D3 sensor efficiency

Started Nov 18, 2007 | Discussions thread
DuncanM1 Contributing Member • Posts: 817
Re: D3 ISO Grossly Overrated?

Bill Janes wrote:

The article by Christian Buil states, "The ISO sensitivity levels
corresponds to an electronic gain extremely different between Canon
and Nikon. Assuming that the quantum efficiency of sensors are
equivalent (it is highly probable in first approximation), the
displayed sensitivity of 6400 ISO for the Nikon D3 #1 corresponds to
a sensitivity of 600 ISO for the Canon 40D, while a sensitivity
displayed at 6400 ISO for the Nikon D3 #2, corresponds to a
sensitivity of 2400 ISO for the Canon 40D. Analysis shows that the
'displayed' ISO announced by Nikon is very over evaluated relative to
Canon competitor."

From the presented data, I find this puzzling. He compared the Canon
40D to the Nikon D3, both operating with a bit depth of 14. What he
calls inverse gain for both cameras at ISO 400 was 0.78 electron/ADU
for the Canon and 2.1 electrons/ADU for the Nikon. The Canon pixel
pitch is 5.7 microns and that for the Nikon is 8.2. The ratio of the
pixel areas is 1:2.22. If quantum efficiency, fill factor, etc are
similar for the two cameras, the Nikon should collect 2.22 times more
photoelectrons than the Canon at equivalent integration times, and
this should affect the gain proportionally. The actual gain ratio is
1:2.7, roughly as expected.

Can anyone explain Christian's analysis?

I think he made a mistake and did not factor in the 14bit RAW ADU of the D3, and underestimated the actual sensitivity of the D3 pixels by a factor of 4.

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