Photographic Lens Design - Types of Lenses

Types of Lenses

The type of lens being designed is significant in setting the key parameters.

• Prime lens - a photographic lens whose focal length is fixed, as opposed to a zoom lens, or it is the primary lens in a combination lens system.
• Normal lens - a lens with a focal length about equal to the diagonal size of the film or sensor format, or that reproduces perspective that generally looks "normal" to a human observer.

• Wide angle lens - a lens that reproduces perspective that generally looks "wider" than a Normal lens. The problem posed by the design of wide angle lenses is to bring to an accurate focus light from a wide area without causing internal flare. Wide angle lenses therefore tend to have more elements than a normal lens to help refract the light sufficiently and still minimise aberrations whilst adding light-trapping baffles between each lens element.

• Extreme wide angle lens - a wide angle lens with an angle of view above 90 degrees. Extreme wide angle lenses share the same issues as ordinary wide angle lenses but the focal length of such lenses may be so short that there is insufficient physical space in front of the film or sensor plane to construct a lens. This problem is resolved by constructing the lens as an inverted telephoto, or retrofocus with the front element having a very short focal length, often with a highly exaggerated convex front surface and behind it a strongly negative lens grouping that extends the cone of focused rays so that they can be brought to focus at a reasonable distance.

• Long focus lens - a lens with a focal length greater than the diagonal of the film frame or sensor. Long focus lenses are relatively simple to design, the challenges being comparable to the design of a prime lens. However, as the focal length increases the length of the lens and the size of the objective increase in size and length and weight quickly become significant design issues in retaining utility and practicality for the lens in use. In addition because the light path through the lens is long and glancing, the importance of baffles to control flare increases in importance.
• Telephoto lens - an optically compressed version of the long focus lens. The design of telephoto lenses reduces some of the problems encountered by designers of long focus lenses. In particular, telephoto lenses are typically much shorter and may be lighter for equivalent focal length and aperture. However telephoto designs increase the number of lens elements and can introduce flare and exacerbate some optical aberrations.
• Zoom lenses - variable focal length lenses. Zoom lenses cover a range of focal lengths by utilising movable elements within the barrel of the lens assembly. In early zoom lenses the focus also shifted as the lens focal length was changed. All modern zoom lenses are now confocal, meaning that the focus is maintained throughout the zoom range. Because of the need to operate over a range of focal lengths and maintain confocality, zoom lenses typically have very many lens elements. More significantly the front elements of the lens will always be a compromise in terms of its size, light-gathering capability and the angle of incidence of the incoming rays of light. For all these reasons, the optical performance of zoom lenses tends to be lower than fixed focal length lenses.
• Catadioptric lens - catadioptric lenses are a form of telephoto lens but with a light path that doubles back on itself and with an objective that is a mirror combined with some form aberration correcting lens (a catadioptric system) rather than just a lens. A centrally-placed secondary mirror and usually an additional small lens group bring the light to focus. Such lenses are very lightweight and can easily deliver very long focal lengths but they can only deliver a fixed aperture and have none of the benefits of being able to stop down the aperture to increase depth of field.
• Anamorphic lenses are used principally in cinematography to produce wide-screen films where the projected image has a substantially different ratio of height to width than the image recorded on the film plane. This is achieved by the use of a specialised lens design which compresses the image laterally at the recording stage and the film is then projected through a similar lens in the cinema to recreate the wide-screen effect. Although in some cases the anamorphic effect is achieved by using an anamorphising attachment as a supplementary element on the front of a normal lens, most films shot in anamorphic formats use specially-designed anamorphic lenses, such as the Hawk lenses made by Vantage Film or Panavision's anamorphic lenses. These lenses incorporate one or more aspheric elements in their design.