Testing my GH2's sensor

Started Feb 20, 2011 | Discussions thread
EXR Senior Member • Posts: 1,437
Re: Testing my GH2's sensor

Kolen wrote:

Hi, from your result, only 160, 320, 640, etc. are native ISOs. I am thinking if DxOmark will then underestimated the sensor performance since they measure the performance at 160, 200, 400, etc.

This shouldn´t make any difference. DXO measures noise, or to be more accurate signal-noise-ratio.

Amplifying a signal will of course decrease SNR, but it shouldn´t matter if you do it analogue or digital. It is rather the opposite, each analogue device will potentially increase noise, so an analogue amplifier could increase noise.

A digital multiplication (which is the equivalent of an amplification) will never increase noise.

When we analyse the DXO-data a bit closer beyond just reading the numbers in front we can see a few things.

Generally the GH1 is better per pixel, which is to be expected with fewer pixels on the same area.

Resolution-normalized both sensors show almost exactly the same performance in most areas.

But there are some interesting things to see.

An ideal sensor should show a 3dB difference of SNR when you increase/decrease ISO by 1 stop.

When we look at the GH1 sensor it matches almost perfectly with the theory, with one exception. ISO 800 is better by almost 1dB (equivalent to 1/3 stop) then expected. But this path doesn´t continue, ISO 1600 and 3200 are about in line with the other ISOs. Looking at the full SNR-data we see that the brightest areas show indeed the expected 3dB difference between ISO400 and 800. The 2 curves get closer at lower grey-values.

This would indicate lower read-noise for ISO800, although there is no plausible explanation why this should happen suddenly and only at ISO800. (and the curves start to get closer at 50% grey, which is a bit to much to be just read-noise)

Anyway, this "extraordinary" ISO800-performance is mainly responsible for the "victory" of the GH1 in the low-light area.

Lets take a look at the GH2.

The first thing we can see is, that we have just 2,7dB difference between ISO200 and 400, a bit lower than expected. This indicates somewhat higher read-noise for the GH2.

Looking at the graphs for the Full SNR-data of both cameras we can see the lines of the GH1 stay straight a bit longer in the area below 1% Grey.

This backs up our theory, that the GH2 indeed has a very slightly, but measurable, higher readout-noise throughout the ISO-range. (possibly a sight-effect of the higher readout-speed)

We also see the GH2 is missing the lowest ISO, starting at 160 instead of 100 (130 in reality). This is mainly responsible for the higher DR-score of the GH1.

We also see a slightly disappointing performance of the GH2 at the highest ISOs.

Between ISO 6400 and 12800 it loses almost 6dB in SNR, which is almost twice as much as expected. I don´t know any useful explanation for this behavior.

In the end the 2 sensors seem to use very similar technology and the difference seems to be very small. Maybe the just had a "good" sample of a GH1 and a rather "bad" sample of the GH2 for the test.

The pure sensor-quality seems to be more or less equivalent between those 2 cameras.

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