Simon, Its kinda like skinnythick was saying re: "angles" but there is more to it. There's a thing called distortion (specifically geometric distortion), its very inherent in some specific types of lenses. And cheap lenses have more of it then others. Then couple this with where the object is in the field...alot of math,ok now I'm sure I'm boring everyone. Just suffice it to say that even without photoshop alot can be accomplish utilizing these lens "defects". This is one time where a cheap or specific type of lens is a persons best friend. Don't ask me how I know this, let's just say I know good bit about lenses.