.061. PMID full text), Brain, Behavior and Evolution 50 (4): 253–259, 19110427.

See also



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doi:10.1159/000113339, fernaldlab/pubs/1997%20Fernald.pdf [23] ^ Goldsmith, T. H. (1990), "Optimization, Constraint, and History in the Evolution of Eyes" (PDF), The Quarterly Review of Biology 65 (3): 281, doi:10.1086/416840, [24] ^ Nilsson, D (2008). "Eye Evolution: the Blurry Beginning". Current Biology 18: R1096. doi:10.1016/ j.cub.2008.10.025. edit • "Anatomy". History of Ophthalmology. anatomy.htm. Retrieved on 23 April 2005. • Kandel ER, Schwartz JH, Jessell TM. Principles of Neural Science, 4th ed. McGraw-Hill, New York (2000). ISBN 0-8385-7701-6 [1] The precise number depends on the author


External links
• Evolution of the eye • Diagram of the eye • Webvision. The organisation of the retina and visual system. An in-depth treatment of retinal function, open to all but geared most toward graduate students. • Eye strips images of all but bare essentials before sending visual information to brain, UC Berkeley research shows • A collection of resources for eye anatomy. • Anatomy of the eye - flash animated interactive.

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