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In humans, the pupil is circular, but its shape varies between species; some cats, reptiles, and foxes have vertical slit pupils, goats and sheep have horizontally oriented pupils, and some catfish have annular types. [3] In optical terms, the anatomical pupil is the eye's aperture and the iris is the aperture stop.
The pupil of the human eye can range in size from 2 mm to over 8 mm to adapt to the environment. The human eye can detect a luminance from 10 −6 cd/m 2, or one millionth (0.000001) of a candela per square meter to 10 8 cd/m 2 or one hundred million (100,000,000) candelas per square meter.
As the pupil enlarges, more peripheral rays enter the eye and the focus shifts anteriorly, making the patient slightly more myopic in low-light conditions. The effect of spherical aberration (a fourth-order aberration) increases as the fourth power of the pupil diameter.
Humans, other primates, and dogs have round (spherical) pupils, which are useful for general all-round vision. Cats, snakes, crocodiles, and alligators have vertical slits which are useful for ...
When pupils are dilated, it means that they grow larger in size. This can happen for a number of different reasons, including drug use and arousal. 7 reasons why your pupils may be dilated, from ...
The iris consists of two layers: the front pigmented fibrovascular layer known as a stroma and, behind the stroma, pigmented epithelial cells.. The stroma is connected to a sphincter muscle (sphincter pupillae), which contracts the pupil in a circular motion, and a set of dilator muscles (dilator pupillae), which pull the iris radially to enlarge the pupil, pulling it in folds.
This type of eye is common in mammals, including humans. The simplest eyes are pit eyes. They are eye-spots which may be set into a pit to reduce the angle of light that enters and affects the eye-spot, to allow the organism to deduce the angle of incoming light. [1] Eyes enable several photo response functions that are independent of vision.
The human eye, showing the iris and pupil. In 1802, philosopher William Paley called it a miracle of "design."In 1859, Charles Darwin himself wrote in his Origin of Species, that the evolution of the eye by natural selection seemed at first glance "absurd in the highest possible degree". [3]