PBS TV has featured a documentary about the Turin Shroud with various theories about its origin. It included a new to me theory that the Shroud may be a medieval photograph of a dead and possibly crucified body.
The original Shroud is a light colored cloth with a faint brown image. The more familiar image is a photographic negative of the Shroud that brings out much more detail which was not possible to see until the Shroud was first photographed in the 1800's.
The PBS film claims that chemicals and lenses existed several hundred years ago that could have produced the faint brown images of the face, and the front and back sides of the body and head on the Shroud.