|Canon 70D "duel pixel" sensor architecture. (Canon Inc.)|
Canon versus Nikon: the age-old debate continues. Depending on your outlook on life, either one is definitely better than the other, or the practical difference between the two is not worth mentioning. It's easy to say that the truth is in between those extremes, but what is more true is that at least for now, Canon and Nikon on diverging in sensor philosophy. I don't mean in the age-old ISO versus resolution debate, but about more subtle design considerations under the hood.
Ostensibly, the 70D is poised to become Canon`s premiere enthusiast-level video machine in the near future. The new dual-pixel sensor will (at least on paper) allow for motion tracking autofocus on APS-C at a performance level that was only previously enjoyed by the Sony SLT cameras, with the added bonus of being able to achieve focus across the whole area of the image frame. In order to achieve this, the new sensor architecture is essentially two different chips merged into one. The top layer is like that of a sensor from a 20.2mp camera, but the bottom read-out layer is like that of a 40.4mp camera. Even though technology improves, the additional amount of circuitry has implications on not only the architecture of the sensor chip, but the whole imaging pipeline of the camera itself.
This image of a mainboard showed up on a few days ago, and is purported to be the new DIGIC 5+ image processor driving the EOS 70D. As with previous Canon's, the Analogue to Digital Converters (ADC's) seem to be located off the image sensor, unlike the Sony designs used in Nikons, where the ADC's are integrated into the circuitry of the of the image sensor.
Recent Canon sensors have used ADC’s located outside off the image sensor, much like the one pictured above. What is means is that a handful of ADC processors (with the 5DmIII, 8 are used) share data among all the columns on the sensor by multiplexing the analog signal out to the ADC board. Minor differences between these analog read-out channels produces patterns in the output signal which may be visible when the signal to noise ratio is low, is in the shadows. Because the analog pathway from light gathering to ADC is longer than it is with the Sony chip's, the Canon chips tend to have more image noise at low ISO. This is because the analog signal is vulnerable to picking up stray signals from the surrounding circuitry; the longer the signal stays in analogue form, the more chance of inducing extra noise. The difference tends to narrow at higher ISO.
Sony chips use ADC's that are integrated into each pixel column on the sensor, and run with a chip clock speed that is significantly lower rate than the Canon implimentation. The signal fluctuations are still present, but because there are thousands of ADC's running at lower speed instead of a handful running at high speed, the signal variation is low and distributed more evenly across the the whole image readout. This results in less pattern noise, aka "read noise". This also improves their low ISO dynamic range compared to the recent Canon chips. Dynamic range is the difference between the saturation capacity of the lightwell and the noise floor; because the Sony chips have such low read noise, the noise floor is lower. This has two implications for still images. The first is the idea of "ISO-less" shooting. The second is that it tends to inflate the DxOMark scores for sensors with on-chip ADC's, as the DxO composite scores are weighted to favour low ISO sensors.
However, even though the Sony method is better for still photography, that doesn't necessarily mean that it is the best architecture overall. With the ADC's located on-chip, heat generation can be an issue with higher frame rates, meaning that additional cooling considerations need to be factored into the design. The implications for continuing with an off-chip ADC go something like this:
- Lower power use on the sensor; less dark current. Good for video and astrophotography.
- ADC can be designed independently of the image sensor
- Additional space taken up in the camera body by ADC mainboard. Modern DSLR's are already pretty cramped.
- Still image quality compromised at the benefit of improved video performance.
It will be interesting to see what the Canon EOS 70D image quality turns out to be, as the dual-pixel architecture, at least on the face of it, seems like a brilliant idea. The release date won't be until the end of August 2013, so it will be the whole summer before a clear picture starts to emerge.