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.

Saturday, 25 August 2007

More Sony Speculations

Warning: Take the following with a large grain of salt. I do not think that this will have a very high chance of actually occurring but might just represent one possible avenue for Sony to develop their Alpha system.

I have already made a few predictions about what I expect and hope the Advanced Amateur camera will be like. I will make a few predictions about what the Flagship camera may be like when it is rumoured to come out some time next year, possibly at photokina. The primary focus will be on the sensor. I do not think that the sensor in the D3 will be used in a Sony body. The conventional readout using 12 high speed ADCs does not fit in with the sensors that Sony are developing. The coyness of Nikon to name the manufacturer also indicates that Sony has had very little input into the sensor.

If the size of the Flagship sensor is as expected (36mm x 24mm), I expect that it will be based upon the IMX021 and thus would be 28MP with a pixel pitch of 5.49 microns. It would have column parallel ADC readout giving 7fps if a suitable mechanism is found for AF and AE, either by a suitably beefed up reflex mirror or Live View. It may deliver 14 bit RAW files, not neccessarily because the DR warrants it but because of marketing concerns. Super Steady Shot will be included, the PMA presentation reinforced Sony's committment to providing this. There are no show-stopping problems in implementing this for a 135-sized sensor, merely a matter of mass, power, sufficient image circle (135-format lenses tend not to have a perfectly sharp-edged 43.3 mm image circle but have sufficient margin to allow for sensor movement) and control strategy.

Several shooting modes would be available, including a 7MP 14fps mode, either cropped to APS-C size or a subsampling of whole frame. These would be in Live View mode. A selectable 12/14 bit RAW file would also be provided. For single shot mode, a high precision sampling option would reduced frame rate but possibly justify the 14 bit RAW file. It's pretty safe to say that the image buffer will be large, possibly 1GB giving 20 RAW or 100 JPG shots. Sony may use a DNG file format, losslessly compressed. Other possible features are weather sealing, 100% viewfinder, interchangeable viewscreens, wireless transfer and control, and geotagging (GPS).

With the Flagship, Sony will want to make a strong statement. By providing the highest pixel count in a 35mm-style DSLR, Sony would signify its committment to the Alpha mount. Coupled with Zeiss optics, the combination would be a serious professional set-up.

Thursday, 23 August 2007

Nikon's D3 and D300: Implications for Sony?

Nikon shooters have had some very good news today, the long awaited and rumoured D3 and D300 have been unveiled to a rapturous audience. They both boast impressive specifications and many other sites have covered these in depth so I won't reproduce them here. I will speculate and point out some features of the new Nikons which may have some bearing on Sony's DSLR efforts. Given the long history between Nikon cameras and Sony sensors, hopefully the latest information on the new models will provide some insight into the machinations behind Sony's closed doors.

We'll start with the D3. From Rob Galbraith, Nikon says that the sensor is a 36mm x 23.9mm CMOS sensor with 8.45 micron square pixels and a 12 channel, 14bit readout giving at least 9 fps in full readout mode and 11 fps in cropped mode. It is stated that the sensor is an original Nikon design, though it is unknown who actually fabricates it.

From the above, we note a few things. The 12 channel readout is different from the parallel column ADC system used in the IMX021. This is the conventional way of reading out a focal plane array and uses a few high speed analogue to digital converters to quantize the photo-electron signal in each pixel. The 14-bit ADC is interesting, I am not sure whether it truly represents a greater than 12 bit dynamic range or merely a marketing, "keeping up with the Jones'". We can estimate the full-well-capacity at about 55k electrons, similar to the Canon 1DII. The minimum sampling rate at 9fps is 9MHz per ADC. To exceed 12 bit dynamic range (DR), we require a read noise of better than 13.4e which is plausible, though whether this can be achieved at 9MHz is to be seen. The switchable 12/14 bit resolution may indicate that Nikon engineers do not consider the system having greater than 12 bit DR.

The D300 sensor is the most relevant to Sony's plans as on the surface, it looks like the IMX021 (left) recently announced by Sony. The D300 sensor is a Sony-made 12MP APS-C sized CMOS sensor with 5.49 micron square pixels and 14 bit readout at at least 8fps. The IMX021 only specifies 12bit output but at 10fps. It is unknown whether the D300 sensor employs parallel column ADC.

For such a smaller pixel (half the area and presumably FWC), the read noise would have to be half that of the D3 sensor to achieve the same DR. Hence, it is unlikely that the DR exceeds 12 bits, making the 14 bit ADCs superfluous, unless parallel column ADC and multi-sampling have been employed to significantly improve read-noise. A rough estimate gives a 36 multi-sampling with an ADC frequency of 1MHz at 10 fps leading to a reduction in read noise of 6.

It is this latter scenario which is promising for a future Sony DSLR. Though Nikon have a habit of ordering bespoke sensors from Sony (e.g. sensors for the D70s, D2x), the IMX021 seems to be a wholly Sony in-house development, hence the ability to use the parallel column ADC architecture for low noise, high DR imaging may not be proprietary to Nikon.

We will hopefully be able to infer more when Pentax makes its announcements soon, otherwise we will have to wait till Sony make their rumoured announcements early September.

PS A useful introduction to noise sources in CCD imaging is here. Much of the material on signal analysis applies equally well to CMOS sensors.