Editing Lens (section)

===Types of simple lenses=== <!--Many redirects point to this section title-->

[[File:Lenses en.svg|Types of lenses|alt=Types of lenses|thumb]]
Lenses are classified by the curvature of the two optical surfaces. A lens is ''biconvex'' (or ''double convex'', or just ''convex'') if both surfaces are [[wikt:convex|convex]]. If both surfaces have the same radius of curvature, the lens is ''equiconvex''. A lens with two [[wikt:concave|concave]] surfaces is ''biconcave'' (or just ''concave''). If one of the surfaces is flat, the lens is ''plano-convex'' or ''plano-concave'' depending on the curvature of the other surface. A lens with one convex and one concave side is ''convex-concave'' or ''meniscus''. Convex-concave lenses are most commonly used in [[corrective lenses#Lens shape|corrective lens]]es, since the shape minimizes some aberrations.

For a biconvex or plano-convex lens in a lower-index medium, a [[collimated light|collimated]] beam of light passing through the lens converges to a spot (a ''focus'') behind the lens. In this case, the lens is called a ''positive'' or ''converging'' lens. For a [[thin lens]] in air, the distance from the lens to the spot is the [[focal length]] of the lens, which is commonly represented by  {{mvar|f}}  in diagrams and equations. An [[extended hemispherical lens]] is a special type of plano-convex lens, in which the lens's curved surface is a full hemisphere and the lens is much thicker than the radius of curvature.

Another extreme case of a thick convex lens is a [[ball lens]], whose shape is completely round. When used in novelty photography it is often called a "lensball". A ball-shaped lens has the advantage of being omnidirectional, but for most [[optical glass]] types, its focal point lies close to the ball's surface. Because of the ball's curvature extremes compared to the lens size, [[optical aberration]] is much worse than thin lenses, with the notable exception of [[chromatic aberration]].

{|
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|[[File:lens1.svg|left|390px|Biconvex lens]]
|[[File:Large convex lens.jpg|right|250px]]
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For a biconcave or plano-concave lens in a lower-index medium, a collimated beam of light passing through the lens is diverged (spread); the lens is thus called a ''negative'' or ''diverging'' lens. The beam, after passing through the lens, appears to emanate from a particular point on the axis in front of the lens. For a thin lens in air, the distance from this point to the lens is the focal length, though it is negative with respect to the focal length of a converging lens.
{|
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|[[File:lens1b.svg|left|390px|Biconcave lens]]
|[[File:concave lens.jpg|right|250px]]
|}
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The behavior reverses when a lens is placed in a medium with higher refractive index than the material of the lens. In this case a biconvex or plano-convex lens diverges light, and a biconcave or plano-concave one converges it.

[[File:Meniscus lenses.svg|thumb|right|upright=0.6|Meniscus lenses: negative (top) and positive (bottom)]]
Convex-concave (meniscus) lenses can be either positive or negative, depending on the relative curvatures of the two surfaces. A ''negative meniscus'' lens has a steeper concave surface (with a shorter radius than the convex surface) and is thinner at the centre than at the periphery. Conversely, a ''positive meniscus'' lens has a steeper convex surface (with a shorter radius than the concave surface) and is thicker at the centre than at the periphery.

An ideal [[thin lens]] with two surfaces of equal curvature (also equal in the sign) would have zero [[optical power]] (as its focal length becomes infinity as shown in the [[#Lensmaker's equation|lensmaker's equation]]), meaning that it would neither converge nor diverge light. All real lenses have a nonzero thickness, however, which makes a real lens with identical curved surfaces slightly positive. 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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