I know EF-S lenses are designed for certain Canon cameras (XT up to 40D), but what is the advantage of them? I was originally made to understand that the EF-S lenses took into account the 1.6x crop factor so that the EF-S 17mm-85mm was indeed effectively a 17mm-85mm equivalent and not effectively 27mm-136mm. The more I read, the more I'm beginning to feel that this is not the case. If this is not the case, then why would I want an EF-S lens as opposed to the more widely compatible EF lenses?
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Cost and weight.
Publishing this again.......
The APS-C format cameras such as Canon’s Digital Rebel series and their 20D through 60D series plus the 7D have a smaller sensor than a 35mm film frame. If you limit the lens selection to those lenses designed to fill a 35mm frame (such as Canon’s EF series lenses), you will find that there are no ultra-wide-angle lenses for the APS-C camera.
At the time of this writing, the shortest zoom lens focal length in the EF mount lens family is 16mm. There are two primes that are a bit shorter, and one of those is a "fisheye" lens. NONE of these lenses are what the average photographer would call "affordable".
To design an ultra-super-wide-angle lens such as a 10mm (non-fisheye) lens for a 35mm film camera is a VERY expensive proposition, which is why there are none.
By making some changes to the design criteria - reducing the "film" area to be covered by the lens, and allowing the lens to project deeper into the mirror box (moving the rear element of the lens closer to the "film"), it becomes much more economically possible to design lenses for the task. Thus, the EF-S family of lenses was born, the “S” standing for Short back focus.
The EF-S lens mount is purposely designed to be different from the standard EF lens mount so that you cannot mount the EF-S lenses on cameras that were not specifically designed for them. If you modified the mount of an EF-S lens to be able to put it on a 35mm film camera, there would be a high probability that the mirror would crash into the rear element of the lens at certain focal lengths (the mirrors in the APS-C cameras are significantly smaller than those in 35mm cameras and “full-frame” DSLR’s). Also, the 35mm film frame would have a dark circle around the edges and the image would be inside the circle (known as vignetting).
Now that you have some understanding of what the EF-S lenses are all about, you need to understand that focal length is focal length is focal length. An EF 50mm prime lens designed for a 35mm camera, when used on an APS-C camera, will provide you with EXACTLY the same image size as an EF-S 18-55 lens set to 50mm. If you could cobble together a mount for a Hasselblad 50mm lens and use it on a 20D, you would again have the same size image as a result.
NO CHARACTERISTIC of any lens changes when you mount it on different format cameras. Focal length (or focal length range for zooms) never changes. Aperture range never changes. The only thing that would change is the apparent field of view, and that change is not a function of the lens but it is a function of the size of the sensor or film that will record the image. There are some changes to the depth of field, but that is also a function of the size of the sensor or film that records the image.
EF-S lenses, by the way, will only fit on the Digital Rebel series cameras, the 20D, 30D, 40D, 50D, 60D, and the 7D as of this writing.