The "bent barrel" look is called, interestingly enough "barrel distortion" and occurs to some degree in all wide angle lenses.... While items in the center of the frame are straight, straight lines along the edges end up curved at the focal plane of the lense.... If you take a photo of a rectangle, it ends up looking like a barrel.... hence the name.... The extreme case is with a "fish-eye" (180 degree) lense where the edges of the image are bent so much they become a circle....

Bob