It’s Done with Mirrors

A key component in any SLR camera is the mirror located immediately behind the lens. This mirror reflects all or part of the light that passes through the lens upwards towards the viewing mechanism. There are exceptions, as in the case of some odd-ball designs like the old Olympus E-300/EVOLT design, which has a sideways-flipping mirror. Other exceptions include dSLR-like interchangeable lens cameras like the Sony Alpha SLT-A55 and later models, which have semi-transparent mirrors that allow most of the incoming light to continue towards the sensor all the time, but which bounce some illumination upwards to an autofocus system (such cameras use electronic viewfinders rather than optical viewfinders like dSLRs).

As you learned in Chapter 1, that viewing system can consist of a focusing screen that images the picture, plus a prism or set of mirrors that takes the screen’s upside-down and reversed view and orients it properly. During exposure, the mirror flips up, allowing the light to continue unimpeded to the sensor. Figure 4.1 shows a highly simplified version of the path of light through a digital single lens reflex. The exposure meters (in the pentaprism/pentamirror housing) and autofocus sensors (on the floor of the mirror box) illustrated in Chapter 1 are not shown in this mirror-centric illustration.