Advances in the compact, lightweight body of the α55 mainly derive from the stationary translucent mirror inside. Using an electronic viewfinder instead of an optical one also keeps the body compact. The space this opens up under the pentaprism houses the autofocus sensor (which is on the bottom of conventional cameras), and in its regular place is a mechanical unit. In this way, completely revising the internal layout enabled more efficient use of space, making the camera about 23% smaller and about 26% lighter than the previous model (α550). Targeting the metal parts that make cameras heavier, the frame structure was simplified and new materials introduced, which makes the α55 frame 40% lighter than the α550 frame.

Although use of recycled plastic has traditionally been focused on external parts, the α55 takes this a step further. Even mechanical components that demand exacting structural precision are made with recycled plastic. Using different types of recycled plastic depending on the functional requirements of parts significantly expands the scope of recycled material in products. More than three times as many parts in the α55 than in the α550 are made with recycled plastic. Lighter and less resource-intensive, the α55 incorporates more recycled plastic than any other Sony SLR.

With a stationary translucent mirror where regular SLRs have a mirror that tilts up and down when shooting, the α55 constantly channels light from the lens into the autofocus and image sensors. This means that autofocus (AF) is constantly working, even during shots, so the camera can track fast-moving subjects better and offer faster continuous shooting. In movie mode as well, as you check the image on the LCD screen, the camera applies the phase-difference detection method to keep subjects in focus. Thanks to this, the images captured remain in clear focus at all times.