The focal length and sensor size
The focal length and sensor size | ||
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The focal length inscribed on a lens assumes that the lens will be used on a camera body with a 35 mm–size sensor or film; this applies even to the lenses designed specifically for smaller sensors. Currently, although there are an increasing number of cameras on the market containing a full-frame sensor, they are in the minority, and most DSLRs have sensors smaller than the 35 mm frame. A lens designed for a 35 mm–size sensor will project a circular image just larger than the sensor. If the sensor is smaller than 35 mm, then the circle is too large, and only the central portion of the projected image is used which is usually the area of the lens giving the best results. This effectively magnifies the quoted focal length of the lens, usually by around 1.5× or 1.6×. This is sometimes referred to as the 'crop factor.' Thus, a 100 mm focal-length lens, when used on a DSLR with an APS-C-size sensor, will have an effective focal length of 150 mm; a 200 mm becomes effectively 300 mm, etc. For this website, the focal lengths quoted will be the figure quoted on the lens (i.e., the 35 mm equivalent). This will need to be multiplied by the crop factor of your particular camera model if you are not using a full-frame sensor. The exact figure will be found in the technical specifications section of your camera manual. | ||