EF vs EF-S practical difference ?

McArth wrote:

I know this topic might have been asked several times, I have read most of them, but have not understood one point. So asking here, to get a clear idea.

I learned that EF-S lenses are designed specifically for APS-C lenses. They have lighter weight and lesser complications and glass compared to typical EF lenses. And also, they cost less in making, fine.

Basically yes. There are often design and manufacturing economies in making a lens for a smaller sensor.

Some people even claimed EF-S focuses better on APS-C but I don't know about that.

No, that's false.

But what I have noticed is, even if we mount an EF-S lens on an APS-C body, crop factor still exists. Like 10mm becomes 10*1.6 = 16mm.

The first part is correct - the crop factor applies to any lens you fit, whether EF, EF-S, or another brand, or even a lens fitted via an adapter. This is because focal length is a property of the glass only.

But 10 mm doesn't "become" 16 mm.

A 10 mm lens on a 1.6x crop sensor has the same field of view as a 16 mm lens on a full frame sensor. We therefore say it has an "equivalent" focal length of 16 mm, but it doesn't "become" 16 mm. It's still 10 mm.

The equivalent focal length merely provides a convenient reference point, so we can compare not only full frame and crop DSLRs, but also compact cameras.

(Some manuals explain that there is a smaller image circle as well, so we lose less light - does this matter when it crops? )

That's just confusing matters. I'd ignore it. Any lens of a given f-number will cast as much light on your sensor as any other. (Ignoring technical differences such as vignetting and transmittance which are outside the scope of a quick answer.)

So my question is, what's the point of buying an EF-S lens, if the crop factor still exists?

For the reason you stated at the beginning - economy. Also because design trade-offs may mean an EF-S lens (or other manufacturer's equivalent such as Sigma DC lenses) can be given a larger aperture or a greater zoom range, for example.