Focal length, refractor lenses and mirror lenses  

When a lens is focused at infinity, the distance between the lens and the focal plane (i.e., the plane where the film or sensor are located, and onto which the image of the subject is projected) is equal to the focal length of the lens. In fact, this is the way the focal length of a lens is defined. This is easy to verify for lenses that consist of a single, thin optical element. An optical element is a single sheet of glass or other transparent material, the thickness of which typically changes across the lens surface. The lenses used in eyeglasses, for example, consist of a single element. In this type of lens, the nodal point (or pupil) of the lens is located at the centre of its optical element, and the focal length is measured between the nodal point and the focal plane. 

All modern lenses consist of several optical elements, and are quite thick about their optical axes (i.e., there is a considerable distance between the surfaces of their front and back elements. Usually, the nodal point of these lenses is not located at their mid-point, and often is located altogether outside the lens itself.

There are practical reasons for building lenses with these properties: for instance, in an SLR (and DSLR) camera, the mirror assembly is located between the focal plane and the back lens element. Thus, the back lens element cannot be placed close to the focal plane. In a single-element lens, this would place a limit on the minimum focal length of a lens. If the nodal point of the lens is located in the air behind its back element, instead, this constraint is avoided, and lenses with a very short focal length can be built. This type of lens is called retrofocus. For instance, in many SLR cameras the minimum distance between the focal plane and the back element of the lens is approximately 40-50 mm, but retrofocus lenses to fit these cameras can have a focal length as short as 8 mm.

Another practical application of this principle is in super-telephoto lenses, which may have a focal length of 1200 mm or more. It is possible to built a lens of this focal length that has its nodal point located well in front of the lens, so that the lens itself can have a physical length much shorter than 1200 mm (typically around 800 mm, for a variety of reasons). This makes a super-telephoto lens lighter, cheaper and easier to transport.

Most camera lenses actually possess two nodal points, one relative to the front of the lens, and one relative to its back. If you look into one of these lenses through either end, the image of its diaphragm appears to be located at a different distance from the front or back elements, and the two apparent distances do not add up to the actual length of the lens barrel.

Macrophotography and close-up

At close range, the focal length of a lens affects its working distance and focus distance. This is the most important factor in choosing a macro lens for a particular situation.


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