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Author Topic: what ISO and CPIA say about the "ISO" setting on a camera, and other measures  (Read 6921 times)


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For the D800, this minimum Ssat is 74, seen for camera EI setting of either 50 or 100 (so setting EI=50 use the same analog gain as EI=100, and just adjusts the light metering to choose twice the exposure level in auto modes, and probably adjusts default raw-to-JPEG conversion appropriately.)

If instead you mean sensitivity in the sense of "ISO speed", more closely resembling film speed measurement by been base on shadow handling and visible noise levels, then the official procedure is something like making a sequence of shots of gray cards at various exposure levels (adjusting illumination and or exposure duration), measuring the SNR of the output for each one, and finding the exposure level at which the SNR is 40:1. The result of that might surprise many people: for both an IQ280 and a D800, this will be overwhelmingly base an adequate photon counts, unrelated to the exposure index setting: it would be roughly the exposure level at which the photon count is 1600 (giving a shot noise count of 40, which overwhelms the dark noise in those sensors.)  At the base ISO-speed described above, the photon count is 12.7% of full well capacity, so:

The ISO speed of these sensors is roughly (base ISO speed) * (full well capacity * 0.127)/1600

Does anyone know the FWC for either of these sensors?  At a guess it is about 30,000 to 40,000, for which the ISO speed would be about 2.5 to 3 times the base ISO speeds that I estimate above.

BJL, thanks for an excellent analysis. I did some sensor analysis of the D800e using the methodology outlined by Roger Clark and derived a full well of 59,000 e-. Sensogren, using curve fitting from the DXO data, gave a value of 47972.


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