Wednesday, 12 March 2014

GM1 shutters

The Lumix GH3 was among the first to feature electronic shutter. However, since the shutter reads the image sequentially row for row rather slowly, causing a number of artefacts, the feature is not very useful.

The Lumix GM1 also has the electronic shutter, of course, but is it any different? In this article I will look into it.



Speed of shutter curtains


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:



To understand how this is related to the speed of the shutter curtains, consider the illustration below. It shows what happens when the mechanical curtain shutter is engaged. Let's say, as an example, that the curtain shutters move across the sensor in 1/100s, and that the exposure is 1/500s. It would go like this:


1. As the exposure commences, the first shutter curtain starts moving downwards, exposing the sensor.

2. After the duration of the shutter speed has expired, the second shutter starts moving downwards. In my example, the shutter curtains travel across in 1/100s, so the shutter speed of 1/500s corresponds to 1/5 total opening.

3-6. Both shutter curtains continue downwards, exposing each row of the sensor for a total of 1/500s.

7. As the second curtain shutter closes, the exposure is finalised.

If the scene is stable during this operation, there are no issues. However, if the subject moves horizontally, it will be skewed. Or if the light flickers, you will get horizontal stripes.

The electronic shutter works in the same way. There are no mechanical curtains with the electronic shutter, of course. But you could visualize the process in the same way.

The first shutter opening corresponds to the first sensor rows initializing, getting ready for collecting the exposure. And the second shutter corresponds to stopping the exposure, row for row.

The test


Since I am in Europe, my lights flicker at 100Hz. Here are some images taken in this light:

CameraElectronic shutterMechanical shutter
Lumix GM1

ISO 25600, f/2.8, 1/640s

ISO 25600, f/2.8, 1/640s
Lumix GH3

ISO 1600, f/2.8, 1/640s


ISO 12800, f/2.8, 1/200s

These results are quite easy to explain. With the Lumix GM1, I get four stripes horizontally. Each stripe corresponds to 1/100s, hence, the total exposure takes 1/25s.

The mechanical shutter is faster, and there appears to be one light cycle during the exposure, corresponding to a shutter curtain speed of 1/100s.

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.

The mechanical shutter of the GH3 is of course a lot faster. In this article, I have measured the shutter curtain speed to be around 1/230s.

Here are the results:

CameraElectronic shutterMechanical shutter
Lumix GM11/25s
1/100s
Lumix GH31/10s1/230s
Lumix GX71/15s1/500s?
Lumix GH41/25s?1/500s?

Consequences


So what does this mean? First of all, using the electronic shutter can cause various artefacts. If you take pictures indoor in fluorescent light, you could risk getting stripes. To avoid them, use a shutter speed which corresponds to the flicker frequency. If you are in an NTSC country, like the US, set the shutter speed to 1/120s, 1/60s or 1/30s, or slower. That should be safe. In a PAL country, use 1/100s, 1/50s, 1/25s, or slower.

If you have moving subjects, you could also risk getting skewed pictures with the electronic shutter. Even the mechanical GM1 shutter is slower than usual, and could give you slightly distorted images. Here are a couple of examples. The vehicles are moving at approximately the same speed:

Electronic shutter Mechanical shutter
ISO 800, 28mm, f/5.6, 1/500s ISO 400, 28mm, f/5.6, 1/250s

As you see, both shutters are prone to distorting moving objects, but the mechanical shutter is generally the most safe.

In addition, the relatively slowly moving shutter curtains means that the flash sync speed is rather poor. You can use a max shutter speed of 1/50s while using the flash. This limits the possibility to use fill flash in combination with a large aperture outdoors. You would typically miss this if you want to take a backlit portrait outdoors using a large aperture lens, like the Lumix G 20mm f/1.7. Otherwise, a max flash sync speed of 1/50s is not that much of a problem.

Image quality


An interesting Panasonic sensor data sheet has popped up. We don't know for sure but it is widely believed that this sheet describes the sensor sitting in the Lumix GX7 and GM1. Some also speculate that it will be in the upcoming Lumix GH4.

The sensor sheet states that a full scan of the sensor can be done in 1/14.7s (12 bits) or 1/22.5s (10 bits). The latter is consistent with my measurement in this article. This finding leads us to think that the image quality may suffer when using the electronic shutter mode. After all, you only get 10 bits readout from the sensor, rather than 12 bits (presumably) with the mechanical shutter.

To test this, I have taken the same image with the electronic and mechanical shutters. All the images were taken at ISO 400, 1s, f/4:

CameraElectronic shutterMechanical shutter
Lumix GM1
Lumix GH3

Are there any differences? Hard to say with these images. We can take a closer look at the shadow areas. I developed the RAW images in Silkypix, and lifted the shadows 3 stops. Click to enlarge the picture:


Based on this test, it is actually quite clear to me that there is more noise in the shadows in the images taken with the electronic shutter, for both cameras.

But keep in mind that this was a pretty extreme example: I did lift the exposure by three stops, which one would certainly not do often.

What this means in practice, is that using the electronic shutter probably limits the effective dynamic range. This could matter when you have both dark and bright areas in the frame at once: Using the electronic shutter could limit your possibility to see details in the shadows. So if the dynamic range of the scene is wide, don't use the electronic shutter.

Other cameras


The Lumix GH3, G5 and G6 all have the slower electronic shutter, reading the sensor at approx 1/10s.

Newer cameras like the Lumix GX7 (1/15s) and GM1 (1/25s) have the improved, faster electronic shutter. The GX7 also has a faster mechanical shutter unit, capable of 1/360s flash sync.

