A reflex finder is a viewfinder system with a mirror placed behind a lens. The light passing through the lens is reflected by the mirror to a ground glass. The image formed on this ground glass can be observed directly, giving a waist level reflex finder or through an redressing optical device (set of mirrors or prism) for eye level viewing, giving an eye level reflex finder.

With a reflex finder, you can focus the image on the ground glass and frame your picture at the same time. It is common to find a device on the center of the ground glass to help precise focusing, for example a split image device or a microprism device. Today's reflex cameras usually incorporate autofocusing.

The cameras using a reflex finder can be distinguished in:

  • single lens reflex or SLR cameras, with one lens for both viewing and taking the picture
  • twin lens reflex or TLR cameras, with one lens for viewing and one lens for taking the picture