Arnoud Quanjer Wildlife Photography

When camera manufacturers started designing digital Single Lense Reflex cameras (SLRs), they decided that those digital SLRs should be about the same physical size of their analog 35mm SLRs. For that reason, they concluded that they could use the line of lenses they already had for their 35mm SLR's on the new digital SLR's.

All lenses designed for 35mm cameras project an image circle onto the film that covers the 24x36mm rectangle. The 35mm camera records the portion of that image circle that is defined by the opening behind the shutter for the film (24x36mm in size). A digital SLR with an APS-C sized sensor only records the smaller area (approximately 15x22mm) of the image circle projected by the lens.

When you put a 100mm lens on a 35mm camera and take a photograph, then put the same lens on a DSLR such as the Rebel XT and take the same photograph - same subject, same position for the camera - with the same lens, and then enlarge both photographs to the same size print (4x6 inches, for example), it will appear as though the photo from the Rebel XT was taken with a longer lens. That is because the image recorded by the Rebel XT was of a SMALLER PORTION of the image circle projected by the lens - cropped, if you will - compared to the image recorded by the 35mm camera. A very important fact to realize is that nothing about the characteristics of a given lens change when you put it on different format camera bodies (such as 35mm film or an APS-C digital body such as a 20D). The focal length stays the same. The maximum aperture and the aperture range stays the same. The only thing that changes is the field of view, and that is because the APS-C cameras record a smaller portion of the image projected into the camera by the lens.

The "crop factor" has only one valid use. Here's an example: Joe took a photo of Mount Rushmore with a 35mm camera from a particular place using a 200mm lens. You want to replicate that photo with your 20D. What focal length do you need to do that from the same location that he took his photo? Divide the 200mm by 1.6 and you get the answer - 125mm.

The extension factor differs between manufacturers and even models of cameras. The Canon 10D, 20D and 30D for instance have an extension factor of 1.6x while the 1D Mark II and Mark III have an extension factor of 1.3x the difference in these cameras is the size of the sensor. The Canon 5D and 1Ds Mark II don't even have an extension factor, their sensor is the exact same size as a 35mm frame. These cameras are know as 'Full Frame' cameras (FF).