Camera Lens Focal Length
How focal length affects photograph composition: adjusting the camera's distance from the main subject while changing focal length, the main subject can remain the same size, while the other at a different distance changes size.When a photographic lens is set to "infinity", its rear nodal point is separated from the sensor or film, at the focal plane, by the lens's focal length. Objects far away from the camera then produce sharp images on the sensor or film, which is also at the image plane. Photographers sometimes refer to the image plane as the focal plane; these planes coincide when the object is at infinity, but for closer objects the focal plane is fixed, relative to the lens, and the image plane moves, by the standard optical definitions.
The focal length of a lens determines the magnification at which it images distant objects. The focal length of a lens is equal to the distance between the image plane and a pinhole (see pinhole camera model) that images distant small objects the same size as the lens in question. Combining this definition with an assumption of rectilinear imaging (that is, with no image distortion) leads to a simple geometric model the photographers use for computing the angle of view of a camera.
Note that some simple and usually inexpensive cameras have fixed focus lenses which cannot be adjusted.
Focal lengths are usually specified in millimetres (mm), but older lenses marked in centimetres (cm) and inches are still to be found. The angle of view depends on the ratio between the focal length and the film size. Due to the popularity of the 35 mm standard, lenses are often described in terms of their 35 mm equivalent fields of view. Traditional ranges of 35-mm-equivalent focal length describe a normal lens (typically 40–50 mm), wide-angle lens (typically 35 mm and less), and telephoto lens (typically 85 mm and more). Use of this "equivalent" is particularly common with digital cameras, which generally use sensors smaller than 35 mm film, and so require correspondingly shorter focal lengths to generate equivalent images.
