![]() ![]() The model of a shape changing lens of humans was proposed by Young in a lecture on the 27th Nov 1800. Those models that stress the importance of external forces acting on a more passively elastic lens and other models that include forces that may be generated by the lens internally. The models can be broadly divided into two camps. When considering vertebrates, aspects of all models may play varying roles in lens focus. Direct experimental proof of any lens model is necessarily difficult as the vertebrate lens is transparent and only functions well in the living animals. What is less well understood is how the subtle, precise and very quick changes in lens shape are made. Varying forms of direct experimental proof outlined in this article show that most non-aquatic vertebrates achieve focus, at least in part, by changing the shapes of their lenses. Land based animals and the shape changing lens Therefore eyes evolved in water have a mechanism involving changing the distance between a rigid rounder more refractive lens and the retina using less uniform muscles rather than subtly changing the shape of the lens itself using circularly arranged muscles. The small difference in refractive index between water and the hydrated cornea means fish and amphibians need to bend the light more using the internal structures of the eye. ![]() Generally mammals, birds and reptiles living in air vary their eyes' optical power by subtly and precisely changing the shape of the elastic lens using the ciliary body. Peripheral distortions are also present in animal lenses though are typically covered by the iris (anatomy) The previous video of the eye lens changing shape with focus is placed into context as the lens in this video is placed into the context of a virtual eye. Most image distortions near the center are due to model being limited to 512 faces to make up the lens. 3D reconstruction based on measurements taken from a 20 year old human male focusing from 26mm to infinity (4.85 dioptre change). As a result animals living in air have most of the bending of light achieved at the air/cornea interface with the lens being involved in finer focus of the image. The air/cornea interface involves a larger difference in refractive index than hydrated structures within the eye. The eyes are therefor required to bend light different amounts leading to different mechanisms of focus being used in different environments. Virtual eye showing the contribution to focus of different components.ĭue to the nature of optics the focused image on the retina is always inverted relative to the object.ĭifferent animals live in different environments having different refractive indexes involving water, air and often both. Very even systematic curvature of parts of the cornea and lens produces this systematic bending of light onto the retina. The real image formed from millions of these points of light is what animals see using their retinas. To get a good image of these points of light on a defined area requires a precise systematic bending of light called refraction. Note objects in some size ranges and distances do not require the light path to bend noticeably to achieve focus.įocusing the light scattered by objects in a three dimensional environment into a two dimensional collection of individual bright points of light requires the light to be bent. Changing the axial length of the eyeball.įocusing mechanisms The path of light through the eye calculated using four refractive indexes, cornea and lens curvatures approximating components of real eyes. Changing the position of the lens relative to the retina. The main ways animals may change focus are: In this, distances vary for individuals from the far point-the maximum distance from the eye for which a clear image of an object can be seen, to the near point-the minimum distance for a clear image.Īccommodation usually acts like a reflex, including part of the accommodation-vergence reflex, but it can also be consciously controlled. Focusing ability of eye Minimum (top) and maximum accommodation (bottom).Īccommodation is the process by which the vertebrate eye changes optical power to maintain a clear image or focus on an object as its distance varies. ![]()
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