• How Camera Lenses Affect What The Viewer Sees

• How camera lenses affect what the viewer sees

A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, concentrating or diverging the beam. A simple lens is a lens consisting of a single optical element. A compound lens is an array of simple lenses (elements) with a common axis; the use of multiple elements allows more optical aberrations to be corrected than is possible with a single element. Manufactured lenses are typically made of glass or transparent plastic. Elements which refract electromagnetic radiation outside the visual spectrum are also called lenses: for instance, a microwave lens can be made from paraffin wax.

The archaic spelling lense is sometimes seen, but Merriam Webster's medical dictionary is the only major dictionary that considers this to be correct.

The oldest lens artefact is dated to c.640 BC, a rock crystal lens found at excavations in Niniveh. The earliest written records of lenses date to Ancient Greece, with Aristophanes' play The Clouds (424 BC) mentioning a burning-glass (a biconvex lens used to focus the sun's rays to produce fire). The writings of Pliny the Elder (23вЂ"79) also show that burning-glasses were known to the Roman Empire, and mentions what is possibly the first use of a corrective lens: Nero was said to watch the gladiatorial games using an emerald (presumably concave to correct for myopia, though the reference is vague). Both Pliny and Seneca the Younger (3 BCвЂ"65) described the magnifying effect of a glass globe filled with water.

The word lens comes from the Latin name of the lentil, because a double-convex lens is lentil-shaped. The genus of the lentil plant is Lens, and the most commonly eaten species is Lens culinaris. The lentil plant also gives its name to a geometric figure.

Excavations at the Viking harbour town of FrÐ"¶jel, Gotland, Sweden discovered in 1999 the rock crystal Visby lenses, produced by turning on pole-lathes at FrÐ"¶jel in the 11th to 12th century, with an imaging quality comparable to that of 1950s aspheric lenses. The Viking lenses concentrate sunlight enough to ignite fires.

Widespread use of lenses did not occur until the use of reading stones in the 11th century and the invention of spectacles, probably in Italy in the 1280s. Nicholas of Cusa is believed to have been the first to discover the benefits of concave lenses for the treatment of myopia in 1451.

Most lenses are spherical lenses: their two surfaces are parts, with the same axis as each other, of the surfaces of spheres. Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat). The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through the physical centre of the lens.

Toric or shero-cylindrical lenses have surfaces with two different radii of curvature in two orthogonal planes. They have a different focal power in different meridians. This is a form of deliberate astigmatism.

More complex are aspheric lenses. These are lenses where one or both surfaces have a shape that is neither spherical nor cylindrical. Such lenses can produce images with much less aberration than standard simple lenses.

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 convex, A lens with two 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. It is this type of lens that is most commonly used in corrective lenses.

If the lens is biconvex or plano-convex, a collimated or parallel beam of light travelling parallel to the lens axis and passing through the lens will be converged (or focused) to a spot on the axis, at a certain distance behind the lens (known as the focal length). In this case, the lens is called a positive or converging lens.

Both converging and diverging lenses have a principal axis through the centre of the lens and perpendicular to it. The principal focus F (aka focal point) lies on the principal axis, at the focal distance (focal length).

Rays from a distant object travel parallel to the principal axis and are refracted through F. (In the case of a diverging lens, they appear to have come from F on the same side as the object).

Constructing ray diagrams

There are three rays required to construct a ray diagram for a lens (see Ordinary Level Physics by A F Abbott, p272-273 in my edition).

(1) Rays parallel to the principal axis are refracted through the principal focus F.

(2) Rays through the principal focus F are refracted to emerge parallel to the principal axis.

(3) Rays through the optical centre of the lens do not change course.

Diverging lens

Construction

Focusing power

The greater the curvature of the lens, the greater its strength or power. Obviously it also reduces its focal length at the same time (by implication then, a short focal length means a powerful lens).

So power = 1/focal length (in metres) Units: dioptres (D)

We say converging lenses have a positive power, while diverging lenses have a negative power.

The human eye

The cornea itself has a power of about +43D. The eye lens varies from about 17D at its thinnest (unaccommodated) state, to about +31D at its thickest (accommodated) state. So the eye’s total power ranges from about +60D to about +74D.

Even so, there is a limit to how close an object can be focused. This is called the near point, and it gets further away with age вЂ" this is called presbyopia, and is the reason why older people tend to hold their newspapers at arm’s length until they succumb to wearing reading glasses!

The eye’s sensitivity to light

The retina is made up of two types of cells, rods and cones.

Rods are sensitive to low light levels. Cones are colour sensitive; the