What is the advantage of Canon EF-S lenses over EF?

Question

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 I was originally made to understand that the EF-S lenses took into account the 1.6x crop factor so that the EF-S 17mm-85mm was indeed effectively a 17mm-85mm equivalent and not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?

Answer 1

It is not the case ... you are right.  The length of a lens is an actual measurement.  The effective lengths are given for those with vast experience using 35 mm SLR's and think of lens lengths when choosing the one they need when shooting various subjects.

EF-S lenses are optimized for the smaller sensor sizes found on all but full frame DSLR's (at present the Canon 1Ds Mark III and 5D and the Nikon D3 and D700)

Because they do not have to cover a full 36 x 24 mm frame, they are less costly to manufacture, so if you are not going to buy the $8,000 1Ds Mark III, you can save money buying EF-S lenses that fit into your photographic needs.

Answer 2

Fhoto is correct. A friend and I took apart his kit lens (after I gave him the 17-35 mm f 4 L zoom to replace it)--because we wanted to see what was inside. And it was tiny. It was not design to cover the full frame 135 mm format--and indeed, I don't think the largest element we saw was half the size of a dime.

On the other hand, I look through my 135 mm f 2 lens (didn't take it apart!!) and it's huge.