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.

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

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.

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

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

What advantage does it have for APS-C Canon bodies?

The camera applies the crop factor. Focal length is focal length. An APS-C lens may not cover the “full-frame” image circle, but the lens, itself, is not performing the actual cropping, to the final aspect ratio. 10mm does not “become” 16mm. The 10mm angle-of-view is “seen” as being equivalent to the angle-of-view of a 16mm lens on a 1D/5D/6D.

To add a bit of perspective to this, Canon, until somewhat recently, produced 1D-series cameras with a an intermediate-sized APS-H sensor, with a 1.3x crop factor.

An EF-S lens may actually be useful on the larger-sensor cameras, but because EF-S lenses protrude farther into the camera, the camera’s mirror assembly, as it moves, will strike the lens. Some have modified EF-S 10-22mm lenses, enabling them to function from about 16mm, to 22mm, in 5D cameras, with no crop factor. (CAUTION: Zooming-out such modified lenses will allow the rear elements to be struck by the mirror!)

Third-party lenses, made specifically for Canon APS-C cameras, do not protrude deeply enough to be struck by the mirror assembly of “full-frame” cameras. These lenses will function on “full-frame” cameras, but the corners and edges will normally vignette heavily, especially at the shorter focal length of the zoom lenses. (All lenses vignette, to a degree, but not as heavily as seen when viewing the world through an APS-C lens on a “full-frame” camera.)

Nikon DX lenses, made for Nikon APS-C cameras, are safe to use on Nikon FX (“full-frame”) cameras, but will tend to vignette, as described above. Nikon FX cameras can be set to DX mode, which crops the image, in-camera, so this vignetting does not show.

Is there a “point” to buying EF-S lenses? Good question. You will receive a wide range of answers. In my case, I bought only one new EF-S lens, the 10-22mm, because I anticipated adding a pre-owned 5D rather soon. Notably, the EF-S 10-22mm lens compared very favorably to the EF 16-36mm L lenses of the time, so I planned to keep using it on a 7D after I bought a 5D, and this is exactly what I did, for about seven years of frequent use, until a partial separation of barrel assemblies occurred, in late 2017 or early 2018.

I think the chief advantage of EF-S lenses is that overall size and weight are reduced. The average consumer/user does not like large, heavy cameras and lenses. Another advantage is that moving the lens elements closer to the sensor, all else being equal, is ideal, from an optical engineering point of view. I do not know enough about optical engineering to take this latter part further, so will not try.

On a practical level, I have continued to use Canon APS-C cameras, to the present day, even though I bought my first pre-owned 5D in late 2011, added pre-owned 1D Mark II N cameras in late 2012 and early 2013, added a 5Ds R in 2016, and added a 5D Mark IV this year. My “on-duty” kit, when I retired from police service in early 2018, was a pair of 7D Mark II cameras, a 100/2.8L Macro IS, a 24-70/4L IS, a Macro Ring Lite II, and several Speedlites, from 580EX to 600EX-RT, mostly contained in a Pelican 1510 Case. This does not mean that I am an expert, of course, , just that APS-C cameras can be used for serious purposes.

Again, I am not an expert, and not an engineer. Some have said that I “talk like an engineer,” but, well, the math became too difficult, so I never went into engineering. I recently retired, and wonder, sometimes, whether I could get remedial training, and re-wire my brain, to better understand math, in my late fifties.

I put “full-frame” in quotation marks, because it is a relative term. Now that we have digital sensors larger than the 36mm x 24mm dimensions of the 35mm film negative, should we call them “fuller frame?”  Nikon avoids such issues by simply using the term “FX” when referring to full-36x24mm-frame size.