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.

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.

It was the most popular type of camera for decades, even until film was replaced by a digital sensor (whence the D in DSLR). But then the mirrorless camera was born.

From mirrors to LCDs

In a way, mirrorless cameras are very much like the familiar point-and-shoot cameras, in which the lens sits directly in front of the shutter and film or sensor. This solves the problem of how to get the view from the lens to both the user’s eye and the image capture mechanism: the image hits the sensor, which then relays that image to the LCD facing the user.

The main way to frame a shot on point-and-shoots is via the display on the back, but because of poor quality screens, lenses and sensors (everything, really), the cameras were never taken seriously. For serious photography an SLR was still mandatory. However, as sensor and image processing technology matured and shrank, it became possible for these cheaper cameras to do the job which had traditionally been filled by larger, more sophisticated pieces of equipment.

So while the early digital point-and-shoots we had in the late 90s and early 2000s were mirrorless after a fashion, the term is now usually used to refer to cameras with interchangeable lenses, manual controls, and often an electronic viewfinder — a small eyepiece in which a tiny display is put, so the user can frame and shoot just like in the old days. In fact, an early term for mirrorless cameras was EVIL – “electronic viewfinder interchangeable lens.” But some didn’t have EVFs, some didn’t really have interchangeable lenses, and so on. What they all had in common was — you guessed it — no mirror.

The primary advantage of mirrorless camera systems is their compact size. Because they require few or no moving parts inside (some retain a mechanical shutter, for instance), they can be thinner and lighter, or add other functions, like in-camera image stabilization. There’s also a motivation to use high-quality displays and intuitive touch controls, since they will be relied upon more than in SLRs.

Smaller cameras, however, tend to have smaller image sensors — which can, but doesn’t necessarily, adversely affect image quality. This differs from camera to camera and often depends more on the quality of the lens; but that’s a WTF for another time.

Another drawback is that electronic viewfinders and LCDs aren’t necessarily preferable to an optical viewfinder. After all, when you are seeing the light as it comes through the lens itself, there is no lag or color bias. On the other hand, since you’re getting info direct from the sensor, what you see is what you get, minimizing guesswork in exposure and focus.

Lastly, smaller bodies also mean less room for the switches, dials, and other old-school physical controls prized by many photographers. It’s largely a matter of taste, but it’s also hard to deny the utility of controls you can handle without looking at them.

Mirrorless cameras have helped invigorate a flagging photography market long divided into “cheap” and “serious.” Compact, powerful, and intuitive, the mirrorless camera is in many ways the best of both worlds.