Tag Archives: speed

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NEX-FS700 Significantly reduced shutter when in Super Slow Mo!

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UPDATED WITH NEW FRAME GRABS FROM STROBE LIGHT AT BOTTOM.

One of the things that did concern me slightly about the FS700 was how would the sensor behave in Super slow Mo. The sensor is a CMOS sensor, so I expected it to exhibit rolling shutter artefacts, which it it does indeed do when in standard shooting modes and S&Q motion. It’s not bad, but you can make the pictures skew and when you try to shooting something like a spinning propellor you can get some weird effects, especially at higher shutter speeds. However when you switch the camera to Super Slow Mo the rolling shutter effects appear to go away. I was able to shoot propellors, do fast pans, shake the camera about etc and there was little sign of the usual rolling shutter artefacts.

FS700 25P 1/100

Just take a look at these two frame grabs. One shot done at 25P with a 1/100th shutter, the other done at 100fps with a 1/100th shutter, so in both cases the shutter speed is the same, so you would expect the rolling shutter artefacts to be the same, but clearly they are not. In standard mode the fan exhibits a typically lop sided, asymmetrical look and the fan blades appear curved, the upper and lower fan blade both bent towards the right of the frame. But in Super Slow Mo mode the fan blades are straighter and the fan is a lot more symmetrical with noticeably less bias towards the right, notice in particular the differences in the lower fan blade.

FS700 Super Slow Mo 1/100th shutter

You can tell the shutter periods are the same as the amount of motion blur and spreading of the fan blades is near identical, so it’s not a shutter speed difference, this is clearly a sensor scan difference. This is very interesting and requires further investigation as it suggests that the sensor read out process is different in the high speed mode. It is probably just a significantly faster scan rate, but it could also possibly be a global shutter of some kind. It’s just a shame that you can’t access this read out mode for normal shooting.

UPDATE:

FS700 Flash band at 25fps 1/100th shutter.
FS700 Flash band at 25fps 1/100th shutter.

Here are a couple more frame grabs done with the strobe focussing flash from a Canon DSLR. In both cases the shutter speed is 1/100th of a second so you would expect the width of the “Flash Band” to be the same. The narrower the band, the slower the sensors scan speed. These frame grabs suggest the scan speed is around twice as fast when in Super Slow Mo. It’s not a global shutter, but certainly a nice improvement. This is 100% repeatable.

FS700 Flash band in Super SlowMo 100fps, 1/100th shutter

You can take advantage of this for normal speed shooting by setting the camera to SSM and  recording the SDior HDMI feed to an external recorder.

Speculation: There is a little more aliasing when shooting in SSM. Is there some line slipping going on perhaps during SSM? This would allow a faster scan speed as fewer lines of pixels are read and thus might account for both the slight aliasing increase and the faster read out speed.

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Shutter, shutter speed and shutter angle.

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So you have your nice camcorder, EX1, EX3, AF100, F3 or whatever and it has a function called the shutter. This function may have different ways of being set, fractions of seconds or angle. What is it that the shutter does and what’s the difference between angle and fractions. Also why is it important to know what the frequency of the local mains electricity?

180 degree film shutter

Lets start by looking at the difference between shutter speed expressed in fractions of a second and shutter angle. Shutter angle comes from film camera days when the film cameras shutter was a simple spinning disc with one half of the disc cut away to allow the light to pass from the lens to the film. The other half of the disc would rotate around, blanking off the film so it could be advanced to the next frame. If you consider that a full circle is 360 degrees, then half of a full circle is 180 degrees. So for each frame cycle with a 180 degree shutter, light is allowed to pass from the lens to the film for half of the frame rate (180 being half of 360).

Taking a frame rate of 25 frames per second, each frame lasts 1/25th of a second. Half of that is 1/50th, so with  a 180 degree shutter the exposure at 25P is 1/50th of a second. There is no difference in the way the shutter works, it is just a different way of expressing the shutter timing.

Film camera 90 degree shutter

If we take that and look at a different angle, this time 90 degrees we can see from the picture that this is now one quarter of a full circle (90 is one quarter of 360 degrees). So at 25 frames per second the exposure is one quarter of 1/25th which is 1/100th and so on.

So why use angle instead of a fraction of a second? Well here’s the thing. If you set you shutter speed to 1/50th, then no matter what your frame rate the shutter speed will be 1/50th. The Sony EX and XDCAM cameras can shoot at various frame rates (as can many other cameras). It is traditional when shooting progressive, trying to create a filmic look to mimic the way a film camera behaves, so for this you would use a shutter that is open for half of the frame rate, ie. 180 degrees. When you set the shutter speed using an angle, when you change the frame rate the shutter speed will also change. Set to 180 degrees it will always be half of the frame rate. So going from 25P to 30P will change the shutter speed from 1/50th to 1/60th. Which neatly brings me on to the next bit….

Why it’s important to know the local mains frequency.

We normally take it for-granted in our home countries, shooting at our home frame rates that the pictures will be OK. But if you travel to a country where the mains frequency no longer matches the cameras base frequency then you may experience problems with flickering or strobing pictures when shooting under artificial lighting. Sometimes you will see light and dark bands slowly rolling up and down the picture. This happens because if you take your camera, lets say set to PAL (50i/25P) to the USA, the US mains frequency of 60hz will drift in and out of sync with the camera from one frame to the next. As many artificial lights brighten and dim in sync with the mains electricity you can appreciate that for one frame the lights may have one brightness and then the next frame the brightness may be different. You will possibly experience problems when the mains frequency cannot be evenly divided by the cameras shutter speed. For example shooting 30P will give problems when the mains is 50Hz as 30 will not divide evenly into 50.

So how do you counter this? Well you need to change your shutter speed to an even fraction or even multiplier of the mains frequency. So shooting 30P in a 50hz country you can use: 1/50th, 1/100th, 1/200th etc (mains frequency, frequency multiplied by 2, multiplied by 3 etc). Note that when shooting 60i you can’t normally have a 1/50th shutter so your limited to 1/100th or higher. When shooting 25P or (50i) in a 60Hz country you should use 1/60th, 1/120th, 1/240th etc. For 24P (23.98) you will often have to use the shutter when using consumer or industrial lighting using the same shutter speeds as give above, dependant on the local mains frequency.