Key Features / What's new?
The mirror in the A55 is semi-transparent. According to Sony, it allows approximately 70% of the light that strikes it straight through to the 16.2MP CMOS imaging sensor. The remaining proportion is reflected upwards to a large 15-point phase detection AF array. This equates to a roughly 1/2 EV reduction in light transmission for live view/image capture but means the AF sensor is always receiving light.
This isn't an entirely new idea - fixed, semi-transparent mirrors have been built into SLRs before, but the system in the A33 and A55 is a major new development. What makes the A33 and A55 unique is that they can maintain phase-detection AF at all times, even during an exposure.
They manage this by removing the optical viewfinder entirely and only having to direct light to the phase-detection AF sensor array. For packaging reasons this has been moved from its conventional position at the base of the camera's mirror box, to its roof. Roughly 30% of the light that strikes the main mirror is reflected upwards, onto the AF sensor array. Meanwhile, in the absense of an optical finder, the main imaging sensor provides a full-time live view image via an electronic viewfinder in its place.
There is still a conventional mechanical shutter so operation isn't silent, however.
The optical system of the A33 and A55 is unique, and represents a significant step forwards from conventional SLR design. Both cameras are built around semi-transparent mirrors, which allow light to pass through them, for the purposes of live view and exposure, but also reflect a portion of the light onto a new phase-detection AF array. Here's a detailed look at this and other new technology used in the SLT A33 and A55.
* Please note: these images are for illustration and do not show the exact position/angle of the mirror as it exists inside the A55/A33.
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