Wednesday, 29 August 2007

The Beta Mount?

The trend in consumer camera sensors seems to be heading towards greater on-chip processing and capabilities due to CMOS active pixel sensor (APS) architecture. This has now filtered upstream to DSLRs in the form of Live View, the ability to see a video frame-rate output from the sensor before final image capture. Sony have been pushing hard recently with parallel column ADC in order to increase readout rate and simultaneously lower readout noise. It is not inconceivable that in the near future, an interchangeable lens camera based on the alpha mount and lenses will be developed which omits the reflex mirror of a DSLR, replacing framing, AF and AE functions by Live View and a suitable electronic viewfinder (EVF).

For this to happen, several things need to be improved. The video refresh rate should be 60 fps or higher to allow accurate tracking and to avoid the feeling of lag. The power consumption should be low, comparable or lower than the power consumed by AF and AE. The EVF needs to have sufficiently high resolution to allow for manual focusing. To reproduce the viewfinder size and magnification of the Dynax 7, for example, may require something on the order of 2MP, roughly HDTV resolution. The technology is getting there, the same technology which is going into HDTV projectors could be adapted for EVF displays. The parallel column ADC approach allows greater flexibility in readout modes.

There are several advantages from a manufacturing point of view, by forgoing the reflex mirror and pentaprism, considerable bulk, mass and mechanical complexity can be eliminated leading to cheaper, lighter, and potentially more reliable cameras. Electronic shuttering would allow flash sync at all shutter speeds, viewfinder blackout would be reduced, and the frame rate would not be limited by mechanical parts. Noise and vibration would also be reduced leading to quieter and sharper operation. Live view would improve exposure and allow detailed manual focusing. Of course, there will be reluctance from traditionalists, perhaps sceptical about the clarity and responsiveness of EVFs, but the time may come where these objections may be overcome by improvements in electro-optics.

So presupposing sufficiently good EVFs and sensor performance, we can ask how best to adapt the Alpha mount to such a system. The absence of a mirror box means that the lens mount can be located much closer to the sensor. New lenses, especially wide angle lenses could take advantage of such geometry obviating the need for retrofocus design. However, the alpha mount and lenses are designed for a 44.5 mm distance between flange and sensor plane, meaning that these could not be directly used on such a short mount. The answer would be to design an adapter, basically an extension tube which would convey all the focusing and communication between body and lens. In this way, full compatibility could be maintained with existing lenses yet allow for optimised designs to take advantage of the reduced mount distance.

The new mount, for the purposes of this discussion called the Beta mount, could have a flange to sensor plane distance considerably shorter than the Alpha mount. Rangefinders have mount distances about half that of SLRs, 27.95mm (Leica M) vs 46.5mm (Nikon F). The Beta mount could have a mount distance even shorter, say 20mm, leaving enough room for adapters for all other mounts.

Beta mount lenses could follow EF lens design and have focusing and aperture electronically controlled by the body with power supplied by the body. This would further reduce the mechanical complexity of the body as well as simplifying the mechanical design of the mount. For alpha mount lenses attached via the adapter, focusing and aperture control could be retained by incorporating actuators within the adapter itself.

All of the above is speculation but could represent an avenue for Sony to make the best use of the advances in electro-optics yet maintain compatibility with legacy users.

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