Probably because you are not used to them. Your eyes have to adapt to the 3D image of a Porro. The flat field of most roofs is easier to get used to but once you become accustomed to the Porro view you will never go back to the flat "pie plate view" of a roof. A roof prism is like looking at a picture on the wall. A Porro is like being "inside" the picture. With a roof you "examine" something like it is under a microscope. With a Porro you are "in" the picture with the object you are looking at.Porros give me eye strain, except the little reverse porros.