Lenses, elements, groups and symmetry

A lens consist of several elements. Individual elements are cemented together to form a group in order to improve the optical imaging properties, i.e. they form an optical unit.

To reduce imaging errors, symmetrical lens designs, which have a largely symmetrical lens arrangement around the optical center, i.e. around the aperture of the lens, are a common type.

Lens types

The basic type of many modern lens designs is the triplet, also known as the Cooke type:

This lens has rapidly evolved into the Tessar, a lens with four elements in three groups, which in principle is still used today, especially in the field of telephoto lenses:

Further improvements of the Tessar lead to the Sonnar, which still shows the origin of the triplet, but uses much more complex groups. Today it is used especially in the lower tele range:

Symmetrical lens designs are mostly based on the Gaussian type.
The four-lens design – in an improved form, in particular by using multi-lens groups – is used today in the range of fast normal focal lengths according to the planar principle and for macro lenses. Other types of this type include the Symmar and the Super Angulon. The Planar and related types are used for normal focal lengths, i.e. 50 mm lenses for 35 mm and 80 mm for medium format.

The symmetrical design greatly reduces many aberrations, such as distortion.

Wide-angle lenses have a basic problem: they have a very small image width, i.e. a short distance between the rear lens element and the film. However, SLR cameras must be able to place and move their oscillating mirror exactly in this intermediate space. A solution to this problem is offered by the retrofocus design of wide-angle lenses: a convex-concave front lens is used in conjunction with a concave rear lens, so that the image width is increased by widening the optical path. This design principle is used, for example, in the Distagon.

In a sense, the opposite problem occurs with very long focal length lenses. Lenses above the normal focal length are called telephoto lenses. More precisely, a telephoto lens is characterized by the fact that the distance of the last lens element to the image plane is smaller than its focal length (while the retrofocus construction increases this distance).
The Sonnar is the typical lens construction for light telephoto lenses. It consists of three assemblies and uses achromatic lenses.
The classic construction for a telephoto lens is the Tessar. It is based on the three-lens Cookc optics.