With the entry-level Sony A55 gone the way of the dodo, the company had to fill the gap in its DSLT (digital single lens translucent) camera line-up. The just announced A57 is the official successor to the A55, which was initially released in 2010 and was the first camera to feature Sony’s translucent mirror technology.
Sony’s SLT cameras have a DSLR-style body and a mirror, but unlike DSLRs, the mirror doesn’t move. The translucent mirror allows most light to hit the sensor while some of it is reflected to a phase detection autofocus sensor. Because of this setup, DSLT cameras feature electronic viewfinders (EVFs) rather than optical ones.