Sony SLT camera

The Sony α33 has a translucent mirror allowing light through to the sensor at all times.

A Single-Lens Translucent (SLT) camera is similar to a DSLR but using a beam splitter instead of a solid mirror, this beam splitter being used for a different purpose than the mirror in a DSLR. The major difference compared to DSLRs is that SLT cameras do not have an optical viewfinder, instead they rely on an electronic viewfinder, using the image collected by the sensor. They cannot be considered true DSLRs because a DSLR, as described here, uses a mirror to show in a viewfinder the image that will be captured.

While SLT cameras resemble DSLRs in gross form and function and even use the same lenses and accessories, they are fundamentally different. A DSLR allows the user to directly view the image that is passing through the lens while an SLT does not use the mirror to show the image in the viewfinder, the image is formed on the sensor and transmitted to the electronic viewfinder; the mirror is only used for focusing.

A semi-translucent mirror allows the majority of the light to pass through to the sensor while reflecting a portion of the light onto a phase-detection autofocus sensor in the top of the camera, allowing the camera to take photos without any mirror movement (and associated vibrations), as well as full time phase detection auto focus.

Users of SLTs must frame their shots using an electronic viewfinder, similar to mirrorless interchangeable lens camera (also called EVIL cameras). However, EVIL cameras don't have mirrors and use contrast-detect autofocus, which is a major difference compared to SLT. Since the autofocus array is constantly receiving light, SLT cameras are able to use phase-detection autofocus during video recording, while DSLR's and other interchangeable lens cameras have to use slower contrast detect autofocus for video and live view.

The first SLT cameras were brought to market by Sony. The mirror in these cameras is fixed in position during normal operation which is a major difference compared to DSLR, although they can be moved for sensor cleaning.

SLT compared to other digital cameras

Conventional DSLR

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.
• More compact body

Disadvantages:

• Lack of optical viewfinder
• Less light reaching sensor, due to portion of the light being reflected to phase-detection autofocus array (approximately 1/3 EV 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.
• 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 (Adaptation (eye)#Insufficiency).
• 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

Main article: Mirrorless interchangeable lens camera

Compared to mirrorless interchangeable lens camera, SLT's major advantages are:

• 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.
• 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 55
• Sony Alpha 33

See also

Pellicle mirror Digital single-lens reflex camera Mirrorless interchangeable lens camera Four Thirds System Rangefinder camera

Optics Single-lens reflex camera Twin-lens reflex camera Digital photography