18. The Lens Conversion Factor

Another important point that has not yet been discussed is the lens conversion factor, a basic concept in the digital SLR camera.

The focal length of lenses (35mm, 50mm and others) is determined in relation of the format of the final image, so the "standard" is based on the most popular format, i.e., 24x36 mm. So when sensors are used whose matrix is smaller than 24x36 (the EOS 300D sensor measures 15.1x22.7 mm), the match with the focal length of lenses is no longer assured. When the image surface is reduced, the angle of the field is reduced correspondingly. In practice, a 35 mm lens to be used to shoot in 24x36 format, when used with a 15.1x22.7 mm sensor produces an image with a much smaller angle of field than if the surface was really 24x36 mm. The increase can be calculated by using a conversion factor based on the size of the sensors. So the sensor used in the EOS 300D implies a factor of 36/22.7 = 24/15.1 = 1.6X. In concrete terms, a 35 mm lens is "multiplied" by 1.6, turning it into a 56 mm lens "as the 24x36 equivalent."

The focal length of lenses designed for digital use is thus given for the surface of the sensor, but this does not represent very much in the public's eyes. A well-informed amateur will implicitly realize the implications of a "35-105 mm zoom lens," but will have much less understanding of the esoteric 17.4-34.33 mm lens.

Thus, for the promotional launch the EOS 300D is being supplied with a 18-55 mm although the exact 24x36 equivalent would be 28.8-88 mm. To generalize, the zoom produced by a 24x36 format camera fitted with a 28.8-88 mm lens is ABSOLUTELY IDENTICAL with the zoom produced by the EOS 300D fitted with a 18-55 mm. Do you follow?

18-55 mm to 35 mm = 28-90 mm on the 300D

In practical terms, this arrangement has its consequences. Fans of telephoto lenses will be delighted, because their lenses will let them "see even further." A 300 mm becomes, as if by magic, a 480 mm! On the other hand, those who stick mainly to wide-angle lenses will be disappointed that the images are skewed. If you only swear by the 24 mm lens, you'll need to get a 15 mm to be able to obtain the same field of vision from the outset. When you learn what the price is for a good quality 15 mm, you may decide to think again. The magical solution developed by Nikon and Canon is to produce lenses that are specifically designed for digital cameras. Canon's 18-55 mm lens, for instance, generally produces a "standard" equivalent of 28-90 mm. These lenses are much less expensive and, of course, they can't be used with a 24x36 format camera, because the luminous surface is not large enough to cover a surface of 24x36 mm.