Hi Hanne - The purpose of the lens distortion correction tool is to correct for the distortion introduced by a particular lens, be it barrel distortion, pin cushion distortion, or more complex distortions such as "mustache" distortion. For example, if you use a lens that exhibits barrel distortion to take a perfectly centered and squared up photo of a rectangle, the image recorded by the camera will come out looking a bit like a barrel. The purpose of the lens distortion tool is to move pixels around slightly to turn the barrel shape back into a rectangle. In other words, it counteracts the original distortion (eg, barrel) by introducing a different distortion (eg, pincushion) that is the mathematical "opposite" (aka, inverse transformation) of the original.
For this process to work properly, (a) you must use the preset for that particular lens; and (b) the correction *must* be applied to the entire frame, not a cropped portion of the frame. If you need to crop the image you ALWAYS do it after the lens distortion correction has taken place.
In fact, if you are referring to the lens distortion correction tool that is in ACR, you will notice that the program reads the EXIF data in your image, and if it determines that you have already edited the image (eg, cropped it), the lens distortion correction tool will be grayed out, so there is no easy way to, for example, apply a Mayima lens correction to an cropped version of the image taken by the same lens, and certainly not to an image taken using a completely different lens, like your Canon glass.
That being said, there are other distortion correction tools besides the one in ACR, and it's easy to force some of these to apply the wrong distortion correction -- you simply specify the wrong lens. The problem is that doing this just stacks one distortion on top of the other, so unless this is being done for purely artistic effect (eg, exaggerating existing barrel distortion to make a lens act more like a fisheye), one should never specify the wrong lens.
Note, all of the above pertains to just lens distortion, not the geometric distortions that arise when the plane of the subject is not perpendicular to the optical axis and/or is not centered on the optical axis. These latter distortions will turn rectangles into trapezoids, circles into rounded tear-drop shapes, etc. These have absolutely nothing to do with the use of a lens. These occur even in pin-hole cameras. These "geometric" distortions can be corrected using other tools in PS, NOT the lens distortion correction tools.
HTH,
Tom M