F-stops, Focal Length and Lens Aperture
Sensors from different manufacturers vary widely in size, but in general they're smaller than a piece of 35mm film. In order to project the image onto a smaller sensor, the focal length is shortened by the same proportion.
If your camera has an APS-C sensor (Nikon DX DSLRs, Sony NEX…) it has a crop factor of 1.5 - meaning you multiply the lens focal length by 1.5 to get its equivalent 35 mm-format focal length. For Canon APS-C cameras that number is 1.6, for Micro Four Thirds cameras it's 2.0 and for the Nikon 1 series it's 2.7. That means a 35 mm lens would give a field of view equivalent to 56 mm on an APS-C camera like a Canon 70D and equivalent to 70 mm on a Micro Four Thirds camera like the Olympus OM-D E-M1.
35mm cameras, a 50mm lens gives a natural view of the subject. Increasing the focal length increases the magnification, and objects appear to get closer.
The following table provides an overview of what focal lengths are required to be considered a wide angle or telephoto lens, in addition to their typical uses.
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