How does the D3 achieve such high ISO?

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

Marianne Oelund wrote:

ejmartin wrote:

Marianne, where are you getting your figure of 22K electrons at
saturation for the 1D3 at ISO 200? Mine has 71K electrons at
saturation for ISO 100 and about 38K at ISO 200. 22K seems awfully
low, more like the value for the 1D2.

Can you reconcile your values to Roger Clark's 12-bit unity gain ISO
value of 1000 for the Mk III? That would imply 20,500 electrons
full-scale at ISO 200.

That has as its source the same gain measurement. "Unity gain ISO" is simply the gain times the ISO at which it is measured, normalized to 12-bit ADU's in Clark's conventions. So Clark's original 2.46 electron/14-bit ADU at ISO 100, is about 1000 for the 12-bit normalized unity gain ISO. Now with 4.8 electrons/14-bit ADU at ISO 100, his unity gain ISO is around 1900.

That translates into about 37K electrons at ISO 200 raw saturation.

As for the change in Clark's data, he had measured the gain at ISO
50, not realizing that ISO 50 is really ISO 100 overexposed by a
stop. So values for gain above ISO 50 are double what he intially
reported, and read noise in electrons consequently also doubled for
all ISO's above 50. (There was clearly a problem, since if you took
the original figures seriously, the 1D3 had the worst photon
collection efficiency of any Canon DSLR since before the 10D).

Thanks for the explanation - I was wondering. Has this also caused
an error in his unity-gain ISO calculation?
If the Mk III unity-gain ISO is actually closer to 2000, then the D3
and Mk III would be back to rough parity regarding read noise, with
the D3 retaining an advantage in shot noise.

Indeed, that's how I see it. The D3 has an advantage of about 60% in per-pixel photon collection efficiency over the 1D3, though that becomes about 15% when referred to a per-area basis. The 1Ds3 and 1D3 have comparable efficiencies on a per-area basis.

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