In digital cameras, the field of view of the lenses is smaller than that of traditional DSLRs because the sensor size is smaller than the classic 24×36 film, which is still the reference point for manufacturers.
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Remember that the focal length of a lens is the distance between the lens on which the light rays of the framed scene converge and the focal plane on which they are projected. The width of the angle of view depends on the focal length of the optics: shorter focal lengths correspond to wider angles of view. Referring again to the old Leica format, a wide-angle lens has a focal length ranging from 20 to 28 mm, a medium telephoto from 85 to 135 mm, and a telephoto from 180 upwards. A lens is called “standard” when the diameter of the rear coverage circle is in a 1:1 ratio with the diagonal of the film, and generally its field of view approaches that of the human eye which, although it spans an angle of about 180°, actually sees sharp only 25% of the scene. Since a 24×36 negative has a diagonal of 43.21 mm, the lens considered standard (even if somewhat approximated) is the 50mm, with a front angle of view equal to 47°. Obviously, in a lens defined as standard, the larger the size of the negative to be covered, the more its focal length must increase: on 6×6, for example, this is 80mm; on 6×7 it is 90mm; and on 10×12, a film with a diagonal as large as 161mm, the 150mm is considered standard.
The angle of view is the width of the scene that the lens can capture, and short focal lengths correspond to wider angles than those provided by long lenses. Therefore, a 50mm lens, which will be standard on a 24×36 film, will correspond to a medium telephoto on the sensor of a digital camera while it will be a wide-angle on a large format view camera.
Here is an interesting table that better than words clarifies the relationship between sensor size and the angle of view obtained with various focal lengths:
When talking about crop factor in digital cameras, it refers to the cropping the image undergoes because it passes through a lens designed to cover a 24×36 dimension but actually finds on the focal plane a much smaller sensor. To know the effective focal length of lenses, it is therefore necessary to multiply their “real” focal length by the so-called “crop factor” which varies from sensor to sensor: inNikon it is 1.5x, inCanon it ranges from 1.3x to 1.6x, in Olympus and Panasonic 2x, while in expensive full-frame cameras, which have a sensor as large as a 24×36 film, the crop factor is 1x, meaning the recorded image corresponds to what would be obtained with a 24×36 SLR.
Pubblicato in Photography
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