Turns out the Earth was flat after all.

At least until about a year ago. Then something changed. The world embraced the third dimension, lining up by the millions to see "Avatar" (and a dozen-or-so other 3-D movies), welcoming the arrival of 3-D TVs at electronics stores and witnessing the launch of the world's first digital camera that shoots in three dimensions. Just last week, DirecTV flipped the switch on the world's first 24-hour 3-D channel.

The revolution is here.

Except it's hardly a revolution at all.

And it wasn't a revolution in the 1950s, when the public first willingly donned ugly glasses in movie theaters. It wasn't even a revolution at the turn of the last century, when millions of people gazed in awe through strange-looking devices called stereoscopes.

Three-dimensional photography dates back more than 160 years. Queen Victoria was a big fan, dazzled by stereo images at an exhibition in 1851. There are also some nice 3-D photos of President Abraham Lincoln.

Since its creation, 3-D photography has gone from hot to cold to hot to cold to, well, you get the idea. All the while, a dedicated group of photographers, hobbyists and collectors continued to carry the torch, toting double-lens cameras and wearing strange specs.

On the crest of perhaps a new wave of 3-D popularity, the National Stereoscopic Association, a group of more than 3,000 enthusiasts, holds its annual convention about an hour west of Cleveland, in Huron, starting Wednesday.

Among the events: exhibits, photography outings, workshops, contests and an "adult theater" program that features nude 3-D slides, magazines and "other adult stereo items to offer for sale to an enthusiastic audience."

But the highlight of the convention will be Saturday night at the awards dinner, where a unique guest will address the audience and give a presentation — yes, in 3-D — about his book of historic stereo photos and his love of the art form.

The special guest? Brian May, a 3-D photo collector and lifelong stereoscopy devotee, who not too long ago earned his doctorate in astrophysics.

Most people know him as the guitarist and founding member of the British rock band Queen. (He also wrote a number of Queen's hits, including "We Will Rock You.")

"I'm the world's biggest fan of 3-D," May says in a recent phone interview from London. "It's been magic to me all my life really."

What is 'stereoscopy'?

The concept is simple. Humans have two eyes, roughly 2 1/2 inches apart. The left eye sees one image; the right, a slightly different one. Put together by our brains, the two single images become one rich three-dimensional world.

When photography was born, around 1840, the images, as amazing as they were, were only in two dimensions, literally lacking depth. Inventors began tinkering with mirrors and then lenses to put two photographic images — the left-eye view and the right — together.

By the early 1850s, inexpensive hand-held stereoscopes ("stereo" being derived from the Greek word stereos, which means "solid") were common. Users could insert a stereo photo card into the viewer, look in and see the two photos on the card become one 3-D image. In a world without radio, movies or even electricity, the public was in awe.

By 1856, the London Stereoscopic Co. had sold half a million stereoscopes and offered "upwards of 10,000" stereo views, according to an ad placed in a newspaper that year.

"As soon as you see the 3-D image, there's just no contest whatsoever," says May. "It's so much closer to reality than a normal photograph is. It's quite staggering."

Even today, in a high-definition world of nonstop visuals, people who look into a stereoscope for the first time react the same way, May says.

"Suddenly it clicks and there is that moment where people go 'wow,' " he says. "The 'wow' is the great rewarding thing for me, because they suddenly, at that point, they get it."

For decades, stereoscopes remained popular. With photo cards, people could gaze at faraway cities, exotic wildlife, even planets. An evening with stereographs remained a popular entertainment for decades, until the 1920s, when movies and radio become the rage.

The 3-D craze returned in the 1950s, fueled largely by innovations in photography, says George Themelis, a 3-D photo expert in Brecksville. (After a career as a materials scientist, Themelis now spends his days repairing stereo cameras and selling photo equipment on the Internet.)

It came back for a bit with the "Magic Eye" craze of the 1990s (remember the colorful blobs on the Sunday comics pages?) that left many people with crossed eyes and migraines.

"The popularity comes and goes," Themelis says. "When something new comes, like the 1950s when color slides became popular, it grows. Now with digital photography, it's popular again."

A virtual frame

Part of the reason the history of 3-D is marked with booms and busts is because of its amazing effect on the public, says May.

It becomes so popular, so quickly, that amateurs move in to make a profit.

"People jump on the bandwagon without knowing what they're doing," says May. "The result is people get headaches and they never want to see another 3-D movie. I'm sure that happened in the '50s, and I think it's in danger of happening now."

Although May always carries a 3-D camera and even snapped stereo photos while touring the world with Queen (you can view some of them here), he says he prefers "the Victorian form, where you just sit with a stereoscope."

Read more on 3-D photography A new dimension for an old art form….