I know EF-S lenses are "optimized" for crop sensors but what are the exact advantages to using an EF-S lens? Does it give better color, sharpness, depth of field, etc...?
In addition, do EF-S lenses still have the crop factor magnification as regular EF lenses do?
EF-S are not just optimized for APS-C cameras, they are made for those only. In other words, they will NOT work on full-frame models or even APS-H ones. The imaging circle the project is smaller which lets them be made lighter and more compact than equivalent full-frame lenses.
The FLM (Focal-Length Multiplier) still applies when comparing the angle-of-view with full-frame lenses. Remember, the focal-length does not change because of the camera but it is the field-of-view that gets reduced.
EF-S lenses are designed ONLY for crop APS-C cameras. They will not work on a full-frame body, and could damage the latter if attached.
Because the APS-C sensor is smaller than full frame, the image circle needed to be projected onto it does not need to be so big. With this in mind, the optics are designed to only target a smaller area. On Canon lenses, you will notice a 'flange' at the back of the lens, which extends just a tiny bit into the camera body when attached (about 3-4mm). This is fine on an APS-C body camera as the sensor, and therefore mirror is smaller, so when the smaller mirror flips up, it still clears this flange. Were you to attach an EF-S lens to a full frame camera such as the 5D MkII, the larger mirror would hit the flange and in best case scenario your picture will turn out half-dark, but in the most likely scenario you will damage the mirror box assembly!
So do not use an EF-S lens on a full-frame body!!
Annoyingly (said in jest), Nikon crop (DX) lenses do not have this problem. You can freely attach a DX lens to a Nikon FX (full frame) body, and the camera automatically senses it, shows a new frame line in the viewfinder that shows you your area that will be recorded, then only this area is saved to the file! Very clever.
The advantages of EF-S lenses:
Having the rear element sit closer to the film allowed Canon to use scaled down versions of existing lens designs as a starting point, cutting development costs.
The need to project a smaller image circle allows wide angle designs to be lighter by having smaller glass elements as vignetting is not as severe.
Having the correct size image circle helps with flare, as does having a lens hood designed for the camera sensor size. Projecting a larger than necessary image circle basically means letting extra light into the camera that doesn't contribute to the image, which is a recipe for flare as this light can bounce back off the rear element and onto the sensor.
Projecting a smaller image circle allows sharper designs to be produced, the lenses used in compact cameras have tiny image circles but are able to resolve many more line pairs per mm than SLR lenses. So all being equal (which it never is!) an EF-S lens on APS-C camera would be slightly sharper than an EF lens on APS-C. It wouldn't be shaper (in terms of line pairs per picture height) than an EF lens on FF, but that's another question - for more information see:
With all other things equal, in a DSLR, will a larger sensor produce a sharper image?
The way I see it, the main advantage of EF-S (or third-party equivalent) lenses is that their zoom ranges actually make sense on 1.6-crop cameras. A 10-20mm EF-S lens is a perfectly reasonable ultrawide zoom; a 17-50mm EF-S lens is a perfectly reasonable normal zoom. For full format lenses, there are such things as the 16-35 f/2.8 zoom, which frankly has to short a zoom range to do service as a normal zoom and is too long to work as a wide-angle lens, or the 17-40 f/4 which has the same problems and is too dark to boot. Similarly, you have the otherwise very good 24-70 f/2.8 and 24-105 f/4, both of which are way too long at the short end to be more than a poor substitute for a normal zoom on a crop camera. They are bad enough on a 1.3-crop 1D actually!
(This does not apply to the longer lenses though. The EF-S 55-250mm lens could be replaced by one of the very nice 70-200s or a 70-300 without any great loss in capability. And as for prime lenses there is such a lot to choose from in the full-format lineup that the dedicated EF-S variants are not dramatically useful.)
In theory, they can be smaller because they project a smaller image circle.
In practice, the difference in weight and size is going to be negligible.
I can't find an apples-to-apples comparison in terms of EF-S to EF. What I have found is a comparison between EF and micro 4/3 lens. The Olympus 300/2.8 with a 4/3 image circle weighs 7 lbs, while the Canon 300/2.8 with a a full 35mm image circle weighs 5 lbs. I don't understand that one at all.
The Canon EF-S 18-200/3.5-5.6 is 21 oz; the Canon EF 28-200/3.5-5.6, 18 oz.
The Canon EF-S 18-135mm f/3.5-5.6 is 16 oz; the Canon EF 28-135 f/3.5-5.6 is 19 oz.
These aren't really fair comparisons because the range is different, but they are as close as I can find.
The lenses still need to be of a retrofocus design to allow the mirror to clear. That's why comparable lenses are so much larger than with the non-retrofocal lenses used on the Leica M series, for example.
EF-S lenses are in general smaller, lighter, and often cheaper than the EF-equivalents.
Also, the EF-S lenses have more appropriate focal lengths for APS-C sensors.
