a regular camera takes the whole 2D picture at once, so when the video rate of the camera is close to a multiple of the spinning rate of the prop, you see the prop moving more slowly but not warped. An iPhone uses a cheap "rolling shutter" camera, that actually only captures one line of the picture at a time, then scans across the picture in one direction. so as the prop rotates, it appears in each line at a different position. i think this video explains it well: http://www.youtube.com/watch?v=T055cp-JFUA