How does the D3 achieve such high ISO?

Started Feb 17, 2008 | Discussions thread
ejmartin Veteran Member • Posts: 6,274
Re: 1D3 full well?

ohyva wrote:

Second I'm still sceptic about these well capacity "measurements". No
direct measurement method AFIAK (unless you probe into the chip which
I think has not been done (and would probably to some extend
interfere with the masurement). If done indirectly based on noise
figs, then I'd really love to know how all the different noise
sources are separated. And how to ensure there is no in-sensor
digital domain NR applied - which is reported by Sony to be one of
the methods how they have reduced the noise levels in their new CMOS
sensor technology.

There are three common noise sources -- the electronic noise in the readout and amplification circuits, thermal noise in those same circuits, and photon shot noise. They are all independent and can all be measured by varying the exposure.

The thermal noise is proportional to the time of exposure, and can be measured in a series of longer and longer exposures with the lens cap on; it is typically irrelevant until exposure times exceed a second or more.

Electronic noise is there independent of exposure, it just comes from reading out the sensor, and so can be measured at very short exposures with the lens cap on (negligible thermal noise, no photon signal).

Photon noise varies proportional to the square root of the number of photons collected, and can be measured by a set of exposures at higher illumination levels where the other noise sources are negligible (of course, one can subtract them out for a slight improvement in the accuracy).

So they can all be separated from one another by varying the exposure, since they all vary differently with exposure.

As for whether NR is applied, all Canons and probably other CMOS sensors use a method to cancel noise in the readout of a pixel. This is not NR as most people think of it, ie smearing out neighboring pixels to smooth out the noise grain; rather it is done for each pixel independently and involves no comparison with neighboring pixels. Noise reduction that involves averaging over nearby pixels is easily detected by looking at the Fourier transform of the image; if this sort of NR were being performed one would see the noise at high spatial frequencies take a nose dive relative to lower frequencies. I have examined raw images of the D3 in this way and see no evidence of such NR -- the noise spectrum is flat all the way out to the limits of the sensor resolution (Nyquist frequency).

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