A simple way to put Canon's sensors on top.

Started Feb 2, 2014 | Discussions thread
Press Correspondent
Press Correspondent Veteran Member • Posts: 3,362
DR at high ISO is limited by properties of light

HumanTarget wrote:

Press Correspondent wrote:

Great Bustard wrote:

At higher ISO settings, Canon sensors are as good as the best of them. So, if Canon were to offer an optional ISOless interface, increase the bit depth of the capture files to 20 bits, and have the cameras shoot permanently at ISO 3200 in the ISOless shooting mode, they'd match or beat Sony's sensors in terms of noise and DR.

By making the ISOless UI optional, it would still allow those who find the current noise and DR levels to be "good enough" and prefer setting the ISO themselves to continue as before without any bother.

At low ISO the DR is limited mostly by the sensor read noise. At high ISO the DR is limited by the well capacity and therefore the photon noise. In other words, at high ISO the DR limitation is in the light itself even before it hits the sensor. Therefore increasing the number of bits in the electronics would not increase the DR at high ISO.

Shot noise does not play a part in DR; it is defined as the full well capacity divided by the read noise. Otherwise you wouldn't be seeing measurements of 14 stops of DR in the D800.

I should have phrased it better indeed, but my point stands nonetheless while your definition is incomplete. Per DXOmark.com:

Dynamic Range

"Dynamic range is defined as the ratio between the highest and lowest gray luminance a sensor can capture. However, the lowest gray luminance makes sense only if it is not drowned by noise, thus this lower boundary is defined as the gray luminance for which the SNR is larger than 1."

Noise in the above definition consists of the read noise, quantization noise, and photon noise of the "lowest gray luminance" (with thermal and other types of noise often disregarded). Let's take 6D as an example. What is the lowest gray luminance at ISO 3200? The read noise is 2.3. The lowest signal should exceed the square root of itself plus 2.3 plus 0.5 for the quantization noise. This solves to approx. 5 photons = (SQRT(5)+2.3+0.5). The well capacity is 2478, so the DR is Log2(2478/5) = 9 stops (8.95). You could see the DR of 10 elsewhere (10.1), but it does not change the fact that the sensor DR at high ISO is limited mostly by the well capacity while increasing the resolution of the ADC would not change a thing.

Now consider the read noise is zero, what is the DR then? The lowest luminance would be 2 > SQRT(2)+0.5 and the DR is 10 stops (10.3) defined purely by light EVEN BEFORE LIGHT HITS THE SENSOR, which was my original point. The pixel level DR at high ISO (or, more precisely, in low light) is limited by the quantum properties of light, not by the precision of the ADC. There is no way to get a 20-stop DR at the 6D pixel size at ISO 3200.

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