Field of view- APS-c and Full Frame
What changes when you fit an EF lens designed for a 35mm film camera onto an EOS digital camera? Essentially nothing. Values such as focal length and aperture are characteristics of the lens and cannot be changed by the camera.
However, the instruction manual for an EOS consumer digital camera might say that the “35mm-equivalent focal length is equal to approximately 1.6 times the marked focal length”. What this really means is that if you had a 35mm camera and the digital camera side by side, the focal length of the lens on the 35mm camera would need to be 1.6x the focal length of the lens on the digital model for both cameras to give the same viewfinder image.
The reason you need lenses of different focal length to get the same field of view is that most EOS consumer digital cameras have an image sensor that matches the APS-C format (22.5 x 15mm). This is smaller than the frame of a 35mm film (36 x 24mm). If you shoot with a lens of the same focal length on both a 35mm camera and most EOS digital cameras, the digital camera will see less of the scene.
In practical terms, this means that if you want to shoot pictures on an APS-C camera with a similar field-of-view to the pictures you shot with a 35mm camera − and with the same subject distance − you need a shorter focal length lens on the digital camera. For example, if your favourite lens for 35mm photography is 28mm, you need to divide this by 1.6 to find the focal length of lens for similar results on the EOS 30D. It is about 18mm.
If your favourite lens on a 35mm camera is 20mm, the equivalent for an APS-C camera is 12.5mm. The EF-S10-22mm f/3.5-4.5 USM lens was introduced to provide a true wide-angle lens for APS-C digital cameras.
The EOS-1D cameras use an APS-H format sensor that measures 28.7 x 19.1mm. The focal length factor for this is 1.3x.
The EOS-1Ds cameras and the EOS 5D use full-frame (36 x 24mm) sensors.