EF vs EF-S practical difference ?

Steve Balcombe wrote:

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.

Pardon my vocabulary here sir. Yes, I learned the same thing, it's field of view.

"Become" is a bad word typed in haste out of curiosity to know the answer.

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.)

But you know, some people even said, not just focal length, but f stop is considered as f*1.6 for crop sensors. That's why I was wondering if this is true. Even a video of Tony Northrup shows the same thing about f stop *1.6.

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.

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So finally, it is economy then. Means people who might upgrade to FF after few years, are better off buying an EF then ? if we can afford.