The Single-Lens Translucent camera (SLT) is somewhat like a DSLR however it uses a beam splitter in place of a a solid mirror, and the beam splitter is used for other purposes than the DSLR mirror. The primary difference when compared a DSLR is
that a SLT camera has no optical viewfinder, instead it relies upon an electronic viewfinder, and uses the image obtained by the image sensor. It cannot be considered a DSLR because a DSLR, employs a mirror to display the image to be acquired in a viewfinder.
The Sony α65 has a translucent mirror letting light constantly into to the sensor.
Although an SLT camera may resemble a DSLR in outward looks and operation and uses the identical accessories and lenses and, it is fundamentally different. A DSLR allows direct viewing of by user of the image which is moving from the lens whereas an SLT doesn't have a mirror that shows the image to the
viewfinder, instead the image is created on the image sensor and relayed to an electronic viewfinder while the mirror is merely there for focusing purposes.
The semi-translucent mirror lets most of the light continue on through to the image sensor while it reflects some of the incoming light to a phase-detection AF image sensor located in the top part of the camera, permitting the camera to shoot photos without having any mirror movement (along with any related vibrations), along with phase detection full time AF.
SLT users must compose their shots by use of an electronic viewfinder, not unlike mirrorless interchangeable lens cameras (sometimes called EVIL or MILC cameras). However, MILC cameras don't contain any mirrors and simply use contrast-detect AF, which is a huge difference as compared to an SLT camera. Since their autofocus array is continually receiving incoming light, SLT cameras have the capacity to employ phase-detection AF while recording video, however a DSLR along with other types of interchangeable lens cameras must use the slower contrast detect AF for their live view and video.
The initial SLT cameras were introduced by Sony. Their mirror's in these cameras remains in a fixed position during normal functions which is a huge difference as compared to DSLR cameras, although they may be moved for cleaning the sensor.
SLT compared to other digital cameras
Advantages: Nonstop phase-detection auto focus even while recording video, during live view or a continuous shooting mode. There's no camera shake from any mirror movement, The viewfinder doesn't blackout while shooting photographs, along with a shorter shutter lag time. The
white balance, exposure values, and many other of the camera settings can be simply verified directly in the electronic viewfinder (while able to be fine-tuned) before clicking the shutter and the camera bodies can be more compact.
Disadvantages: There's no optical viewfinder, while less light reaches the
image sensor, because a part of the light is reflected to the phase-detection auto focus array (almost 1/3 EV in the latest designs). The refresh rate is limited by the amount of time required by the sensor to create an image; and in poor lighting conditions, causes extreme viewfinder image shuttering while panning (e.g. if it requires 1/4 sec for the image sensor to obtain enough light to create the image thus the EVF will update at 4FPS).
There's no real-time updating of viewfinder during high rates of shooting, the viewfinder simply displays the last image captured instead of showing where the lens is actually pointed; mainly a concern when shooting fast-moving targets. The EVF has nowhere near the dynamic range of the image sensor, as a result the EVF doesn't properly display what will be acquired. The brilliant light from the EVF in present viewfinders can create eyestrain when employed over long time frames and can have an effect on adapted dark vision, even creating temporary nyctalopia. Over time, the surface of the mirror can become dirty, resulting in the mirror scattering light rays and degrading recorded images.
Pellicle mirror SLT
The mirror is more resistant
The viewfinder is bright
Camera bodies can be more compact
No optical viewfinder
Compared with Mirrorless interchangeable lens cameras
When compared with mirrorless interchangeable lens cameras, the major advantage of SLT's are quicker auto focus (phase-detection vs contrast-detect) and continuous phase-detection auto focus even during capture plus phase-detection auto focus in live view, even while recording video.
The SLT's disadvantages when compared with mirrorless interchangeable lens cameras are larger camera size, more costly. less light reaching the image sensor, because part of the light is reflected to the phase-detection auto focus array (around 1/2 EV in present layouts).