Testing my GH2's sensor

Started Feb 20, 2011 | Discussions thread
Kolen
Senior MemberPosts: 1,015Gear list
Like?
Re: Testing my GH2's sensor
In reply to EXR, Apr 22, 2011

EXR wrote:

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.

Thanks for interpreting the data. It is clearer. I think at least GH2 has much better control on banding noise then. This alone can compensate to any slightly lower score in the DxOMark.

I heard that Topaz Denoise has banding removal. I think I should try it out soon.

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

 Kolen's gear list:Kolen's gear list
Canon EOS 5D Mark II Sony Alpha NEX-C3 Nikon D800E Olympus PEN E-PL5 Canon EF 200mm f/2.8L II USM +32 more
Reply   Reply with quote   Complain
Post (hide subjects)Posted by
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark post MMy threads
Color scheme? Blue / Yellow