A simple camera lens bulges on both sides, and is consequently known as biconvex. Lenses of this type concentrate light passing through the glass element onto single a point. The distance from the optical centre of the lens to the equivalent point in front of the lens at which light would theoretically converge is known as the lens's focal length. This standard measurement is used for all lenses which cause light to converge, including the more advanced compound designs.
Unfortunately a simple biconvex, spherical-surface lens manufactured using single-density glass is prone to various types of optical distortion. These are sufficiently severe to render very blurred the image produced. This fundamental problem is cured to a large extent by the use of compound lenses. These consist of several different elements, some containing optical glass that varies in density and others of an aspherical design, assembled in groups of five or more individual lenses. The result is the modern camera lens which produces very sharp and almost distortion-free images. However, even these modern wonders of computer design are not perfect.
Some lenses produce so-called barrel distortion oat the outer edges of the field of view. This renders straight lines bulged outwards so that a rectangle would approach the shape of a barrel. Others produce so-called pincushion distortion which is a similar effect realized in the opposite direction. The result is that the sides of a rectangle tend to bow inwards.
Other types of distortion include axial and lateral chromatic aberration. Axial chromatic aberration occurs in a lens because light of different wavelengths (colours) is focused at slightly different focal points. The shorter wavelengths of the blue part of the visible spectrum are typically focused in front of the nominal plane of focus, and the longer wavelengths of the red part of the spectrum are focused behind the plane of focus. The effect manifest itself as a minor colour fringing dispersed evenly across an entire image. Lateral chromatic aberration is caused by different wavelengths of light being diffracted by varying amounts as they pass through lens elements. The continuous spectrum of light is consequently separated into its constituent colours so that red, green, and blue wavelengths are focused at different locations in the plane of focus. The effect is not normally noticed at the centre of a lens, but may become more pronounced towards its outer edges.
Another source of image degradation in some lenses is spherical aberration. This is caused by rays of light passing through the periphery of a lens being brought to marginal focus at a shorter focal distance than that for rays passing through the lens closer to its optical axis. The effect is most apparent in thin lenses.
Diffraction may degrade image quality when very small apertures are used. Rays of light encountering sharp edges such as those found on the blades of an iris diaphragm may be deflected. The effect becomes more pronounced as the light waves strike the edges of the blades at a more acute angle, and is noticed first at the longer (red) wavelengths. With lenses designed for 35mm film cameras, and the smaller DX-format digital sensors, diffraction typically becomes an issue at apertures smaller than f/11. It also becomes more noticeable as the focus distance is reduced.