See under reflex camera.
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Definition for viewing lens (2 of 2)
a camera in which the image appears on a ground-glass viewer (focusing screen) after being reflected by a mirror or after passing through a prism or semitransparent glass; in one type (single-lens reflex camera), light passes through the same lens to both the ground glass and the film, while in another type (twin-lens reflex camera), light passes through one lens (viewing lens) to the ground glass and through a second lens (taking lens) to the film, the lenses being mechanically coupled for focusing.
Origin of reflex camera
First recorded in 1925–30
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for viewing lens
a camera in which the image is composed and focused on a large ground-glass viewfinder screen. In a single-lens reflex the light enters through the camera lens and falls on the film when the viewfinder mirror is retracted. In a twin-lens reflex the light enters through a separate lens and is deflected onto the viewfinder screen
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012