Sensor Crop Factor Calculator

Crop factor is a single ratio that lets lenses be compared across camera systems: the diagonal of the 36 by 24mm reference frame, which is 43.27mm, divided by the diagonal of the sensor in question. Because a lens projects a circular image and a smaller rectangle samples less of it, that ratio describes how much narrower the angle of view becomes. The common values follow directly from sensor dimensions: 23.5 by 15.6mm gives a 28.2mm diagonal and about 1.53x for Nikon, Sony and Fujifilm APS-C, 22.3 by 14.9mm gives about 1.61x for Canon APS-C, 17.3 by 13mm gives 21.6mm and exactly 2.0x for Micro Four Thirds, a 1-inch type sensor works out near 2.7x, and the 43.8 by 32.9mm sensor in current medium format bodies gives about 0.79x because it is larger than the reference. Multiplying focal length by the crop factor gives the equivalent focal length, and multiplying the f-number by the same factor gives the aperture that would produce matching depth of field and matching total light collection on full frame. The equivalences are useful but frequently over-read. The crop factor is a diagonal ratio, so it is exact only when the two formats share an aspect ratio; comparing a 4 to 3 Micro Four Thirds frame against a 3 to 2 full frame using the diagonal alone gives horizontal and vertical equivalents that do not both match. Nothing physical about the lens changes, since focal length is a property of the optics and a 50mm lens still projects a 50mm image circle with 50mm perspective characteristics regardless of what sits behind it. The equivalent aperture applies to depth of field and to the total number of photons gathered, not to metered exposure, which depends on illuminance per unit area and is therefore unchanged: f/4 at 1/250 and ISO 400 is the same exposure on any format. Diffraction scales with the format too, so a smaller sensor reaches its diffraction-limited aperture sooner, typically around f/8 on Micro Four Thirds where full frame tolerates f/16.

The reference frame everything is measured against was an improvisation. Oskar Barnack, working at Ernst Leitz in Wetzlar, adapted 35mm motion picture film for still use in the years around 1913 and 1914 by running the film horizontally and doubling the 18 by 24mm cine frame to 24 by 36mm, giving a larger negative from cheap and widely available stock. The Leica went on general sale in 1925 and the format became dominant over the following decades, which is why its 43mm diagonal, conveniently rounded to a 50mm normal lens, still anchors the whole equivalence system today. Crop factor became a working concept only when digital capture arrived at sizes smaller than that film frame, because full-size silicon was initially far too expensive to yield. The Kodak DCS professional bodies of the 1990s and then the Nikon D1 in 1999, with a 23.7 by 15.6mm sensor, put photographers in the position of using familiar lenses with unfamiliar angles of view, and the shorthand of multiplying by 1.5 spread through camera club and magazine writing to explain the difference. Canon reintroduced the 36 by 24mm format digitally with the EOS-1Ds in 2002 and made it broadly affordable with the EOS 5D in 2005, which is where the retronym full frame comes from. Olympus and Kodak announced the Four Thirds standard in 2003 and Micro Four Thirds followed in 2008, deliberately choosing a 2x factor for smaller bodies and lenses. The more rigorous equivalence framework covering aperture and total light, rather than field of view alone, was argued out publicly in the 2000s in enthusiast and review publications.