goes in and out of vogue every so often. In the US it's considered (at least by me) to have gone through several distinct phases:
began about mid-century when photographers were still selling their images on their original plates (as opposed to making prints). The photographer shot two images, placed them in a frame that held the pair side by side, and a pair of spectacles folded out so that they viewed as a stereo image. A lot of photographs from mid-century are in stereo, including many from the US Civil War. Most are reproduced now as a single iamge and you'd never know they were originally half of a pair.
The writer Oliver Wendell Holmes (father of the famous Supreme Court justice) is popularly credited with making the "Holmes viewer," (sometimes called a stereopticon) which was the famous wooden jig that had a hood and lenses on one side, a stand for a stereo card (typically called a stereograph or view) on the other, and a handle underneath so it could be easily held in one hand. This appeared late in the 19th century and took off, largely because it was convenient, but also largely because of the availability of mass-produced stereographs. Photographers roamed the world, took photos in stereo pairs of anything and everything, and the prints were mass-produced and sold in volume. Tens of thousands of views were available and they were cheap, and a lot of people enjoyed looking at views as a leisurely past-time.
The Holmes viewer continued on well into the 1930s, but it was losing steam by the First World War. Books and magazines printed more photos and fewer drawings, and new generations looked back at Holmes viewers as old fashioned and tired, opting instead for post cards, cigarette cards, baseball cards, periodicals, and other interests.
Around 1950, an instrument firm named The David White Company started a sideline from making surveying equipment and developed a stereo camera based on 35mm film. The camera was small, very well made, and best of all it took Kodachrome—Kodak's wonderful color slide film. The camera took a pair of half-frame images so it made almost as many exposure pairs as a normal 35mm camera would make on the same length of film. When mounted in cardboard slides (one slide held both images) and placed in the simple viewer, a bright, colorful, 3D image could be seen. The crispness and color wiped away all the quaint memories of drab gray cards viewed under gaslight. The camera was called the Stereo Realist and the images became the Realist format. And no matter how hard they tried, everything else looked like (and was) a copy of the Stereo Realist.
The fad really took off when celebrities of the day got hold of it. Harold Lloyd's career as a movie star was long over, but he loved his Realist, shot many images, collected more, and used his considerable charm as he exhibited his slide shows. Friends and other movie stars bought Realists, like Edgar Bergan, Fred Astaire and Dick Powell. But the big one, the one that really sold the camera with an unintentional endorsement, was Dwight Eisenhower. In 1952 Life magazine put him on the cover about a story of his return to Europe, and the photo showed him with a Realist around his neck.
The Realist format had its problems. The better cameras (like the Stereo Realist) were on a par with the other mid-tier cameras of the day, but many of the imitators were poor. The Realist format was really meant for slides— if you wanted prints, you had to have them specially printed and use a Holmes-style viewer. The regular Realist slide viewer was nice but only good for one person to use at a time; to show them simulatenously to your friends you needed a special stereo projector (which were and are expensive), a special screen (which were and are expensive), and polarized glasses. Plus setting everything up and getting it going wasn't the easiest thing in the world, and stereo effect didn't always work as well on screen as it did in the hand viewer.
Personally, I think the fact that it was based on people making their own images, as opposed to buying professionally made images, that kept it from moving beyond a passing fad. By far most people bought Holmes-era stereographs from Keystone and Underwood & Underwood, they didn't shoot and print their own views. But Realist-format images were almost always consumed by their own makers. The only other stereo format that rivals the Holmes-era stereographs for popularity has been the View-Master system; and View-Master's sales were always commercially-made reels; very few people made their own at home.
Around 1980 a little Scottish company began making an interesting little camera called the Nimslo that took 35mm film. But unlike the Realist, it had a different strategy. It had four lenses up front instead of two, making four images (and thus four different pair combinations). The idea was that you would send the film in to special printers who would print the images onto special lenticular paper; the result was a print that could be seen as a 3D image without a viewer.
It didn't work very well. The camera got a fair amount of attention but it languished. And it would have passed into history mostly ignored except that a couple of other companies tried to make something about the same, such as the Nishika, but they were very expensive and didn't solve any of the problems. And they too passed. But I felt enough of a presence was made that it rated it's own "era."
I am unaware of any digital system that has really taken hold. For a little while after the movie Avatar made a big splash as a hit 3D film, there was a lot of talk that 3D television was going to finally take over; it didn't. Around that time, Fujifilm made a 3D still camera I wanted called the FinePix Real 3D W3 for awhile but it came and went. I have a Vivitar T135, but it wasn't much account. I see a couple more oddities for sale (or more likely, just placeholders) on Amazon but nothing I'd ever heard of. Matterport makes something interesting but it's priced far out of my budget. I believe that most people who are making stereographs these days are either scanning film from film-era cameras, or using modern single-image digital cameras either in pairs or with a slide bar.
My cameras are largely from the Realist era, except for the View-Master Personal, Tri-Vision, Nimslo, Nishika and Vivitar.