Optical systems for use in digital cameras are provided. The systems can be switched between three optical paths (1,2,3): a first path (1) for through-the-lens (TTL) viewing; a second path (2) for recalling previously taken pictures from a microdisplay (16); and a third path (3) in which pictures are taken using an electronic sensor (15), e.g., a CMOS sensor. The optical systems employ a zoom objective lens unit (10), e.g., a 3:1 zoom, an erector lens unit (12), and an eye lens unit (13). They can also employ an image size adjusting lens unit (11) for equalizing the TTL and recalled images presented to the user. To facilitate integration with the electronic components of a digital camera and to provide a “book” format for the camera, the first and second optical paths (1,2) lie in a single plane and each includes two folding mirrors (17,20 for the first path; 18,20 for the second path) and the third optical path (3) has a single folding mirror (19) so that a portion of that path is in the single plane and a portion is orthogonal to that plane.