A reflex camera is any camera that uses a mirror to reflect light from the lens up to a viewfinder, letting you see exactly what the lens sees before you take the photo. The word “reflex” comes from the reflection of light off that internal mirror. This simple but elegant design has been the backbone of serious photography for well over a century, and understanding how it works helps explain why these cameras remain popular even in the digital age.
How the Mirror and Prism System Works
Light enters through the lens and hits a mirror angled at 45 degrees inside the camera body. This mirror reflects the light upward onto a focusing screen. Without any further correction, the image on that screen would appear reversed left to right, because a lens naturally inverts the image both horizontally and vertically. The 45-degree mirror fixes the vertical flip, but the lateral reversal remains.
That’s where the pentaprism comes in. This five-sided glass prism sits above the focusing screen and bounces the light twice internally, flipping the image back so left is left and right is right. The corrected image then travels through the viewfinder eyepiece, giving you a natural, real-time view of your scene with no electronic processing and no delay.
When you press the shutter button, the mirror swings upward out of the light path, the shutter opens, and light hits the film or digital sensor directly. This is why the viewfinder briefly goes black during exposure: the mirror that was feeding it light has moved aside. Once the exposure is complete, the mirror drops back into position and the viewfinder image returns.
Single-Lens Reflex vs. Twin-Lens Reflex
There are two main types of reflex cameras, and they solve the same problem in different ways.
A single-lens reflex (SLR) camera uses one lens for both viewing and capturing the image. The mirror flips up when you shoot, temporarily blocking the viewfinder. This design became the dominant format for decades because what you see through the viewfinder is precisely what ends up on film or the sensor, with no parallax error from offset lenses.
A twin-lens reflex (TLR) camera has two lenses stacked vertically. The top lens feeds light to the viewfinder through a fixed mirror, while the bottom lens handles the actual exposure. Because the viewing mirror never needs to move, the viewfinder image stays visible even during exposure. This mechanical simplicity also means less vibration and virtually no shutter lag from mirror movement. TLRs were popular from the 1930s through the 1960s, particularly for medium-format portrait and street photography, but fell out of mainstream use as SLR technology improved.
Why Optical Viewfinders Still Matter
Modern mirrorless cameras have replaced the mirror and prism with an electronic viewfinder (EVF), which is essentially a tiny screen showing a live feed from the sensor. This approach has real advantages: you can preview exposure, white balance, and depth of field in real time. But the optical viewfinder in a reflex camera offers something an EVF cannot: a zero-latency, zero-power view of the world.
Even high-end mirrorless cameras running their viewfinders at 120 frames per second have a small amount of display lag. For most photographers this is negligible, but some find it disorienting, particularly when panning quickly to follow fast action. A poor EVF with low refresh rates can even cause motion sickness. The optical viewfinder in a reflex camera shows photons bouncing off your subject in real time, with no processing step in between. It also doesn’t drain the battery, which is one reason DSLRs tend to last far longer on a single charge than their mirrorless counterparts.
A Brief History
The reflex mirror concept predates photography itself, appearing in camera obscura devices centuries ago. The first production SLR with a brand name was Calvin Rae Smith’s Monocular Duplex, made in the United States in 1884. Early SLRs were bulky and slow, with mirrors that had to be manually reset after each shot. The design matured through the mid-20th century as instant-return mirrors, through-the-lens metering, and autofocus were added. By the 1990s, the SLR was the default choice for professionals and serious amateurs alike.
Digital SLRs (DSLRs) replaced film with an image sensor but kept the mirror and prism system intact. Canon, Nikon, and Pentax dominated this era. Starting around 2010, mirrorless cameras began offering comparable image quality in smaller bodies by removing the mirror box entirely. Today, most major manufacturers have shifted their development focus to mirrorless systems, but millions of DSLRs remain in active use.
Caring for the Mirror
The mirror inside a reflex camera is a front-surface mirror, meaning the reflective coating sits on the exposed face of the glass rather than behind it like a bathroom mirror. This makes it optically precise but physically delicate. A scratch on this surface is permanent and cannot be repaired.
For routine dust, a simple air blower is usually enough. If you notice smudges, a soft lens cloth or a cotton swab with a small amount of lens cleaning solution works well. Apply minimal pressure, wipe gently, then follow with a dry swab to remove excess liquid. Sensor cleaning swabs paired with a streak-free cleaning solution designed for delicate optics are another safe option. The key rule: if you can’t remove something with light pressure, do another pass rather than pressing harder. A bit of dust on the mirror won’t affect your photos since the mirror moves out of the way during exposure, so aggressive cleaning is rarely necessary.
Who Still Uses Reflex Cameras
Despite the industry’s shift toward mirrorless, reflex cameras remain a practical choice for several groups. Wildlife and sports photographers who rely on long battery life in the field appreciate that an optical viewfinder draws no power. Photographers who already own extensive lens collections for Canon EF or Nikon F mounts may see no reason to switch systems. And the used market is flooded with high-quality DSLRs at a fraction of their original price, making them an excellent entry point for beginners who want interchangeable lenses without a large investment.
Film SLRs have also seen a resurgence among hobbyists drawn to the tactile, deliberate process of analog photography. Many classic bodies from the 1970s and 1980s are fully mechanical, requiring no batteries to operate, and accept lenses that are still widely available.