I have a Sony body with a 24MP APS-C CMOS sensor. It will store images in the following resolutions:
- 6000x4000 (24MP)
- 4240x2832 (12MP)
- 3008x2000 (6MP)
When comparing the 24MP and 6MP cases, it's easy to see that the camera could effectively sample from four physical pixels to produce each stored pixel. Based on this fact alone, I would expect SNR to be twice as high. Or, putting it another way, I could double ISO when recording 6MP and expect the same noise I see at 24MP, all else equal. Is my theory accurate so far?
Now, in practice do cameras that downsample achieve that full theoretical improvement in noise?
There are so many physical and logical layers between the point at which the image impinges the sensor and where it is finally written to a digital image stream I am left wondering if perhaps the camera can do even better when downsampling?
(Conversely, if on-camera downsampling is known to be suboptimal, one would prefer to "pay" to record at maximum resolution and downsample in post-processing, where the full Sqrt[N] improvement in SNR can be achieved.)