Peter MacNab's Loch Ness photograph at Urquhart Castle

Peter A. MacNab was a bank manager. He was also a local councillor and President of the Clan MacNab Society. In the summer of 1955 he was driving home from a holiday in the north with his son. He carried an Exakta camera loaded with 127 film, and a six-inch telephoto lens. Above Loch Ness, he pulled the car up on the road just above Urquhart Castle.

29 July 1955. "It was a calm, warm hazy afternoon," MacNab recalled. He was all ready to take a shot of Urquhart Castle. Then his attention was held by a movement in the calm water over to the left.

He began switching to the telephoto lens. "As I was doing so a quick glance showed that some black or dark enormous water creature was cruising on the surface." Without a tripod, he took the shot in a great hurry. He took a second, very quick shot with a fixed-focus Kodak. Then the creature submerged.

His son had been busy under the bonnet of the car. When he looked in response to the shouts, there were just ripples on the water. The picture shows two long black humps on Urquhart Bay, trailing wakes, with the castle and its reflection beyond. Scaled against the castle, the part above water is more than 18 m long.

MacNab kept the photograph private, out of "diffidence and fear of ridicule." He is said to have destroyed the Kodak negative after he was ridiculed. In October 1958 the Weekly Scotsman ran H. L. Cockrell's Loch Ness picture. MacNab sent in his own, and the paper published it one week later. It also appeared in Constance Whyte's book More Than a Legend, though the dates of the two releases do not sit easily together.

The trouble began with the negative. Roy Mackal asked to use the photograph for The Monsters of Loch Ness, published in 1976, and MacNab sent him the negative. It did not match Whyte's print. The tree at the bottom left was missing. Mackal concluded that the negative had been made by re-photographing a print, a sign, he believed, that the image had been doctored.

Darren Naish, writing for Scientific American in 2013, found the huge size and the "notably (read: suspiciously) dark" humps hard to accept. The castle's reflection sits in a slightly different place in the two versions. MacNab's two cameras cannot explain it, Naish argued, because the moving animal is in exactly the same position in both. The Museum of Hoaxes suggested that painting in the monster, or painting out a boat, could have produced the differing negatives.

Ronald Binns offered a natural reading in The Loch Ness Mystery Solved in 1983. The humps, he wrote, could easily be a wave effect resulting from three trawlers travelling closely together up the loch.

Roland Watson has defended the picture. He argues the versions differ only by cropping and slight enlargement, that the two wakes have different structures, and that the humps stand taller than a fishing boat's wake. He quotes Adrian Shine telling the BBC in 2008 that "there is no definitive proof that the image is a fake."

MacNab never confessed. Over 20 years later he was still sticking to his story on Arthur C. Clarke's Mysterious World. He died in 2002. His photograph remains one of the classic Loch Ness images. And one of the most argued over.