Testing my GH2's sensor

Started Feb 20, 2011 | Discussions thread
Kolen Senior Member • Posts: 1,015
Re: Testing my GH2's sensor

kenw wrote:

Kolen wrote:

By the way, I think I still don't understand it completely. jumping from ISO 600 to 12800 shows that its digital pull is not perfect.

Yeah, I can't figure out what is going on there. At first I thought it might be because by 12800 the SNR 18% curves were becoming read noise instead of photon shot noise limited. I just looked at the full SNR curves though and that doesn't appear to be the case, the entire SNR curve is about 6 dB down starting from saturation - makes no sense to me. I really don't understand what is going on at 12800 - either with the camera or DxO's measurement. If the DxO data is valid one should never use ISO 12800 and should just push ISO 6400 instead. If I have sometime maybe I'll do a test.

Actually Canon is infamous for that too so users are warned by the others not to use ISO 125, 250, etc.

So Canon had done some things differently. They weren't underexposing and multiplying, they were overexposing and dividing. This leads to lower dynamic range. A number of their cameras also have an unusual SNR behavior at lower ISOs. If I recall it is on account of two different read noise sources (before and after the VGA I think). That is also probably making the Canon case a bit different.

I think a video posts somewhere in this post shows similar behavior for GH1/GH2. So that could mean using the ISO200, 400, etc. to measure the noise in DxOMark might underestimate the sensor performance at native ISOs.
I might be wrong, though.

Possible, but the one "native" ISO they did measure (160) is exactly on the trend line for the 18% SNR and actually dips a bit on DR (which it probably should as things are getting read noise limited in that region). Looking at the full SNR curves the "fake" ISOs are behaving exactly as expected compared to 160, no sign of trouble. The outlier is 12800. I don't know what is going on there, whether it is the camera or something with DxO's measurement.

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Ken W
See plan in profile for equipment list

Thanks. I think Canon's ISO is this: ISO125, 250, etc. is pushed from ISO100 and 200, etc. And ISO50, 160, 320, etc. are pulled from ISO100, 200, 400, etc. But I am not sure exactly why Canon shows more noise at ISO125, 250, etc. I think it is because Canon's read noise is quite high, showing up a lot in low ISOs, so pushing at low ISO like ISO100 to 125 results in even more noise than ISO200.
There is a video posted a while ago in this post: http://vimeo.com/18580410

It shows that ISO 250, 500, 1000 and 2000 has more apparent noise. It actually behaves very Canon like that 125, 250, 500, etc. should be avoided. And this tempt me to conclude that 250 from GH2 is pushed from 200, which is not true according to the original post in the very beginning.
Anyway, there is so many mystery.

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