Sensor size and multiplier zoom factor

Some high-end DSLRs have a sensor the size of a full 35-mm film frame (24 x 36 mm), but most DSLRs have sensors that are only two thirds that size, a format known as APS-C (about 15 x 23 mm). In between is the APS-H format of high-end Canons (19 x 29 mm). The rival Four Thirds (4/3) system, developed by Olympus and Kodak, uses digital sensors that are smaller yet, 13.5 x 18 mm.1

You shouldn't feel cheated if your sensor is smaller than "full frame." Remember that "full frame" was arbitrary in the first place. The smaller sensor of a DSLR is a better match to a telescope eyepiece tube (32 mm inside diameter) and also brings out the best in 35-mm camera lenses that suffer aberrations or vignetting at the corners of a full-frame image.

The sensor size is usually expressed as a "focal length multiplier," "zoom factor" or "crop factor" that makes telephoto lenses act as if they were longer. For example, a 100-mm lens on a Canon Digital Rebel covers the same field as a 160-mm lens on 35-mm film, so the zoom factor is said to be x 1.6. This has nothing to do with zooming (varying focal length) in the normal sense of the word.

My one gripe with DSLR makers is that when they made the sensor and focusing screen smaller than 35-mm film, they didn't increase the magnification of the eyepiece to compensate. As a result, the picture you see in the DSLR viewfinder is rather small. Compare a Digital Rebel or Nikon D50 to a classic film SLR, such as an Olympus OM-1, and you'll see what I mean. One argument that has been offered for a low-magnification viewfinder is that it leads snapshooters to make better-composed pictures by encouraging them to look at the edges of the frame.

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