GM1 E. Shutter Lower DR (10 bit)

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kenw
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GM1 E. Shutter Lower DR (10 bit)
4 months ago

In some other threads it has been mentioned the GH4 sensor appears to do 10-bit instead of 12-bit read out for its electronic shutter.  In another thread the question was posed what does the GM1 electronic shutter do.

I ran a quick test taking a shot of the same scene at ISO200 with identical exposures but shooting once with the mechanical shutter and once with the electronic shutter.  A shadow region was then strongly pushed in post processing and I also plotted the RAW histograms to see if 12 or 10 bits were being used in each case.

The results are very clear, the GM1 when using electronic shutter is only 10 bit and there is a definite degradation to DR at base ISO when using the electronic shutter.  Below see images (100% crops of a deep shadow region pushed five stops) and RAW histograms (just the lowest ADC counts).  Clearly more shadow noise in the images and the histograms clearly show the electronic shutter is using 10 bits instead of 12.

This is a bit of a bummer because above 1/500 shutter speed only the electronic shutter can be used which means in bright harsh lighting where you might be tempted to push a deep shadow you will get lower DR.  The trade off here appears to have been the increased read out speed of the electronic shutter compared to previous cameras.  Rather than taking about 1/10 second for the electronic shutter to scan the entire imager in the GM1 (and GH4 apparently) it is closer to 1/25.  This is a good thing since it reduces rolling shutter artifacts.  However, reading an imager faster also usually results in a noise trade off.  This is rather clear from the images below.

Just to head off any "the sky is falling" or "give us our bits back" posts...

At higher ISOs where shadow noise really begins to impact image quality on a routine basis in a big way this is likely less of an issue.  There is so much other noise contributed from other sources that the 10 bit read out is probably just fine.  I doubt I'll take the time to test this but anyone else can easily (take identical images with mechanical and electronic shutter at high ISO, do some shadow pushing and compare).  So likely the only real issue here is if you take low ISO shots and try to make full use of the DR by pushing shadows by quite a bit.

As to the missing two bits there is a very good reason we don't have them.  It is quite likely the faster electronic shutter read out already generates enough noise that two extra bits would do no good.  I won't go into the gory details of ADC design and noise trade offs with read out speed, but it is likely that with careful noise analysis it would be evident that the extra noise we see in the shadows below is not the result of two bits being thrown away (quantization noise, which would be uniform distribution) but rather the result of noise in the ADC front end when running at that read out speed (which would likely be Gaussian distributed instead).

Electronic Shutter

Mechanical Shutter

Electronic Shutter

Mechanical Shutter

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Ken W
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