Canon Digital SLR Crop Factor

Canon digital single lens reflex cameras do not all come with the same size digital sensor or chip.

On Canon full-frame digital SLRs such as the Canon 1Ds and 5D models, the sensor is the same size as a film negative - 36mm x 24mm.

On the Canon 1D models, the sensor is slightly smaller, measuring 28.5mm x 19mm (referred to as an APS-H sensor), while on the Canon Digital Rebels and 30D, 40D, and 50Ds, the sensor is even smaller, measuring 22.5mm x 15mm (referred to as an APS-C sensor).

Normally, with lenses designed for 35mm film cameras and digital SLR cameras, the image projected by the lens fully covers the area of the negative or full-frame sensor. However, when you substitute the sensor with one that’s smaller in area, part of the image flows over the edges of this smaller sensor. It’s like showing slides or an old home movie on a screen that’s too small, so the picture “overflows” the screen.

This can be seen in the example below, where the black lines represent a full-frame sensor, the blue lines an APS-H sensor, and the red lines an APS-C sensor:

The practical implications for photographers are far-reaching, as the image received by the smaller sensors is apparently now magnified - it fits on the full-frame sensor, but is too big for the smaller ones. The effect is similar to using a teleconverter - or post-processing by cropping the image in an image editor. This is great for photographers using long lenses, as their teles now have more reach, but it’s not so great if you shoot wide angle, as your wide angle lens is now more like a standard lens.

This effect on the image caused by smaller sensors is known as the “crop factor” or “focal length conversion factor” - for more information, see this article about the Canon Digital SLR Crop Factor.

For general information about Canon lenses, see these articles:
Canon Telephoto Lenses
Canon Zoom Lenses
Canon Wide Angle Lens