The partially transparent mirror in Sony's SLT (single-lens translucent) cameras offers some interesting features, but what hasn't been clear is the toll it takes on image quality.
DxO Labs released today sensor test results for Sony's SLT-A33 and A55 and found that the mirror soaks up about one-third of an F-stop's worth of light.
This means a shot that otherwise could be taken at a shutter speed of 1/200th of a second would have be slowed down to 1/160th to get the same amount of light on an SLT camera, for example--or, holding other factors unchanged, that ISO sensitivity in dim conditions would have to be increased from 1,600 to 2,000, with the commensurate increase in noise.
DxO Labs made the measurements by comparing the results from the A33 with the compact NEX-5, which uses the same sensor but lacks the partially translucent mirror arrangement.
That type of mirror, also called a pellicle mirror, lets the SLT camera line perform some neat tricks--autofocus can be continuously engaged, which allows the A33 to shoot 7 frames per second and the A55 to shoot 10 frames per second. With a pellicle mirror, autofocus works when shooting video, too.