Definition: Reflex camera

Definition: Reflex camera | Glossary entry

Reflex camera

A type of camera that includes the DSLR, SLR and other cameras that have a ‘reflex’ mirror. The reflex mirror or viewfinder mirror reflects the light from the lens through a special lens (pentaprism lens  External link - opens new tab/page). That lens itself projects the light into the viewfinder. Through it the scene is visible to the photographer. The user of a reflex camera will see the same view through the viewfinder as is recorded by the image sensor.

Reflex camera optical paths

In a reflex camera the optical path has two possible tracks.

  1. Viewfinder path: Light enters the photographic lens. It hits the mirror and reflects up into the viewfinder.
  2. Digital sensor path:The mirror is lifted and the optical path is direct onto the digital sensor where the exposure is made.

The term “reflex camera” indicates the use of a mirror that “reflects” light up into the viewfinder.

Parallel light paths

In a camera with no reflex mirror there are two parallel light paths. Each has their own lens. One goes directly to the film or the digital sensor. This is where it creates the exposure once the shutter opens.

The other path is direct to the eye. The viewfinder shows a view seen only by the user. It is not the same as that which creates the exposure.

The two views, viewfinder and exposure, are slightly separated. This creates a problem. The photographer cannot exactly line the scene up for the exposure. It also means they cannot take into account what effect the lens has on the image. This means an exact composition involves some guesswork. The reflex camera solves this problem.

The parallel path in a non-reflex camera creates a common problem. For example, the photographer lines up a person for the shot. The composition sets the subject comfortably in the viewfinder frame. However, the photographic lens is set slightly lower in the camera body. This results in the photograph frame being recorded slightly lower down. This results in the top of the head cut-off by the top edge of the image.

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Damon Guy (Netkonnexion)

Damon is a writer-photographer and editor of this site. He has also run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photogs.
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