I've always wondered,

The Entombment of Christ,
one of my favourite Caravaggios

if the Shroud of Turin was painted or otherwise manufactured by someone in the 13th century,

how come it's a photo negative?

How would a medieval man, seeking to create a hoax relic, even conceive of the idea? Isn't it much more likely that it would be a positive image, and one, moreover, that is a bit easier to see? If I were someone in the 13th century trying to convince pilgrims to part with their cash, a medieval P.T. Barnum, I would come up with something that would appeal more to the senses.