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Crop Factor and Field of View
Posted By: Ole, 02-01-2009, 09:31 AM

One of the more confusing terms in digital photography is "Crop Factor".

The illustration "A" below shows a lens (say a 43 mm lens) on a full frame camera. The horizontal field of view is illustrated in yellow - the width of the frame (sensor/film) is 36 mm.

Mount the same lens on a DSLR with an APS-C sized sensor and we get a more narrow field of view (illustration "B"). The focal length f is of course unchanged since it is the same lens we just moved to another camera.

If we want the same Field of View on the APS-C sensor camera as on the full frame camera we need a lens with a shorter focal length (f2, illustration "C"). We actually need a lens with 1.5 times shorter focal length, or about 28 mm in the example. That ratio (1.5 in the case of Pentax) between the focal length of the lens used in "A" and the lens used in "C" is called the Crop Factor.

I have used 24 mm as the width of the APS-C sensor for simplicity - according to Pentax' specifications their sensors are 23.5 mm on the longest side.

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Last edited by Ole; 02-01-2009 at 12:15 PM.
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