Read "Turin Shroud" by Lynn Picknett and Clive Prince (1994) isbn 0-06-017224-X. They make the claim that the Shroud is actually the world's first photograph, possibly done by daVinci, the only one smart enough to figure out a way of creating it. I read the book because, while not a Believer, I thought the claim preposterous.
However, they bring out the fact that capturing an image on skin or cloth was well known as far back as Roman times, but that the image could never be fixed - it faded after a while. daVinci came up with a method (think of writing in lemon juice and then heating it to reveal the writing - same "scorching" effect).
They also brought out points I missed or were unaware of: If it was a burial shroud, why didn't the top of the head leave an impression? Instead the front and back are hinged. In the chapter "Getting the Measure of Shroudman" they claim that the head is a seperate image (a solid line of demarcation between it and the body that can't be explained by the cloth being folded under the chin) and is much brighter than the body, indicating a seperate application. The head is too small proportionally - the usual ratio of head to height ranging from 1.75 to 1.85 but the Shroud's proportion is 1.87 on the front and 1.92 on the back. Also, the front image measures 203cm (6'8") and the back 208cm (6'10"). A miracle indeed.
They were able to replicate the way the image was created by capturing an image on cloth treated with egg white and chemicals of the daVinci period, then "fixing" it by exposing it to heat.
I ended up thinking it made more sense than anything else I had read (a lot) about the subject. They also gave interesting insights on how the "Shroudies" (believers) sidetrack info that doesn't agree with their theories. Nothing new there, but it does cast suspicion on the "open-mindedness" of some who say the Shroud is real. Right now, I'm thinking it's a fantastic artifact - the world's first photograph, about 400 years before Daguerre.
At the end, the authors ask any college photographic department to try their method and see if their results can be replicated. Sounds like something neat students specializing in photography could do.
I've read it. It is mere speculation with no basis in fact. None of their conclusions carry any validity. Da Vinci was born 101 years AFTER the Shroud was first displayed in Lirey, France. There are none of the chemicals (or their residues) on the Shroud that Picknett and Prince require to be present to make such a photograph. Any photograph created by the techniques that P&P suggest that Da Vinci could have used, would have long ago faded into nothingness with exposure to light.
Finally, the Shroud is NOT a photograph. It bears only superficial relationship to a photograph. It's more akin to a topographical map. Whatever mechanism created the image on the Shroud did so as a function of distance of the cloth from the skin of the body it covered.
That's a dang good question I've never seen asked before.
In fact, a true shroud would be wrinkled in nearly countless places. The image on the shroud essentially assumes that it was a flat sheet not surrounding the body, but somehow suspended above it.
the only problem with that stretched out theory is that the whereabouts of the shroud are known long before da Vinci - even the carbon dating, which was evidently done on the patched areas, take it back hundreds of years before da Vinci.
I would humbly suggest that anyone who has not 'studied the studies' and read the plethora of books written by the different scientists who were in the team of 1978, do so before making up their minds on 'opinion'.
One good place to start for info and leads is Barrie Schwortz's site at
http://www.shroud.com/
He is one of the scientists on the '78 team, who went as a skeptic.
Believe the Shroud is authentic or a fraud, one thing is true. You cannot offer even a valid opinion if you have not studied the history and the data...
i.e., Don't believe everything you think.