500

The distance between the centre of the lens and the film, the focal length, ensures the object is accurately focused even when it is located at any infinite point. It is measured in millimetres e.g. 500 mm and is marked on the camera mount along with the largest aperture for photographer’s convenience. A focal length of any given camera decides it's category into normal, telephoto, and wide angle. It is essential in photography as it establishes field of view of the camera, which is indirectly proportionate to the focal length e.g. 500 mm. A normal lens that is compatible with 35 mm film with the focal length of 50 mm, gives a field of view that is almost equivalent to that of a normal vision. A telephoto lens is available in a wide range of focal length: 60 mm to 1000 mm. A telephoto lens aids in magnifying the subject, albeit it narrows the field of vision, resulting in images with flatter looks compared to that produced by standard lens.

This type of lens is mostly integrated in SLR camera. It is manufactured by many popular lens makers. While some specifications of this lens may differ, technical configurations such as angle of view (4.9-degree) and closest focus (4m) remain same. Interestingly, this lens has only one aperture possibility that is f/8. Most of the 500 mm lenses have straight helicoids focusing with smallest field being 28 x 19 cm. External features such as weight (590g), length (97mm) and diameter (81mm) are general estimated figures. Some of these may also have an inbuilt hood and 72mm filter, which is screwed in the lens.