![]() ![]() We have to consider that there are high differences between the ocular structures of the avian and human eyes (especially taking into account that the iris is firmly attached to the peripheral anterior lens surface in avian eyes). Then, iris sphincter contraction alone will not affect lens shape. They suggested that contraction of the ciliary muscle pushes the malleable lens against the iris disc and the central lens bulges through the pupil. Rather than actively deforming the lens, contraction of the iris sphincter results in the formation of a rigid disc with a central aperture, the pupil. Levy and Sivak 9 proposed that the role of the iris is somewhat more passive. Walls 7 and Goodge 8 also suggested that accommodative effects of great magnitude are brought about by the action of iris sphincter contraction of the lens. ![]() rather than acting on the lens directly the ciliary muscle facilitates the action of the iris sphincter muscle on the lens. Hess 6 proposed that contraction of the ciliary muscle in birds such as the cormorant improves contact between the iris and the anterior lens surface i.e. Levy and Sivak 9 found that certain aquatic birds possess an exaggerated accommodative ability resulting in a drastic change in the curvature of the anterior lens surface. 6–11 These species used pupil miosis to modify the power of the first surface of the lens increasing its curvature. However, it remains unclear if the physical constriction of the iris causes a change in the optics of the eye as opposed to simply having a smaller pupil.Īs evident example of the effect of the iris on the eye optics, some animal species such us diving birds or sea terns, accommodate by squeezing the lens with the outer portion of the iris. After pupil miosis there are some well studied changes such as reduction in high-order aberrations, 1 increasing diffractions effects, 2 increase depth-of-focus 3, 4 and reduction of the Stiles Crawford effect 5 that modify the optical quality of the eye. It is well know that pupil size affects the optical quality of the human eye. ![]()
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