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There are five main spherical lens forms, or shapes, that determine the imaging characteristics of the lens. They are plano-convex, plano-concave, bi-convex, bi-concave, and meniscus. By their very nature, plano-convex and bi-convex lenses are of positive power. This characteristic is denoted by positive focal lengths. On the other hand, plano-concave and bi-concave lenses are of negative power: their focal lengths are negative. Meniscus lenses can be of either positive or negative power.
- Plano-convex lenses have a positive focal length, which makes them ideal for collecting and focusing light for many imaging applications. This is the most common type of lens element. It can be used to focus, collect and collimate light. It is also useful as a simple imaging lens where image quality requirement is not too critical.
- Plano-concave lenses have a negative focal length and are used for image reduction or to spread light. These lenses diverge collimated incident light. They form virtual images which are seen through the lens. They are often used to expand light or to increase focal lengths in existing systems.
- Bi-convex lenses have a positive focal length and are useful for 1:1 imaging and in multi-element systems.
- Bi-concave lenses have negative focal length, and are used for image reduction and to spread light. These lenses diverge collimated incident light and form virtual images, which are seen through the lens. They are often used to expand light or increase focal lengths in existing systems. They are used in laser beam expanders, optical character readers, viewers, and projection systems.
- Meniscus lenses can increase the numerical aperture of a positive lens assembly, without an undue increase in the aberration.Aplanatic meniscus lenses are used in conjunction with achromatic doublet lenses to form high quality, low-f/number imaging systems.
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Imaging performance of an individual spherical singlet is limited by significant monochromatic and polychromatic aberrations. When used in combination, however, one singlet can cancel aberrations caused by another. The result is a system of higher-quality performance. It is important to note that extremely high-quality performance often requires specially designed components.
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