No, that is fundamentally incorrect. You start with a 20mp sensor, which has 40mp PHOTODIODES.
Jrista, you are just assuming that Canon's dual-pixel
tech is in fact a dual-photodiode tech.
My assumption is that it's already a quad-photodiode tech - and it's equally valid, as neither
one us has info on the actual implementation.
In general, before making any claims for photodiodes and pixels, consider the following:
A 'classic' pixel design has a photodiode plus three transistors (you can read about it on Wikipedia
- a reset transistor for resetting the photodiode voltage
- a source-follower transistor for signal amplification
- a row select transistor
So, one definition of a pixel is a photodiode with three transistors.
The thing is, to improve fill factor and for other design considerations, modern sensors are using transistor sharing.
That is, a single set of the 'classic' transistors is shared between multiple phododiodes.
Transistor sharing is widely used in small sensors.
In the case of these sensors, though, each photodiode has its own microlens.
Thus, the photodiode is
the pixel in these designs.
In short, depending on the implementation, a photodiode and a pixel could mean the same thing.
Canon's 'dual-pixel' tech is assumed to be based on a shared-transistor design.
That is, it is a multi-photodiode design.
But since in a shared-transistor design photodiodes are effectively equivalent to pixels (as explained),
Canon's tech could be called multi-pixel design as well.
So, you can stop correcting people who use dual/quad-pixel terminology, as these could in fact be used interchangeably.
The line between between a pixel and a photodiode is blurred in shared-pixel designs.
And the fact that the two photodiodes are read independently for auto-focus further
indicates that these could very well be independent pixels - if they didn't share the
same microlens and color filter.
Also, your claim that there are exactly TWO PHOTODIODES (and that's it!) is not based on fact.
We don't know for sure if Canon's design is a dual-pixel design (your assumption) or a quad-pixel design
Canon's marking is selling it as a 'dual-pixel' tech likely because it's easier this way to communicate
the concept to the general public.
But we don't know for a fact what the actual implementation is.
So, your TWO PHOTODIODES claim is based on marketing materials, really.
If I were you, I wouldn't put too much weight into these
My assumption for a quad-pixel design is based on simple geometry.
If there are just two photodiodes per pixel, these photodiodes need to be rectangular.
This would be uncommon - if not even a first in the industry.
But with a quad-pixel design, the photodiodes are square just like in any other sensor.
Considering the potential future advantages of a quad-pixel design (e.g. for a non-Bayer sensor),
I'd speculate that Canon would have invested in a quad-pixel design from the start - rather than
designing rectangular photodiodes that later would need to be made square anyway.
Just a speculation, of course - but based on some informed assumptions.