Why did Canon make EF-S lenses
The object of the EF-S lens series is twofold:
â€¢ If the lens is not intended to be used on "full-frame 35-mm size" sensor cameras, but rather those with a smaller sensor size, it can have a smaller image circle, and this makes it easier to design for a certain set of optical properties with a certain optical performance.
â€¢ If the lens is not intended to be used "full-frame 35-mm size" sensor cameras, but rather those with a smaller sensor size (and thus a smaller reflex mirror, assuming an SLR arrangement), then the lens can have a smaller "back focus" (the physical distance from the rearmost point on the lens to the focal plane), which simplifies the design of lenses with smaller focal lengths.
Overall, then, we might expect that the objective is to allow the design of lenses for use only on "smaller-format" cameras that are less costly, smaller, and/or lighter, for given optical properties and optical performance, than if they were designed to be used on all cameras of the genre, including those with a full-frame 35-mm sensor size.
Note that the focal length quoted for EF-S lenses is (nominally) the "actual" focal length of the lens (the only focal length it has). It is an optical property of the lens, and does not presuppose its use in any particular format size camera . The lens has that property on any camera it will fit, or when in its carton.
The stating of the "full-frame 35-mm equivalent focal length" for a lens as it will be used on a camera of some particular format size
, other than "full-frame 35-mm", is intended to allow the real property of interest, the field of view of the lens when used on that camera, to be expressed in a familiar, traditional form which can be compared to work with a full-frame 35-mm format camera. It is not an actual focal length of the lens under any circumstances.