Knock on the senior citizens center door and a woman will answer you with a question: Salsa aerobics or camera obscura? Sounding a bit cryptic — almost like there’s some code or Latin phrase required for entry — it turns out, all that’s needed is some form of ID in exchange for a key to the darkened room up a flight of stairs.

Nothing untoward is happening up there. In fact, what occurs has been described for millennia, as far back as Aristotle’s day. In the middle of that room is a large, flat, white disk. And the picture on that disk is never exactly the same thing.

Welcome to the camera obscura, housed incongruously atop the Santa Monica Senior Recreation Center. Built way back in 1899, the room-sized camera projects a view of the surrounding beach onto a circular screen. It’s like an enormous pinhole camera, except that no images are recorded for posterity. Imagine closed-circuit security monitors, generations before there was TV or video.

Another way to think of obscuras, the precursors to modern day cameras, is like submarine periscopes. There’s a lens up on the roof of the building and you rotate it with a miniature captain’s wheel, like you may find on an ocean-going schooner. It turns in a 360 degree radius and shows you what’s outside of your little darkened room.

We saw several couples walking along the palm-lined promenade, a little kid playing on a cannon guarding Santa Monica from marauders, but were disappointed not to see senior salsa swingers.

The Chinese philosopher Mozi, from 400 BC, referred to the device as a “collecting plate” or “locked treasure room.” As a professional photographer, I like the treasure room description. There was a certain sense of quiet awe in the decidedly low tech chamber. My brother Dean gradually steered the lens around its orbit, collecting ever-changing images of seas and trees. And since no one was banging on the door, demanding their turn at the wheel, we could take our time as well as some photos. As incongruous as the camera atop a senior citizen center was, imagine me snapping pictures of it with my iPhone.

There are many camera obscuras around the world, including the first one I ever visited in Edinburgh, Scotland. The one in San Francisco is a pretty fun one too. We finished our 360 degree spy tour of Santa Monica and casually strolled back downstairs. I think I saw the lady who leads the dance classes. Dean grabbed one of her brochures.

The San Francisco camera obscura looks like a giant obscure camera. photo by By Sanfranman59 via Wikimedia Commons
The San Francisco camera obscura looks like, well, a giant obscure camera. photo by Sanfranman59 via Wikimedia Commons