How thin your pinhole material is is also a factor (yes that looks weird to me too). I made a pinhole out of a small coffee can, where the 5x7 photo paper was oriented horizontally and curved along the backside of the can, and was able to get coverage corner to corner. I used aluminum from a red bull can, and sanded it down really thin. The focal length of the camera was right at 100mm to the center of the paper, but obviously the sides were much closer.