‘Single Lens Translucent’ (sic)
The Alpha 55 and 33 introduce Sony’s solution, which is to use a semi-silvered or ‘translucent’ mirror to transmit about 70% of the lens image to the sensor, and 30% to a new AF module which uses 15 detectors including three cross-type. This is located in the top of the camera, more or less where the focusing screen would be in an SLR.
This new camera design, termed ‘SLT’ or single lens translucent by Sony, seems to have involved enough new design to solve problems which have plagued SLRs for years. One of these is focus module collimation, which can produce front or back focus errors. My own experience with the Alpha 55 has been that when focus is correctly confirmed, it is very accurate indeed, more so than with previous models from the Alpha 100 to 700. The Alpha 900 and 850 offer programmable adjustment of focus for every lens used, overcoming the problem. The Alpha 55 simply doesn’t seem to have the issue – maybe due to the new 15-point, 3-cross sensor focusing module.
I suspect that the translucent mirror may also play a role. Conventional AF involves about eight separate collimation adjustments. The focusing screen (1) must be perfectly located relative to the adjustable sensor plane (2), and the primary 45° mirror must then be perfectly aligned (3). Then there is a secondary mirror behind the main one (4), a return mirror (5) and then the AF module itself which sits on a carriage controlled by three screws. Add to this similar multiple screws to adjust the sensor mounting, and the setup of the lens and the lensmount relative to the whole assembly (7 and still counting). It’s a highly complex system.
In the SLT design, the AF module has no additional mirrors and receives the image head-on from the SLT mirror. Its mounting relative to the image sensor is one critical adjustment, and the SLT mirror is another. These positions are fairly simple when there is no further relationship to consider such as the focus screen (absent). The SLT mirror position could, in fact, be used to fine tune AF. Accuracy of lens mount alignment is no more, nor less, an issue than with other designs and the same goes for sensor adjustment.
The simplicity of the SLT design for collimation and adjustment can only be bettered by direct focus detection from the sensor itself – and the Alpha 580 and 560 both offer this function. They can perform a contrast detect autofocus or an enlarged manual focus, for single shots, with critically accurate results. The 55 and 33 only offer enlarged manual focus as the PDAF system always takes priority.
The final benefit of this system is that there is no moving mirror. This either means no need for a second motor system, synchronised with the lens and shutter operations, or a single motor which drives a complex but fixed sequence of actions. Recent Sony SLRs have used one motor, where Minolta models used two. This has resulted in the loss of mirror lock-up as a function, because the single motor driven firing cycle can not be interrupted at that point.