# Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
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Re: Correction: G3 vs EM5 - Estimates of Ratios of Light Exposure
G3 at ISO=1600 and EM5 at ISO=3200

(For the G3 relative to the EM5), the ratio of the Average RAW channel levels divided by the ratio of the DxOMark "Saturation ISO" ratings is 1.494 EV, but when receiving -0.322 EV less light (due to it's higher Shutter Speed), and transducing -0.236 EV less electrons due to it's lower QE. So, the G3/EM5 ratio of the amount of light received in that case seems to be around:

(For the G3 relative to the EM5), the ratio of the Average RAW channel levels divided by the ratio of the DxOMark "Saturation ISO" ratings is 1.494 EV, but when receiving 0.322 EV more light (due to it's lower Shutter Speed), and transducing -0.236 EV less electrons due to it's lower QE. So, the G3/EM5 ratio of the amount of light received in that case seems to be around:

+ 1.49 - 0.322 - 0.236 = + 0.9358 EV (91.289% more light)

DM, I don't see any squaring of SNRs You can't read off mean values and relate them to electrons/photons/exposure because you do not know each cameras' gain.The only way to get back to relative electrons/photons/exposure is through SNR when most of the noise is shot, which is what I did.

That's interesting, Jack. I do follow your procedure in the quoted text below. So, you are saying that since the ratio between each camera's ADU/Photon gain may be different, then simply dividing the ratio of Average RAW levels by the ratio of DxOMarks "Saturation ISOs" is not sufficient. That seems worthy of consideration. I wonder what Anders thinks about that. Will be interesting to hear from him.

Also, I calculated the difference in QE at these ISOs to be less than 1/6 of a stop, which correlates relatively well with Sensorgen's which you used. On the other hand, I believe mine is more precise because I assume a blackbody radiator at 5000k as the light source, while Bob assumes a green laser

How did you calculate the QE then ? Seems like it would be a factor in the ADU/Photon gain ? The SNR varies by the square-root of QE multiplied together with the number of Photons. How would one be able to separate-out the QE from the number of Photons ? Your stated 11% (below) is quite a bit less than the 17.78% difference that Bob's Senorgen data indicates. How do you relate it to a black-body radiator (as opposed to a monochromatic Green wavelength) ?

QUOTED FROM JACK'S PREVIOUS POST:

... you have confirmed the relative saturation ISOs, but that does not mean that you can use mean raw values directly to estimate the signal (alias exposure, alias the number of photons that hit the sensors). To do that you need good ol' SNR, around a mean of such a level that the vast majority of the N is shot noise. At ISO 3200 and 1600 respectively for our two little cameras that means at around 25-50% of full scale. We find just such a compliant little area in DPR's RAW captures under the letter A of the Kodak Gray Scale. We find that the EM5 at in-camera ISO 3200 has an SNR of 19.39 and the G3 at ISO 1600 has an SNR of 21.04. Squaring these values will give us the number of electrons output by each sensor. That is 376 and 443. Taking into consideration the G3's 11% lower Absolute Quantum Efficiency we can calculate that for the two captures in question the G3 received 0.41 stops more photons,/exposure/signal than the EM5.

So in DPR's test, the one I referred to with the images earlier, the G3 at in-camera ISO 1600 received 0.41 stops more light than the EM5 at ISO 3200, not twice the signal or 1 stop as you suggest. This is fully explained by its 0.32 stop slower shutter speed (1/640 vs 1/800 and possible inaccuracies in the system), as mentioned in my post with the pictures. Since the lens appears to be the same, both cameras were illuminated by the exact same light, within +0.1 of a dB, good job DPR.

All this to say that when these two captures were taken the cameras were receiving virtually the same light, meeting the "same signal, exposure, number of incident photons requirement", within the allowed 1/3 of a stop difference fully accounted for by the different shutter speeds.

So both cameras, the EM5 at ISO3200 and the G3 at ISO1600, were illuminated by the same light, were subject to the same exposure, and would reach saturation at the same absolute luminous exposure, with the usual 1/3 of a stop proviso. A very good match indeed, and imho the most useful of the ones available for the G3 to compare SNR performance with the EM5 at ISO 3200.

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