Sunday, 15 September 2013

180 degree shutter

One could say that video is just like photos, the only exception is that there is a series of still images composing the video stream. This works well for central concepts like focus, selective focus and background blur (bokeh), sharpness and so on.

However, there are some important differences too. One is focus pulling, to change the focus distance during video recording.

Another important video concept is the 180° shutter. Put shortly, this means that the shutter speed is twice that of the frame rate. So if you are in an NTSC country, e.g., USA, you would set the shutter speed to 1/60s when recording at 30 frames per second.

The name "180 degree shutter" comes from the construction of early film cameras. The shutter in these cameras was simply a rotating disc, with an opening that exposed each film frame. When using a disc with a 180° opening, i.e., a half disc, the film would be exposed half the time.

Why 180° shutter?


The point of the 180° shutter is to create motion blur. When you have movement in the scene, you'll want the movement to be slightly blurred. Using a faster shutter speed would yield stuttered motion, where the moving objects appear to be in different spots in each frame.



Here is an example, where you can compare footage recorded at 1/50s (corresponding to 180 degree shutter in PAL countries with 25 frames per second) with footage recorded at 1/400s:



Here is another example:



You see from the example above that the video recorded with a 180° shutter has more fluid motions, and generally looks more pleasing.

How to get 180° shutter?


Here's the thing: Controlling exposure parameters during video recording is not easy, and not always possible, with any Micro Four Thirds camera. If you start the video recording by pressing the red video button, the camera generally does its best to use the exposure parameters it thinks are best.

Generally, in my experience, this means using the lens wide open, unless it is a very bright lens and there is a lot of light. So if you use a kit zoom lens on a sunny day, starting the video recording with the red button will use exposure parameters like f/5.6, 1/1000s, ISO 200. This is very far from the 180° shutter, much too fast shutter speed.

Panasonic Lumix GH camera series


With the Panasonic GH camera series, e.g., the GH2 or GH3, it is quite easy to achieve a 180° shutter. You can use the "Creative Movie Mode", indicated by an "M" and a film camera symbol. In this mode, you can choose the S exposure mode, allowing you to set the shutter speed:

GH3 mode dial set to "Creative Movie Mode"Inside the creative movie mode, set the S exposure mode (top left corner) to be able to set the shutter speed for video recording

A very good news is that the smaller and cheaper Panasonic Lumix G6 also supports this manual control over video exposure parameters, unlike the predecessor G5. This is a good development: The G6 is a good alternative to the GH3 for those who want a more compact and less expensive camera, while retaining most of the features. It also has a sensible design, with a large and ergonomic grip, a tiltable LCD screen for easy video recording, and a high resolution electronic viewfinder (EVF).

Disadvantage of Panasonic cameras


Even if the GH series of cameras from Panasonic are supposedly video optimized, they have a big disadvantage in my opinion: The exposure information is never displayed during video recording. I'm often interested in seeing and controlling the exposure parameters like the ISO, shutter speed and aperture, but they are never displayed in any of the video modes.

The only exception is the fully manual exposure mode available in the "Creative Movie Mode". Here, you set the ISO, shutter speed and aperture manually, and they are shown in the display. However, in this case the camera will not adapt the exposure if the lightning changes, and this is mostly usable for professional studio use.

Even the most basic NEX camera from Sony, the Sony NEX-3N, displays the video exposure parameters during video recording.

Sony NEX-3N displays exposure parameters during video recording

You still cannot control these parameters with the Sony NEX-3N, though, so it is not all well.

When to not use 180° shutter?


If you are recording video footage solely to later go through it and grab single frames for still image use, then the 180 degree shutter does not make sense. In this case, you will often want each frame to be as sharp as possible, i.e., set a fast shutter speed.

Using ND filters


You may have noticed that pro video camcorders, like the Panasonic AG-AF100 have built in ND filters. But why?

The reason is that you'll want to combine both selective focus (background blur, bokeh) and a 180° shutter at the same time. To be able to set a shutter speed of 1/50s or 1/60s on a sunny day, you will often see the need to set a small aperture. This gives you "infinite focus", not always what you want. So using an ND filter, you can set both a large aperture and a slow shutter speed at the same time.

If you are using the Lumix X 12-35mm f/2.8 lens, you may need an ND8 filter to be able to use the lens for video recording on sunny days, to get both selective focus and a 180° shutter.

Conclusion


Even if all Micro Four Thirds cameras support video recording nowadays, not all feature the manual controls you need to be able to record video with a 180 degree shutter. For the best controls, get a GH2 or GH3. But even these cameras don't show you the exposure settings used when in automatic exposure modes, a strange and annoying limitation.

It is strange that the cameras don't help you to achieve a 180 degree shutter, given that this is a central concept for video production. I think Panasonic and Olympus should make the video controls better and more easy to use.

You don't need to strictly set exactly a 180 degree shutter, of course. If there is low light, for example, you could set something like 1/30s to avoid pushing the ISO too high.






2 comments:

  1. There's also a pretty nifty feature hidden away in the Motion Picture menu of most of the Pany cameras called "Flkr Decrease". It allows you to preselect a predetermined shutter speed, which the camera will lock in if you hit record in anything other than Manual Movie Mode.

    For example, even though the G5 doesn't allow for manual control during movie recording, you can stipulate that it uses a shutter of either 1/50, 1/60, 1/100, or 1/120th. Obviously intended as a means of avoiding the banding and flickering issues caused by lights, it's also a handy way to ensure 180 degree shutter if that's of importance.

    I leave it turned on in the GH3, even though I almost exclusively shoot in Manual Movie Mode. It means that if something crops up unexpectedly you can just hit the record button and the camera with faithfully use a 180 degree shutter. The obvious trade-off being that it's manipulating aperture and ISO to maintain correct exposure (or possibly under/over exposing if prevailing lighting conditions are too extreme).

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    1. Thanks, this is very useful information! I had no idea such a mode existed. Too bad the camera still doesn't report which shutter speed it uses, so that you can verify that you are getting a 180 degree shutter.

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