Single-lens translucent camera

Single-Lens Translucent (SLT) cameras stand in-between digital single-lens reflex cameras (DSLR) cameras and mirrorless interchangeable-lens cameras (MILC).
While SLTs' aspect recalls DSLRs in gross form and they use the same lenses (having the same flange-back distance), they are fundamentally different and - functionally - SLTs are possibly closer to MILCs rather than to DSLRs, because SLTs have an electronic view-finder whereas DSLRs' fundamental characteristic is a view-finder allowing the user to directly view the optical image generated by the lens.
SLT cameras have a mirror, but - unlike the one in DSLRs - it does not move, and it is semi-transparent, allowing the majority of the light to pass through to the sensor whilst reflecting a portion of it onto a phase-detection autofocus sensor, this way allowing full time phase detection auto focus.
Like MILCs, SLT cameras have an electronic viewfinder, using the image collected by their sensor. MILCs, though, focus by contrast detection, i.e. the same technology used by compact cameras (and by DSLRs when operating in live view mode and/or to record movies). See Autofocus.
SLT cameras - instead - are able to use phase-detection autofocus all the time, including during video recording, which - in general - is faster and more reliable than contrast detection autofocus. Therefore - at focusing - SLT cameras have an advantage both towards MILCs and towards DSLRs.
Fixed semi-transparent mirrors have been used in single-lens reflex cameras from at least the 1960s (see Canon Pellix. Olympus was the first - in the year 2000 and with its E-10) model - to introduce a similar concept to digital photography. Olympus E10 worked by splitting the light coming in from its lens into two beams: one for feeding the sensor, the other one for feeding in continuum an optical viewfinder, thus doing without a flipping mirror.
The term SLT has been introduced by Sony with the launch of its Alpha 55 model. Its mirror can be removed for sensor cleaning.
SLT compared to other digital cameras
SLT
Advantages:
* Continuous phase-detection autofocus even during video, live view or continuous shooting mode
* Lack of camera shake due to mirror movement
* No viewfinder blackout while taking photograph
* Shorter shutter lag
* Exposure value, white balance and other settings can be easily and directly verified on the electronic viewfinder (and thus tweaked) before taking a picture.
Disadvantages:
* Less light reaching sensor, due to portion of the light being reflected to phase-detection autofocus array (approximately 1/3 EV or 21% in current designs).
* Refresh rate limited by the time it takes the sensor to form an image; in low light this causes severe stuttering of the viewfinder image when panning (e.g. if it takes 1/4 sec for the sensor to gather enough light to form an image then the EVF updates at 4FPS).
* No real-time viewfinder update at high shooting rates, the viewfinder shows the last picture taken instead of where the camera is actually pointed; mostly a concern with fast-moving subjects(Note: Some DSLR's do not update the image at all, as mirror is up when shooting. However, some will give you a glimpse of what is being shot as the mirror and shutter reset for the next shot.).
* The EVF has far less dynamic range than the sensor, so the EVF does not properly show what will be captured.
* The bright light of the EVF in current implementations can cause eyestrain when used over long periods and can affect dark-adapted vision, causing temporary nyctalopia ().
* Over a period of time, the mirror surfaces can become soiled. As a result of this the mirror scatters the light rays and degrades the recorded image.
Pellicle mirror SLT
Advantages
* More resistant mirror
* Bright viewfinder
* More compact body
Disadvantages:
* Lack of optical viewfinder
Mirrorless interchangeable lens camera
Compared to mirrorless interchangeable lens camera, SLT's major advantages are:
* Much shorter delay between touching the trigger and shot taken
* Faster autofocus (phase-detection vs contrast-detect)
* Continuous phase-detection autofocus even during exposure
* Phase-detection autofocus in live view, including during video recording.
SLT's disadvantages compared to mirrorless interchangeable lens camera are:
* Increased camera size / weight.
* Higher cost.
* Less light reaching sensor, due to portion of the light being reflected to phase-detection autofocus array (approximately 1/2 EV in current designs).
List of SLT cameras
* Sony Alpha 33
* Sony Alpha 35
* Sony Alpha 55
* Sony Alpha 65
* Sony Alpha 77

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See Also

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