Sunday, 16 February 2014

Shutter readout speed

The Panasonic GH3 has four different ways of starting and stopping an exposure. In this article, I will look at the properties of these shutter methods:

  1. Mechanical shutter
  2. Electronic shutter
  3. High speed electronic shutter
  4. Video frame readout



To examine the properties of these shutter methods, I made a rotor contraption in Lego Technic. The Lego power functions mini motor is geared up twice using 36t and 12t gears:



By video recording the motor running at 50 fps, I could count that the red connector rotates 13 times per second. The rotor is being geared up 3:1 after the red connector, so this means the rotor rotates 39 times per second.

To examine the shutter characteristics, I photographed the running rotor using these four methods:

Mechanical shutter, f/2, ISO 12800, 1/2000sElectronic shutter, f/2, ISO 1600, 1/640s
High speed electronic shutter, f/2, ISO 12800, 1/1300sVideo frame readout, f/2, ISO 6400, 1/1000s

To photograph these, I needed a fast lens that could focus close. So what better choice than the Olympus 50mm f/2 1:2 macro? It is a Four Thirds lens, but works well with an adapter. The only M4/3 camera to date which can autofocus it quickly is the Olympus E-M1, though.

Another lens choice for this application could have been the Leica 25mm f/1.4 with a macro extender ring.

1. Mechanical shutter


On the GH3, as with most other system cameras, the mechanical shutter is a pair of vertical travelling curtains. An exposure starts with the first curtain starts opening up. Then, after the desired shutter speed has passed, the second curtain starts closing the exposure.

If you have a quick shutter speed, the second curtain may well start closing before first curtain has opened fully, meaning that not the whole sensor is exposed at once, only parts of it. If so, the shutter speed is too quick for flash sync. Normal flash operation can only be done if the shutter is completely open as the flash fires.

The GH3 has a flash sync speed of 1/160s, which is not very good by today's standard. The GX7 has a newer shutter module with a flash sync speed of 1/320s, which is a good specification. Then again, people say that the shutter is rather loud.

This means that the shutter curtains of the GH3 move across the sensor in 1/160s, or probably a bit faster for some margin. I try to measure this by looking at the picture of the rotating propeller:



We know that the propeller is rotating 39 times per second. During the curtain movement, it has rotated about 60°, which is one sixth of a total rotation. Hence, the curtains open and shut in approximately 1/39*6s, i.e., 1/234s. There is of course quite some uncertainty here, but it is probably reasonable that the shutter is somewhat faster than the flash sync speed, 1/160s. So the result is certainly plausible.

The miniature Lumix GM1 has a brand new shutter module. It was designed to be very small, and also to be inaudible. Rather than having springed curtains, they are moved with electronic actuators, which is uncommon. The GM1 has a flash sync speed of 1/50s, which is unusually slow. This means that it is very difficult to use fill flash outdoors, for example, as the ambient light will be more dominating at 1/50s. 

2. Electronic shutter


On the GH3 the electronic shutter is limited to ISO 1600, and a maximum of 1 second shutter speed. This was upped to ISO 3200 for the GX7 and GM1. This feature is useful, however, the sequential sensor readout is slow. To see just how slow, let's look at the picture of the rotating propeller:



We see that a propeller blade crosses over 11 times while the sensor output is read row for row. As the rotor spins 39 times per second, this means that one single blade passes 117 times per second. Hence, the electronic shutter readout takes about 1/11s, according to this measurement. This is consistent with Panasonic material which states that the electronic shutter takes about 1/10s. So my method appears to be spot on.

The upcoming Panasonic GH4 is advertised to have a faster electronic shutter. However, don't hold your horses, it is not a significant improvement. Marketing material says that the electronic shutter runs "faster than 50ms", which is still only twice the speed of the GH3.

A sensor readout speed of 1/10s is quite slow. It can lead to annoying artefacts in your pictures if you are panning, or if you have moving objects. For example like this:



Read more about it here.

The Nikon 1 mirrorless cameras were designed with electronic shutter in mind from the start. The Nikon 1 S1 10MP camera has a 1/60s readout, six times faster than that of the Panasonic GH3. The Nikon 1 J3 14MP camera is even better, with an electronic shutter capable of reading the whole image during 1/80S.

