Sony Alpha A55 review
How Single Lens Translucent Works
Sony Alpha A33 & A55 - How ‘Single Lens Translucent' Works:
The best way to explain how a translucent mirror system works is to compare it side by side to a DSLR system's operation.
In a DSLR camera the light enters from the lens and bounces off the camera's mirror into an AF sensor for fast phase-detection autofocus. Simultaneously, light also bounces into a pentaprism that in turn feeds a preview image to the eye via an optical viewfinder (the pentaprism is there to flip the image from upside down and back to front to the usual way we see). Once the image is framed and the focus set a full shutter press will flip the mirror upwards and out of the way before the shutter itself fires to expose the image sensor (for the time period the shutter is set at) for the final image capture.
An ‘SLT' camera, as Sony is naming the A55, also has a mirror construction - but, as this is translucent, 70% of light can permanently pass through, thus eradicating the need for the mirror to move at all during an exposure. Without the need for this slow mechanical movement it's only the shutter that fires to make an exposure - and this can happen much more frequently, hence the A55's super-fast 10fps shooting. The 30% of light that bounces off the mirror is still available to reach an AF sensor for continuous phase-detection autofocus during live view mode - the first time this has been possible in a digital camera (including the A33 that was released alongside the A55).
This way of doing things does not lend itself to an optical viewfinder due to the lack of light that could hypothetically reach it - this is why the A55 features a built-in electronic viewfinder, or EVF, instead.