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I have a Canon EOS 550D with a APS-C sensor. I have two lenses, (1) Canon EF-S 18-135mm f/3.5-5.6 IS and the (2) Canon EF 50mm f/1.8 Mk II.

If I use the 50mm focal length in the first EF-S lens... I get the exact same image as with the EF lens... How is this possible?? I was expecting the EF-S to take a less-cropped, "wider" picture at 50mm...

I have seen Is an EF 50mm f/1.4 the same as 50mm with an EF-S lens on a Canon 550D? but I am still a bit confused. The guy said "The lens does not change. The f/1.4 50mm EF lens has exactly the same focal length (and everything that goes with that) as your EF-S zoom lens set to 50mm" but then... if the image is supposed to be cropped in my EF lens, is it cropped in my EF-S lens too?

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I have seen photo.stackexchange.com/questions/11613/… but I am still a bit confused –  Beatriz Sep 15 '13 at 23:34
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4 Answers 4

up vote 6 down vote accepted

The difference between the EF 50mm f/1.8 II and the EF-S 18-135mm f/3.5-5.6 IS is not in terms of focal length or angle of view provided by each lens when both are used on an APS-C body. What makes the EF-S lens different is that the image circle projected by that lens is smaller than the image circle projected by the EF lens. The EF lens must project a larger image circle in order to cover the larger sensor of a Full Frame (FF) camera. But the extra part of the larger EF lens' image circle, when compared to the smaller image circle of an EF-S lens, falls outside the edges of the APS-C sensor in your Canon EOS 550D.

Since a Canon APS-C sensor is usually around 22.5x15mm, the diagonal width of the sensor is about 27mm. The diagonal width of a 36x24mm Full Frame (FF) sensor is a tad over 43mm. The EF-S lens only projects a circle just large enough to cover the 27mm diagonal width of the APS-C sensor. AN EF lens, on the other hand, must provide an image circle large enough to cover the 43mm diagonal width of a FF sensor.

I think where you are getting confused is at the assumption that the same Field of View (FoV) is contained in both the 27mm image circle of the EF-S lens and the 43mm image circle projected by the EF lens. This is not the case. The inner 27mm of the larger image circle projected by a 50mm EF lens contains the exact same FoV as the entire 27mm image circle projected by the 50mm EF-S lens. What this means is that if you put the EF 50mm lens on a FF camera, the FoV provided by the same 50mm lens on an APS-C camera will be in the middle 40% (in terms of area) of the frame, and the outer 60% will be from the part of the image circle that is projected beyond the edges of the smaller APS-C sensor when the same lens is used on the APS-C body. On the other hand, if a lens (in this case 17mm) that provides an EF-S sized image circle is mounted on a FF body, the circle doesn't cover the entire sensor.

enter image description here

The white rectangle shows the portion of the image circle that would fall on a Canon APS-C sensor. With magnification at the same level, the same photo would look like this taken with an APS-C Camera,

enter image description here

It is only when we make the image from the APS-C camera the same size as the image from the FF camera that we see the inaccurately named 1.6x focal length multiplier.

enter image description here

See also Why do Canon EF and EF-S lenses exhibit the same crop factor? and this answer to What is crop factor and how does it relate to focal length?

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really a nice desk ;) –  Christian Sep 16 '13 at 7:32
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The simple reason is that the crop factor has to do with the camera body (sensor), not the lens. So any 50mm lens will have the same angle of view.

Some lenses are designed to only be used on crop sensors, which is where you are confused, but the lens doesn't cause the crop factor.

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The EF-S lens is specifically designed to cast an image circle that is sized to your camera's sensor size (APS-C), the EF lens is designed to cast an image circle that is sized for 35mm sensors. What that means is that the EF lens is able to fully cover your sensor, just as the EF-S lens is, so there is no difference. Now, put those same lenses on a full frame camera such as the 5D mk III, then the EF-S would heavily vignette, which is effectively cropping down the image, but the EF lens would behave normally.

Now, as a far as I know, you can't actually put an EF-S lens on a full frame Canon camera, but if you could, it would either actually crop (which is what a Nikon would do) or vignette as I noted. Key is, you need a full frame camera to see it.

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Third-party "APS-C only" canon-mount lenses can be mounted on a FF body, I think (they only use EF), so you could check it out that way. –  Berzemus Sep 16 '13 at 9:34
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The super quick version is that the 50mm projects the image at the same size, but the EF-s lens projects only enough to cover the APS-C sensors while the EF lenses project enough to cover a full frame sensor (even though an APS-C sensor crops within this area.

This is why EF-s lenses can't be used with full frame cameras. (They can also sometimes cause physical problems with the mirror, so don't try it to see if you happen to have a full frame unless you know the lens is safe at the given focal length. An adapter would still be required as the mount is different for safety reasons.)

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It's not the only reason that EF-S lenses can't be used on full frame bodies, however. An EF-S lens may project far enough into the camera body that it would interfere with the mirror in a full-frame body. –  Caleb Sep 16 '13 at 15:54
    
@Caleb - you are correct, and I suppose it is worth mentioning so someone doesn't try it, I simply didn't mention it because it wasn't relevant to the question at hand and is also the primary reason. All EF-s lenses won't project a big enough circle, only some of them will interfere with the mirror and cause physical damage if you try to force it through an adapter. –  AJ Henderson Sep 16 '13 at 16:36
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