Digital Focal Lengths and the Crop Factor

The majority (but not all) digital SLR cameras have a sensor, which is reduced in size when compared to a frame of 35 mm film used in film cameras. The field of view you would see through the same lens will differ between the two camera bodies. Since a cropped sensor is smaller than a frame of 35 mm film, it captures a smaller portion of the area covered. The focal length and magnification of the lens retains the same engraving on the barrel of the lens, for example a 12 mm—24 mm. However, in view of the small sensor you get a 'crop factor', which produces an equivalent focal length. Nikon cameras have a crop factor of 1.5x. So, apply this to the 12 mm—24 mm lens example and in reality it becomes a 18 mm—36 mm (even though it is still engraved with 12 mm—24 mm on the barrel. Confusing I agree!)

Canon cameras have a 'crop factor' of 1.6x so their 50 mm lens in reality has a focal length of 80 mm. Camera manufacturers have designed certain lenses only for use with digital cameras so those 'digital lenses' work under the same principle. Notwithstanding that the Nikon Dx fisheye is labelled on the lens barrel with '10.5 mm' it still equates to a 15.75 mm lens (1.5 x 10.5 mm = 15.75 mm) or 16 mm for short.

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