Until my loved ones convinced me I was insane, I used to troll eBay for families’ collections of 35-millimeter slides. Cleaning out attics or basements, people would auction off hundreds or thousands of slides from family vacations or other important events. I was especially interested in family travel slides from the 1950s and 1960s. Unsurprisingly, I seldom got into an intense bidding war, and could score a thousand slides for $10.

I justified the practice by saying that I’d sometimes find an architectural slide — the needle in a haystack — that was just perfect to illustrate a point in one of my lectures. But in fact, I just loved looking at the images. What I liked was not just the shot of Grand Teton but, even more, the picture of Aunt Sally riding shotgun in the Winnebago. Motel swimming pools, the streets of downtown Detroit — the list is endless.

And I loved imperfections. Nothing was more disappointing than to open a box of slides and find the photographer had professional abilities. I liked harsh, unbalanced colors, unusual depth of field effects, and exposure errors that may have resulted from overenthusiasm but that yielded disappointment when the slides returned from the photo processor.

Slides are one thing. Snapshots are another, more immediate medium. As Michelle Hauser and Andrew Flamm — of Oddfellows Art & Antiques in Mount Vernon, Maine — say in their essay in the brochure that accompanies “20th c. Folk Photos: A Lost Medium” at Washburn Gallery, we are all intimately familiar with snapshots, in such a way that we can readily identify with the meanings certain shots must have had for their takers or possessors. Ms. Hauser and Mr. Flamm sort through thousands upon thousands of anonymous snapshots to find the gems Joan Washburn calls “folk photos,” which, like folk paintings or other folk art, exhibit artistic intent or effects. A selection of their holdings adorn Washburn’s walls.

In the 20th century, the wide availability of cameras gave every man the potential — either by intention or accident — to create fascinating images. Today, of course, the digital revolution has given us the Photoshopped bounty of the Web site Flickr.com, where one may (as I often do) spend hours in a rapture of images. Yet the snapshot aesthetic belongs to an earlier era, when the photographer couldn’t touch up his photos and didn’t know for days how they would “turn out.”

As we see in the images — small snapshots from the 1920s to the 1960s — the range of effects was marvelous. We do not know from one picture to the next whether said effects were intentional or accidental, and we’re left to ponder the significance of that. The price list only gives a simple descriptive name for the picture and approximate date (“Rooftops 1940s”), because that’s generally all that’s known. No amount of research will ever yield further information about the photographer’s intentions. Was the double exposure intentional — from a budding Man Ray in our midst, perhaps — or purely an accident? And so on.

What we do know from looking at the pictures is that many of them are fabulous — beautiful, weird, seductive, disturbing.

The color “Easter Sunday 1960s” is one of the more conventional snapshots: A glum-faced chubby girl in her Easter dress stands in front of her family’s cheap plywood-shingled house, as though she knows she’s putting on airs in her banal environment.

One of the best images is the color “Building with Rainbow 1960s.” It’s hard to tell, because of the lighting, if it is a real rainbow or a golden arch produced from some other source but, either way, its glow bathes what looks like a modern hotel in front of which is a white tail-finned car. The colors, the glow, and the car all roar “1960s,” as few images from fine art photography I’ve seen have.

The black-and-white “In the Mirror 1930s” evokes its decade as neatly as the golden rainbow evoked the 1960s. A lovely, young, and quite fashionably dressed woman stands before her clapboarded house — altogether a more substantial affair than the houses of the aforementioned little girl — and gazes admiringly into a mirror. Did the photographer know that by the angle of the shot we would see her face full only by putting its left profile together with the remainder that is visible only in the mirror? And there’s that pillow oddly sitting on the outdoor walkway. It’s expectant youth in an era of diminished hopes, and is almost heartbreaking.

Best of all is “Girl with Shadow 1939,” a black-and-white showing a very attractive woman with a stiff smile seated on a rug, with her left arm resting on a pillow and her left hand holding a coffee cup. It could be a bohemian setting. The domestic interior is of dark wood dimly lit and it’s easy to see it as a Depression-era rooming house carved from an old brownstone. The shadow of another person, as though a corpse or spectral being, sprawls on the rug before the woman. Artful arrangement or a snapshot that just turned out to be a beautiful and haunting image? That’s folk photography.

Until July 25 (20 W. 57th St., between Fifth and Sixth avenues, 212-397-6780).