Only the Nikon 1 V2 camera features a mechanical shutter at all, usable when you want to be sure the image comes out without any rolling shutter artefacts. Further, the camera can take full 14MP images at a staggering 60FPS rate using the electronic shutter, opening up for very interesting uses.

3. High speed electronic shutter


Just like the GH2, the GH3 is capable of taking a series of small size pictures at a high speed. You'll get 2336x1752 JPEG images at about 3MB each. The GH3 can take twenty of these per second.

Using the same method, I measure the readout speed of these. The rotor blade passes about 6.5 times during the exposure, indicating a readout speed of 1/18s. This explains why the GH3 can only do 20FPS in the high speed shutter mode, while the GH2 could do 40FPS.



Comparing with Nikon 1, again, so those cameras can take bursts of images at 60 FPS for one second. I.e., 60 full resolution images, with RAW capture, during one second. This is very useful for sports photographers who want to capture just the right moment. Very impressive performance from Nikon here.

4. Video frame readout


Recording video is of course related to the electronic shutter: The mechanical shutter is not used at all during video. That would be far too noisy, and is probably not feasible at all.



Looking at the same test again, it looks like the rotor blade passes two times during an exposure. This corresponds to a shutter speed of 1/59s, with 59=117/2. I'm guessing that the actual speed is 1/60s (or slightly faster), since the camera, in the NTSC version, can record 60 frames per second.

If this readout speed is slow, you'll get the famous "rolling shutter" effect. This is the "jello" effect you see when panning the camera. In this test, I compare the rolling shutter properties of the GH2 and GH3, and find them to be very similar. This is as expected: Since both cameras can record 60 FPS, they probably both have a sensor readout of around 1/60s anyway.

Here we have the reason why no Micro Four Thirds camera can record video at a highter framerate than 60 FPS (in NTSC markets): Reading the data from the sensor takes about 1/60s, so no faster framerate is possible. Again, the Nikon 1 cameras beat the M4/3 system here: Capable of up to 1.200 FPS video, albeit only at 320x120 pixel resolution.

Conclusion


ShutterGH3 approximate readout speedOther cameras
1. Mechanical shutter1/230sE-M1: faster than 1/320s.
GM1: faster than 1/50s.
2. Electronic shutter1/10sGH4: 1/20s.
Nikon 1 V2: 1/80s.
3. High speed electronic shutter1/20s (at 20 FPS, reduced resolution only)GH4: Probably 1/40s (at 40 FPS, reduced resolution only)
4. Video frame readout1/60sGH4: Probably 1/60s for 1080p, 1/30s for 4K video

There are still no Micro Four Thirds cameras which can photograph using an electronic shutter without any significant rolling shutter artefacts. The full sensor output still takes around 1/10s to 1/20s (depending on the model), which is too slow for safe use with moving objects. The Nikon 1 series is far ahead in this respect, having a full sensor readout in 1/80s, pretty much eliminating the need for the mechanical shutter.




5 comments:

  1. Bra jobba! Lærerik artikkel : )

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  2. But if I understood it correctly,the new GH4 does 40fps at full resolution when using the electronic shutter, so sensor readout time must be faster than 50ms...

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    Replies
    1. The GH4 can photograph 40fps, as you say, but not at a full resolution. Just like the GH3, the high speed shutter only works at a limited resolution.

      The maximum FPS at full resolution with the GH4 is 12. Not 40.

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    2. Thanks. Many bloggers are stating that the camera is actually capable of 40fps full resolution pictures, but I was unable to find a reliable (official) source stating that. But Bing's translation of Panasonic's Japanese webpage *seems* to confirm your information:

      http://www.microsofttranslator.com/bv.aspx?from=&to=es&a=http%3A%2F%2Fpanasonic.jp%2Fdc%2Fgh4%2Fmobility.html

      Having said that, I don't know how can you be sure the datasheet you link on your GH4 blog entry is about the sensor on the GH4 and not the one on the GX7...

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    3. Thanks for your well thought out test.You must be the rocket scientist I heard about(no sarcasm intended)

      Delete