Camera Lens Focal Length Simplified
A camera lens is also know as photographic lens is an assembly of lenses used in conjunction with a camera body and mechanism of objects either on film or on other media capable of storing an image chemically or electronically. There is no major difference in principle between a lens used for video camera, still camera, a telescope, microscope or other apparatus but the detailed design and construction are different. A lens may be permanently fixed to a camera or it may be interchangeable with lenses of different apertures, focal length, and other properties.
In operation, most camera lenses can be thought of as modified pinhole lenses. The two fundamental parameters of an optical lens are the focal lengths and the aperture. Camera lens focal length determines the magnification of the image projected onto the image plane and the aperture the light intensity of that image. For a particular photographic system the focal length determines the angle view, short camera lens focal length giving a wider field of view than longer camera lens focal length. The wider the aperture allows using a faster shutter speed for the same exposure.
The maximum usable aperture of a lens is specified as the focal length ratio or f-number defined as the camera lens focal length divided by the effective aperture, a dimensionless number. A camera lens focal length is usually specified in millimeters but older lenses might be marked in centimeters. Camera lens may be classified as a: Normal camera lens with an angle view of the diagonal about 50 degrees and a camera lens focal length approximately equal to the image diagonal; wide-angle camera lens with a view wider than 60 degrees and camera lens focal length shorter than normal; and the telephoto camera lens with an angle of view narrower and camera lens focal length longer than normal. Macro camera lens is a special lens corrected optically for close-ups. A macro camera lens may be of any camera lens focal length.
There is a disadvantage of using different camera lens focal lengths. Using different focal lengths when taking photo of a particular subject will have different perspective. Even with the same subject, the results will be different because of the use of different camera lens focal length. Different camera lenses have different use and influence on the perspective of a photo.
Although in practice a photographer will intuitively perceives a natural relation between perspective and the focal length he is using and no such relation actually exists. The immediate reason for this impression is that lenses of different angles of view may be used in different ways. Cameras with tilt/swing facilities do not really change perspective but rather distort the image by oblique projection onto the image plane. Taking photo at a distant scene with a wide angle lens will yield the exact same perspective associated with a telephoto lens, enlarging the same part of the wide-angle image as that photographed with the latter will confirm the fact that when it comes to looking at a photo, the distance from which it is viewed will also influence the way in which it is perceived. The perspective in the image will determine the most agreeable viewing distance.
Lens Focal Length Explained