Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
Jack Hogan
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Relative Average Absolute QE
In reply to Steen Bay, Mar 21, 2013

Steen Bay wrote: At high ISOs (like 2-3 stops above base ISO) the PRNU noise doesn't seem to affect the SNR near full saturation, and according to DxO's SNR 18% graphs the E-M5 has about 1 dB better SNR than the G3. Doesn't that indicate a a bigger difference in QE than 11%?

G3 at ISO 1600 and EM5 at ISO 3200 have DxO Ssats of 1481 and 1489 respectively, effectively the same value. DxO in fact measuresHsat and calculates Ssat according to the standard formula Ssat = 78/Hsat. So if we know that Ssats are the same, we also know that Hsat are the same. If Hsats are the same that means that 100% full scale in the DxO Full SNR Curves is the same luminous exposure, in this case 0.0525 lux-seconds - yet another jolly good reason to choose this ISO pairing for a useful comparison

Armed with that knowledge and the fact that the number of incoming photons per unit area-time is similar, in order to calculate relative Absolute QE you can read what signal each recorded and note the difference. Both cameras have equally sized sensors and the same number of sensels. So you do that by simply checking the ratio of the SNRs at the same % of full scale in an area of the Full SNR DxO curves where you suppose it's mostly shot noise. I measure a difference of about 0.484 dB, which corresponds to 0.162 Stops which means that with the same number of incoming photons, the G3 is recording about 89.4% of the electrons as the EM5.

Hence less than a 1/6 of a stop difference in Average Absolute QE.

Jack

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