LYCOS RETRIEVER 
Lens: Light
built 606 days ago
A lens can be thought of as two rounded prisms joined together. Light passing through the lens is always bent toward the thickest part of the prisms. To make a minus lens (above on the left), the thickest part, the base, of the prisms is on the outer edges and the thinnest part, the apex, is in the middle. This spreads the light away from the center of the lens and moves the focal point forward. The stronger the lens, the farther the focal point is from the lens.
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A negative meniscus lens has a steeper concave surface and will be thinner at the centre than at the periphery. Conversely, a positive meniscus lens has a steeper convex surface and will be thicker at the centre than at the periphery. An ideal thin lens with two surfaces of equal curvature would have zero optical power, meaning that it would neither converge nor diverge light. All real lenses have a nonzero thickness... which affects the optical power. To obtain exactly zero optical power, a meniscus lens must have slightly unequal curvatures to account for the effect of the lens' thickness.
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Behind the pupil and iris are the crystalline lens and the ciliary muscle. The muscle holds the lens in place and changes its shape. The lens is a colorless, nearly transparent double convex structure, similar to an ordinary magnifying glass. Its only function is to focus light rays onto the retina. The lens is made of elongated cells that have no blood supply.
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The Sigma lens, by comparison, is an all-plastic barrel and has a cheap feel to it. The manual focus ring doesn't feel like it's connected to anything, although it obviously works. The front element of the Sigma is ... highly curved and sticks out right to the extent allowed by the lens cap. Fortunately, it's smaller than the diameter of the lens at this point (something I don't quite understand--tell me why it had to be 82mm out front). The Sigma is so light in heft compared to the Nikkor, you'll wonder how it can even take decent pictures. But it does.
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The AcrySof® IQ IOL is designed to conform to the natural shape of the lens capsule. This helps it stay stable and centered in the eye. But what makes the AcrySof® IQ IOL unique is its ability to filter blue light.
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The best way to understand the behavior of light through a curved lens is to relate it to a prism. A prism is thicker at one end, and light passing through it is bent (refracted) toward the thickest portion. See the diagram below.
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