# Sensor performance, quantum efficiency, sensorgen and DXOmark

Started Feb 5, 2014 | Discussions thread
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Re: Sensor performance, quantum efficiency - how is this really measured?

Jack Hogan wrote:

Jason Rickerby wrote:

I must confess, I'm still unclear of the relationship between read noise, well depth and dynamic range. Will camera A, with half the read noise of camera B, at the same ISO and with the same well depth have one more stop of usable dynamic range?

The generic approximate answer is yes but the precise answer depends on the exact definition of DR required and sensor technology, including QE

Reading the OP's paper, there's a lot of of sophisticated math.

Looking at the following Sensorgen pages, http://www.sensorgen.info/CanonEOS_5D_MkIII.html, http://www.sensorgen.info/NikonD800.html I'm still unclear as to where these measurements actually come from.

At ISO 100 with the Canon 5D Mark III and Nikon D800 we have the following:

5D3: Read Noise - 33.1, Saturation - 67531, DR - 11.0

D800: Read Noise - 2.7, Saturation - 44972, DR - 14.0

As A/D converters aren't producing decimal output, how realistic are these Read Noise and DR figures? Isn't it a situation whether you trust the A/D converter low value bits or not? Are these numbers derived from integer data in RAW files? Are decimal figures an estimate due to a known analog gain applied to the sensor readout?

Does the following approximation work?

For the 5D3, 33.1 requires 2^6 and 67531 would require 2^17. That would be a 2^11 difference between saturation and noise. Does this mean sensor gain can be designed to optimize performance of the 14-bit A/D converters?

For the D800, 2.7 requires 2^2 and 44972 would require 2^16. That would be a 2^14 difference between saturation and noise. This is taking 14-bit A/D converters to the limit?

Is the above an approximation of how sensor DR is calculated? Assuming sensor performance is linear, which it isn't, ideally every additional bit equates to a stop of dynamic range? It seems like as ISO goes up, analogue gain is increased and more low value A/D bits are ignored? Is analog gain optimized for A/D performance, ISO accuracy, power consumption/linearity or all of the above? You can't have more stops of camera dynamic range than A/D bits right?

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