In essence only the portion of light entering the lens that actually hits the sensor is altered, meaning the field of view is smaller, and thereby effectively doubling the focal length of the lens. The term 'crop factor' is somewhat misleading as the sensor does not crop the image at all - it simply utilises a smaller section of the lens. The ratio of the image size, combined with the smaller sensor means that Four-Thirds based DSLRs have a 'crop factor' of exactly 2 - that is to say, a 50mm film lens used on a Four-Thirds body gives the equivalent field of view to a 100mm lens on a 35mm film camera body. As a result, all Four Thirds lenses out-resolve in optical terms their competitors in the equivalent focal lengths. Unlike film, digital sensors require light to fall perpendicularly to avoid optical distortions. While other systems emphasize the compatibility of their digital bodies with their legacy lenses from the film era, Olympus chose to start from scratch with a new system optimized for the optical needs of digital sensors. All Four-Thirds compliant lenses produce a telecentric image circle, resulting in photos that are virtually free from chromatic aberrations, vignetting or diffraction. Unlike other systems however, Four-Thirds cameras have the only sensor size optimized for digital. But at 4.2µm, the pixel pitch of an Olympus E-30 12MP sensor is identical to that of the latest Canon EOS 7D 18MP sensor. The Four-Thirds system is often criticised for the small size of its sensor referral to the diagram to the right will show that the sensor size is only very slightly smaller than the currently most popular entry-level DSLR, the Canon EOS 400D. The diagonal length of the 4/3rds sensor is roughly half that of a 35mm film negative. This gives the image a diagonal length of 21.6mm the smallest of the common APS-C sensors used has a diagonal of 24.88mm with dimensions of 20.7mm × 13.8 mm. The actual size of the 4/3 sensor - specified by Olympus, and a design constant in the 4/3rds system white paper - is 18mm × 13.5mm, with an imaging area of 17.3mm × 13.0mm. This ratio was inherited from analogue TV standards. Unlike 35mm film (and therefore most APS-C sized sensors used by other big camera brands such as Nikon, Canon or Pentax) the Four Thirds system uses a ratio of 4:3 for its image sensor - 35mm film has a ratio of 3:2 (whereby the long edge of the rectangular sensor is either 4/3rds or 3/2 the length of the shorter side).
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