Quote Originally Posted by Franglais
No. That's not right.

As someone else stated, a 50mm lens is a 50mm lens whether it is EF or EF-S. But the view you get through it is not the same on a 450D as on a 5D

- The EF lens gives an image circle that fully covers the 24x36mm frame of a "full frame" camera like the 5D.
- The EF-S lens gives a smaller image circle that fully covers the 22.2x14.8mm frame of an APS-C camera like the 450D. If you put an EF-S lens on the 5D you would see that the corners were dark.

Put an EF lens on an APS-C camera and you are only using the central part of the image. The rest is outside the edges of the sensor and is lost. The effect is as though you had put a more telephoto lens on the camera. Multiply the actual focal length of the lens by the crop factor and you get the corresponding focal length if you were using a "full-frame" camera.

A 50mm lens on an APS-C camera gives the same view as a 50x1.6=80mm lens on a full-frame camera (in other words it's a moderate telephoto)

The "normal" view on a full-frame camera is the 50mm. On an APS-C camera the equivalent of a 50mm is a 50/1.6 = 31mm lens.
I knew when I read it back it wasn't clear and your explanation is excellent but Isn't that what I said?

View:

50mm EF on XTi = 80mm EF on 5D which is 35mm?

Therefore 50mm EF on XTi = 80mm EF-S on XTi

Roger