Canon, Nikon, Sony and most of the rest of the camera companies, plus the third party lens makers, like Tamron, Sigma, etc., all make two series of lenses, on designed for full frame digital SLRs and 35mm film cameras, and another range designed for use on their smaller sensor digital SLRs. But there is a lot of confusion about these lenses.
Canon call their lenses designed to only work on the small sensor digitals EF-S. Presumably the S is for small sensor. Nikon call theirs’ DX, since they call their small sensors DX and their full-frame sensors FX. For convenience in this article I will refer to Nikon lenses designed for full frame as FX, even though I don’t believe Nikon does.
When a lens is designed there are a number of criteria that the designer must meet. One of these is what’s called the image circle, the area at the film/sensor plane that is to be fully illuminated, sharp and with aberrations well corrected.
The EF and FX lenses are designed to create an effective image circle that covers the full 35mm frame. This has a diameter of approximately 43mm, which is the distance across the diagonal of the 36mm x 24mm sensor/film size. This is why a 50mm lens is considered ‘normal’ on a full frame 35mm camera, as it is the closest common focal length to the diagonal size.
EF-S and DX lenses are designed for the smaller image circle of the small sensor in the 450D, 50D, etc models. The sensor is 22.2mm x 14.8mm, which has a diagonal distance of 26.7mm. This is why if you put an EF-S lens on a full frame camera you get a circular sharp image with a dark outer area.
Designing a lens for a larger image circle is harder. It typically uses more lens elements and more expensive ones because of the need for more aspheric or special glasses to keep the image sharp and evenly lit over the larger image circle. This makes the lens larger, heavier and more expensive. Basically it comes down to this, creating a lens for the smaller sensor is easier and cheaper than it is for the big sensor at the same level of lens performance. This is also the case with lenses for medium and large format cameras.
A 50mm EF or FX lens on a small sensor camera has the field of view of an 80mm on a full frame camera. A 50mm EF-S or DX lens also has the same field of view as an 80mm would on a full frame camera. If you go to the Canon and Nikon websites they clearly state this, for example “Uses optics designed to take advantage of the small image circle to give a 17-85mm (equivalent to a focal length of 27-136mm in 35mm format), 5x zoom ratio” for the 17-85 EF-S.
Canon’s tilt and shift lenses, for example, are designed with an even larger image circle than full frame, so that with the lens shifted off center and/or tilted, part of the image circle will still cover the whole sensor.