A reflex camera is defined by having a mirror that reflects light from the lens into the viewfinder so you can compose through the taking lens. When you press the shutter, that mirror moves out of the way so light can reach the film or sensor.

So, "reflex" does not mean interchangeable lenses. Many reflex cameras do have interchangeable lenses, but that is a separate feature. Likewise, many interchangeable-lens cameras are not reflex cameras—mirrorless cameras, for example, have no reflex mirror.

The main advantage of a reflex design is that you see the image through the same optical path used to take the picture, which avoids parallax issues found in cameras that use a separate viewing path, such as some rangefinders or optical viewfinder compacts.

Common types include:

• SLR: single-lens reflex
• DSLR: digital single-lens reflex
• TLR: twin-lens reflex