Digital cameras come with different size sensors. Compact cameras have tiny sensors – about the size of the nail on your little finger. On digital SLRs, the sensors are considerably larger, which is why these cameras produce higher quality images, particularly at ISO 400 or more.
Canon digital SLRs have three different sensor sizes, depending on the model:
1. Full-frame sensor, which is the same size as a 35mm neg (36 x 24mm), found on the Canon 1Ds and Canon 5D bodies.
2. APS-H sensor — slightly smaller than full-frame (28.7mm x 19.1mm); found only on Canon EOS 1D bodies.
3. APS-C sensor — smallest of the three (22.5 x 15mm); found on Canon Digital Rebels (400D, 450D, 500D), Canon 30D, 40D, 50D models, and the new Canon 7D.
Lenses made for traditional 35mm SLR cameras are designed so that the circular image cast by the lens just covers the rectangular area of the film (or a digital sensor of similar size).
When you substitute a full-size sensor with a smaller one, more of the subject now fills the frame — as in the picture below where the black rectangle represents a full-frame sensor, the blue rectangle an APS-H sensor, and the red rectangle an APS-C sensor.
The impact of using a lens designed for full-frame sensors on a camera with a smaller sensor is commonly referred to as the “crop factor”, and also as “lens magnification factor” or “focal length conversion factor”.
The implications are enormously beneficial for photographers using long lenses on cameras like the Canon 50D or 500D, but simultaneously rob wide angle lenses of extra width. For more on this, please see my attempt at more fully explaining the Canon digital SLR crop factor, one of a new series of articles under Canon Lens Reports.