THE specifications of the CMOS 13.1 megapixel, 12.3 effective megapixel sensor used in the Sony Alpha 700 and the Nikon D300 continue to provoke discussion. On the one hand, we have Sony’s own announcement of the ‘commercialisation’ of the IMX21 sensor in August, preceding Nikon’s advance publicity by a few days. On the other hand, we have people who refuse to believe that Nikon, who openly bought Sony CMOS technology to create the D2X, is not entirely responsible for its own sensor.
Here is a picture of the Nikon sensor:
And here is a picture of the Sony IMX21:
The two are clearly the same item, bar any minor differences to the type of AA filter, glass cover coating, and modifications to circuitry. It is also possible that Nikon may have a rigid quality control floor below which sensors are rejected on the test bed. This has been a persistent rumour, that Sony ends up using the sensors which fall below Nikon’s stringent requirements. It seems unlikely that Sony would risk their reputation in this way.
In the Sony publicity, it was claimed that the IMX21 has 12-bit on-sensor A to D but can also feed an analog output to be used by an external A to D converter – it has twin output possibilities.
Nikon state in their own publicity that their sensor has on-board A to D conversion, stating 14-bit, but they do not elaborate as to whether they use the 12-bit onboard the same as Sony for their 8fps performance (the sensor can achieve over 10fps with a fast enough processor) and use the analog feed and their own 14-bit external converter – which only runs at 2.5fps. Or, they may have commissioned a modification to the onboard A to D which permits 14 bit. This seems unlikely when the provision was made for higher bit depth external conversion from the start.
Sony’s .ARW2 file format, uncompressed, is 12-bit. Their cRAW compressed format uses 8-bit depth. Nikon’s formats are 14-bit and 12-bit. There is clearly a substantial difference in the A to D for 14 bit but I would suggest that Nikon’s 12-bit output is using the same on-chip process as Sony’s 12-bit. Nikon makes no claims for on-sensor noise reduction but, again, it seems likely this is happening. It appears to be part of the sensor design.
Some out there may have more detailed information, but until we learn of some unique modifications made by Nikon to the actual silicon itself, we can assume that the bit where the photons hit the pixels is pretty much the same item.
– David Kilpatrick