HI, there is a small mirror in the back of the main mirror of my elan II (as in all SLRs I guess), which folds up with the main mirror. What is it for? is it for auto focus?, for light metering?, how does it work?, thank you in advance.......

Jorge.

-- Jorge (jboscolo@ciudad.com.ar), September 27, 2001

Answers

This is for the AF system. The metering takes place in the pentaprism above the mirror, and the AF system lives in the bottom of he camera, underneath the mirror. The mirror in an AF SLR is not a true mirror, but rather is a pellicle mirror (usually 67/33 reflection transmission). The light that is let through is reflected to the AF system by the secondary mirror you've noticed. The primary mirror's reflected light falls onto the focusing screen to form the viewfinder image, and to be metered. Look in the manual of your camera to find out the transmission/reflection of the mirror in your camera.

-- Isaac Sibson (isibson@hotmail.com), September 27, 2001.