Tuesday, 9 September 2014

Worse image quality with electronic shutter?

Recently, an interesting Panasonic sensor data sheet came up. It is believed to describe the sensor sitting in, e.g., the Lumix GX7 and GM1.

The data sheet says that there are two options for electronic shutter readout: 12 bits in 1/15s and 10 bits in 1/22.5s. It has been demonstrated that the Lumix GX7 has a readout speed of 1/15s, and I have measured the speed of the electronic shutter in the GM1 to around 1/25s.

Do recent Panasonic cameras use the faster 10 bit readout, and is it affecting the image quality when using the electronic shutter mode?



To test this, I have taken the same picture using the Lumix GH3, Lumix GH4 and Lumix GM1, all using both the mechanical and electronic shutter modes. The photos were underexposed by two stops, to make the rendering of the shadows more challenging.



All the pictures were taken at ISO 200, the base ISO for these cameras:

Mechanical shutter
Electronic shutter
Lumix GH3
Raw image file
Raw image file

Lumix GH4
Raw image file
Raw image file
Lumix GM1
Raw image file
Raw image file

Looking at the JPEG output images above, there is little difference to find, beyond some unintended differences in the exposure. However, when I process the RAW files in Silkypix, increasing the brightness by three stops, I find some difference in the shadows:


What we see here is that the Lumix GH3 image quality is pretty much comparable when using the mechanical and electronic shutter. However, when using the electronic shutter with the Lumix GH4, you lose some effective dynamic range. There is more noise in the shadow areas. The same goes for the Lumix GM1.

This is because the GH4 and GM1 are designed to use a 10 bit output in electronic shutter mode, to speed up the sensor readout, rather than 12 bits with the normal shutter. Less bit depth effectively means less dynamic range, and more noise in the shadow areas.

So is this a problem?

Keep in mind that the pictures above were deliberately underexposed by two stops, which is a lot. Here is what I get when I don't adjust the exposure.

Mechanical shutter
Electronic shutter
Lumix GH3

Lumix GH4
Lumix GM1

Here are the 100% crops from the same area, after applying a one stop exposure increase in Silkypix:


As you see, when you get the exposure fairly right, there is no difference between using the mechanical and electronic shutter. Increasing the exposure by one stop in post processing using the RAW file is no problem with the electronic shutter mode.

Speed of E-shutter readout


The GH3 electronic shutter had a readout speed of 1/10s, which is very slow. This leads to significant rolling shutter artifacts, that you can read about here. How does the GH4 compare?

One way to test the speed of the electronic shutter is to take a photo at a fast shutter speed in artificial light. For about a century or so, people have been using incandescent light bulbs for electronic indoor lightning. Even when used on alternating current (AC), the light is stable. Since the filament is heated, it emits light also when the alternating current is at zero.

However, traditional incandescent light bulbs are now being replaced with the energy saving fluorescent light bulbs. They tend to flicker at 100Hz (in Europe) or at 120Hz (in the US). The lights don't flicker at 50Hz and 60Hz, as you might expect. This is since during each period, the electrical current reaches two peaks, see the illustration below:



Here are images taken at ISO 1600, 1/400s with both cameras:

Lumix GH3

These results are quite easy to explain. With the Lumix GH4, I get about 3.3 stripes horizontally. Each stripe corresponds to 1/100s, hence, the total exposure takes about 1/30s.

The Lumix GH3 has a much slower electronic shutter. There are ten stripes in the image, which means that the exposure takes 1/10s. This causes a number of problems, making the electronic shutter mode fairly useless.

Here is a summary of recent cameras:

CameraElectronic shutter readout speedBit depth
Lumix GH31/10s12 bits
Lumix GX71/15s12 bits
Lumix GM11/22.5s
10 bits
Lumix GH41/30s10 bits

Panasonic has made the electronic shutters faster, but at the expense of the bit depth. This could cause somewhat worse dynamic range when using the electronic shutter.

Conclusion


In recent Lumix cameras, Panasonic have sacrificed some bit depth over speed when designing the electronic shutter. I think this makes sense, as the slow shutter readout caused big rolling shutter problems with the first generation electronic shutter, as implemented in the Lumix G5, G6 and GH3.

If you need to lift the shadows a lot, the newer implementation using the shallower bit depth is going to give you more noise. However, with most real world use, this is not a problem at all. A one stop increase is no problem at all, but if you need an extreme three stop increase, then you will see a difference.





2 comments:

  1. Great article, as usual.
    Off-topic question, why do you color in the "lumix" letters?

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    Replies
    1. I do it to make the cameras look more stealthy.

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