Semi-transparent stationary reflex mirror in an SLR is nothing new. Canon Pellix in 1965 used such mirror to let about two-thirds of the light from the lens pass directly through to the film, while the rest was reflected to the viewfinder and CdS meter cell for the TTL exposure measurement.
Sony's Single-Lens Translucent technology uses a semi-transparent fixed mirror which diverts about 30% of the incoming light to a phase-detection autofocus sensor, while the remaining 70% strikes the image sensor, which in turn feeds the electronic viewfinder. This design allows for full-time phase-detection autofocus during electronic viewfinder, live view, and video recording operation.
The drawback of these technologies is the loss of light:
- Canon Pellix: about - 0.5 EV in the exposure through the semitransparent mirror and about - 1.7 EV dimmer finder compared to using a fully reflecting mirror
- Sony SLT: about - 0.5 EV in exposure because of the portion of incoming light diverted to the autofocus sensor.
Ricoh were just (11 November 2014) granted a patent on a translucent mirror with adjustable transmission rate: light transmission rate is adjustable from 50 to 100%, i.e. reflection rate varies from 50 to 0%.
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With such a device, you can either, and successively, improve the efficiency of the autofocus sensor in low light by increasing the reflection rate and improve that of the image sensor by increasing the transmission rate (SLT scheme with an electronic viewfinder) or, if you keep the optical viewfinder (Pellix scheme), successively improve the brightness of the viewfinder by increasing the reflection rate then the efficiency of the imaging sensor in increasing the transmission rate.
The better of both worlds.
Last edited by Mistral75; 11-29-2014 at 07:03 AM.