Digital cameras today tend to fall under one of two categories: SLR or mirrorless.
What’s the difference? And why does it matter? To find out, let’s take a quick trip through the history of the medium. It turns out that many of the changes to how cameras work have been attempts to solve a simple problem: How do you show what’s in front of the lens to both the user and whatever’s recording that image?
From glass plates to mirrors
Originally, photographs weren’t on film; instead, you’d coat a metal or glass plate with a photosensitive liquid. This would be placed behind the lens — before you did so, however, you looked through where the plate would later be to frame the photo.
To speed up the process, a second lens was added near the first, through which framing and focus could be adjusted without fiddling with the primary one.
Although these offset lenses only offered an approximation of what the primary lens would see, the design persists to this day. They’re called “rangefinder” cameras, and the lens you look through has been given additional functionality, notably the ability to determine the distance to the subject for the purpose of setting focus.
Top down view of a Yashica-635 Twin Lens Reflex.
Mirrors were added around the turn of the century (the last one, to be clear), allowing for two new camera types. One was a “twin-lens” setup where the mirror bounced the image from the extra lens upwards onto a small translucent screen; a second, similar lens just below opened onto the shutter and the film itself, and focusing the first would also focus the second.
In the other design, the mirror went directly between the lens and the film, reflecting the image through a prism that, in turn, refracted it into your eye. But when you hit the shutter button, the mirror flipped up out of the way and the shutter opened on the film. This was known as the single lens reflex, or SLR. The design was not only more compact, but since the user looked through the primary lens, you could switch that lens out and still see exactly what the film would see.