The upcoming Lumix GH4 is said in Panasonic marketing material to have an electronic shutter capable of "faster than 50ms readout", which corresponds to faster than 1/20s, i.e., consistent with my GM1 finding above. It could very well be that these cameras share electronic shutter technology.

So far, no Olympus cameras have the silent, electronic shutter option.

Conclusion


The electronic shutter of the Lumix GM1 has improved a lot, making it much more useful. The risk of getting serious distortions due to the rolling shutter effect is much smaller with the GM1.

There are indications that the dynamic range is more limited in the electronic shutter mode, due to only reading 10 bits of colour information from the sensor, as opposed to 12 bits when using the mechanical shutter.

Panasonic chose to optimize the GM1 mechanical shutter for size, silence, and less vibrations. Mostly, this is a good thing. There are some limitations in terms of what kind of flash images you can take. And you can risk some rolling shutter artefacts with fast moving objects. Otherwise, what you get is a very silent shutter with little risk of shutter shock.

All in all, I think Panasonic have struck a nice balance here. When people see the Lumix GM1, they expect a light and stealthy camera, not a speed monster. People who are interested in a sports camera would not buy the GM1 anyway, they would probably be better off with a GH3 or a G6.

Further reading


Image quality comparison between the GM1 and GH3.

Autofocus during video comparison between the GM1 and GH3.



12 comments:

  1. How is it possible to shut a burst of 39 images at 40fps in super high speed mode if each exposure takes 1/25s?

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  2. Super high speed mode pictures are at reduced resolution, not full 16MP.

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  3. I can't see the connection between resolution and the number of exposures per second. The FOV is still the same and the shutter curtains have to travel 40 times per second from top to bottom.

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    Replies
    1. Consecutive frames can overlap with electronic shutter. Frame N doesn't have to finish acquisition when Frame N+1 starts. It's like multiple rolling shutters.

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    2. The shutter curtains don't move in the high speed mode. In this mode, you can only use the electronic shutter.

      The resolution is related to the frames per second, since reading out the sensor pixels is what takes time. So scanning the sensor more quickly can only be done in reduced resolution mode.

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    3. Terrifically well reasoned and very useful. Thanks!

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  4. Really interesting.
    I shoot street a lot, the quality of my camera's image is a secondary consideration, as compared to speed of operation and the ability to 'set up and go'. That said I would always like a better, more detailed sharper image. I know Winogrand always shot at 1/1000 of a second, personally I try to stick around 1/640.

    I've often tried to shoot with my phone cam, based on the 'good enough' and the 'the camera you always have with you' model. But due to the effect of people sliding off and through the image, like the above examples of traffic, found it impossible.

    I used a compact, of course electronic shuttered, for a while with good success, far more so than my DSLR, but the files lacked headroom. I've switched to a Nikon J1, which works OK for me, but would like the pop of being able to use some DOF. Firstly, having read your piece above, I don't get why my J1 doesn't suffer the same artifacts as my phone cam... or that you show the GM1having? Or, if I did switch to the GM1, could I expect it to perform similarly (shutter speed wise) to my J1?

    I've gone m43 before, but the camera I used has just a little 'clunky' for my tastes. The GM1 certainly looks a better fit.

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    Replies
    1. When it comes to electronic shutter, the Nikon 1 series is much further ahead than M4/3. The Nikon 1 series have an electronic shutter readout speed of about 1/80s, much faster than the speed of 1/23s (Lumix GM1), or even 1/30s (Lumix GH4).

      Be sure to not confuse the shutter readout speed with the shutter speed. They are independent. The shutter readout speed says how fast the sensor can output the values electronically, row for row. With a readout speed of 1/80s, the first rows are exposed 1/80s before the last rows, regardless of the shutter speed. A slow readout speed causes rolling shutter effects.

      If you want to continue to use the electronic, silent shutter (like the Nikon 1 J1), then stick with the system. I am using the Nikon 1 V3 myself, and the electronic shutter is very good, I rely on it almost all the time.

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    2. Thanks. So even shutter speed isn't just shutter speed. Tsk. I'm surprised that given that line by line the shutter speed tops out at 1/80, I've never had rolling shutter. I've certainly noticed the banding with fluorescent lights. I've had some amazing effects when I shot a band who were using strobe lights.

      I just can't find a compelling reason to drop my J1, despite really wanting a bit more resolution for when I want to crop.
      Cheers again, Brandon

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    3. I have the J1 myself, and I think it is a good camera. The image quality is great!

      I used the J1 for all the images in this review.. I think it performs well!

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  5. Quite sometime later I find myself back here, looking up gm1 rolling shutter...
    ...because I now have the GM1. I found a reason to upgrade when I tripped over and smashed the lens back into the body of my J1 and the insurance cashed out.
    The truth is, real world; once in a while I get rolling shutter, but infrequently and it is relatively easy to avoid, with a modicum of care. However the Nikon AF rains on the Lumix, from a great hight. The J1's AF was spookily preternatural. The Pansonic likes nothing better than to latch onto your subjects background and not let go. Back to zone focus we go.

    The greater truth is, my old camera was just fine and I'd happily recommend the 1 system. It seems odd to me that amongst 'camera guys', it's the system that dare not speak its name. I do wonder if when (inevitably) the J5 drops to peanut money, I'll find house room for it.

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    Replies
    1. I think the Nikon 1 system has some uses. I use it myself, and it is nothing to be embarrassed about. Large sensor cameras are very hip at the moment, and some may frown upon the 1 inch sensor. But I wouldn't worry about it.

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