Tag Archives: shutter

cinematography

Why doesn’t every camera have a global shutter?

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Global shutter cameras are not a new thing. They have been around for a very long time.  The Sony Z750 is 2.5 years old and has had a global shutter since day 1. There are also the HDC-3200 and F5500 4K global shutter cameras.
Screenshot-2023-11-15-at-11.46.52-600x435 Why doesn't every camera have a global shutter?
Global Shutter Sony PMW-F55
 
The PMW-F55 had a global shutter and CCD cameras had global shutters.

And now Sony have announced the new A9 MKIII stills camera that also has a global shutter:
 
Screenshot-2023-11-15-at-12.09.54-600x419 Why doesn't every camera have a global shutter?
Sony’s new A9 III has a global shutter.


So, given that it’s not really a new thing – why doesn’t every camera have a global shutter?

The main reason is noise – and in particular fixed pattern noise that will show up in blacks and deep shadow areas if you try to lift the shadows or use high levels of gain. With a global shutter the signal from every pixel is globally shifted into a memory cell at the end of each exposure period and then those memory cells are read out while the next frame is being capture. Each memory cell will have a slightly different very tiny signal offset and as the arrangement of the memory cells never changes these offsets get added to the signal and appear in the output as a fixed noise pattern. It can be harder to eliminate this fixed pattern noise in post production compared to random noise and it can look very ugly, not at all like film grain.

In addition the readout can be delayed by up to 1 frame more than a rolling shutter sensor as the readout from the sensor to the image processor must wait until after the frame has been captured and shifted from the pixels to the memory cells. This adds additional latency to the monitoring (not really an issue in a photo camera, but more of a problem in a video camera).

IQ IS A BALANCING ACT.

Image quality is never about one single factor. It is about the balance between noise, readout speed, DR, colour, artefacts. But when one issue, such as fixed pattern noise overwhelms any other benefits it tends to become a problem. The F55 was well know for it’s fixed pattern noise, so a good bright exposure was always desirable to avoid the noise. An under exposed F55 was ugly and generally you would always try to shoot 1 or 2 stops brighter than the cameras base ISO. Early tests of the A9 III appear to indicate that it is a bit noisier that other similar rolling shutter cameras and the limited ISO range suggests that the sensors DR is also a bit more limited – this shouldn’t really be a surprise as noise limits the shadow DR. Plus this is a single ISO camera, no dual ISO goodness with the A9 III.
 
So, a high end global shutter camera may well be good to have, but are you willing to give up dual ISO, exceptional low light performance or low noise? Given the A9 III sensor appears to have a native ISO of 250, what about needing to use an EI of 250 to get the best performance out of your S-Log3 or raw video camera when everything else can now be rated at 800 without issue? The F55 was 1250 ISO, but you needed to shoot at around 320-640 EI to get an image as clean as we can now get at 800EI with the newer cameras and there was no way you would want to shoot at 4000ISO/EI with an F55 but now we take for granted the ability to shoot at high ISOs without excessive noise.

I have no doubt that the A9 III is a great photo camera and that it’s global shutter can bring some benefits such as eliminating the need for a mechanical shutter and very high speed flash synchronisation. But these benefits are not essential for a video camera. In the future maybe all cameras will have global shutters, but we are not yet at the point where a global shutter doesn’t have any downsides. The extra memory cells, the extra transistors used to control the movement of the tiny signals on the sensor all add a little extra noise. The sensor might run hotter too especially if used for video. Plus the sensor is probably more expensive to make. So, while I think the A9 III is a welcome addition I don’t think it makes our rolling shutter video cameras obsolete. The majority of films shot on film had a small small amount of rolling shutter caused by the sweep of the cameras rotary shutter across the film.
cameras, Shooting Tips, Technology

What shutter speed to use if shooting 50p or 60p for 50i/60i conversion.

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An interesting question got raised on Facebook today.

What shutter speed should I use if I am shooting at 50p so that my client can later convert the 50p to 50i? Of course this would also apply to shooting at 60p for 60i conversion.

Lets first of all make sure that we all understand that what’s being asked for here is to shoot at 50(60) progressive frames per second so that the footage can later be converted to 25(30) frames per second interlace – which has 50(60) fields.

If we just consider normal 50p or 60p shooing the the shutter speed that you would chooses on many factors including what you are shooting and how much light you have and personal preference.

1/48 or 1/50th of a second is normally considered the slowest shutter speed at which motion blur in a typical frame no longer significantly softens the image. This is why old point and shoot film cameras almost always had a 1/50th shutter, it was the slowest you could get away with.

Shooting with a shutter speed that is half the duration of the cameras frame rate is also know as using a 180 degree shutter, a very necessary practice with a film movie camera due to the way the mechanical shutter must be closed while the film is physically advanced to the next frame. But it isn’t essential that you have the closed shutter period with an electronic camera as there is no film to move, so you don’t have to use a 180 degree shutter if you don’t want to.

There is no reason why you can’t use a 1/50th or 1/60th shutter when shooting at 50fps or 60fps, especially if you don’t have a lot of light to work with. 1/50(1/60) at 50fps(60fps) will give you the smoothest motion as there are no breaks in the motion between each frame. But many people like to sharpen up the image still further by using 1/100th(1/120th) to reduce motion blur.  Or they prefer the slightly steppy cadence this brings as it introduces a small jump in motion between each frame. Of course 1/100th needs twice as much light. So there is no hard and fast rule and some shots will work better at 1/50th while others may work better at 1/100th.

However if you are shooting at 50fps or 60fps so that it can be converted to 50i or 60i, with each frame becoming a field, then the “normal” shutter speed you should use will be 1/50th or 1/60th to mimic a 25fps-50i camera or 30fps-60i camera which would typically have it’s shutter running at 1/50 or 1/60th. 1/100th(120th) at 50i(60i) can look a little over sharp due to an increase in aliasing due to the way a interlace video field only has half the resolution of the full frame. Particularly with 50p converted to 50i as there is no in-camera anti-aliasing and each frame will simply have it’s resolution divided by 2 to produce the equivalent of a single field. When you shoot with a “real” 50i camera line pairs on the sensor are combined and read out together as a  single field line and this slightly softens and anti-aliases each of fields. 50i has lower vertical resolution than 25p. But with simple software conversions from 50p to 50i this anti-aliasing does not occur. If you combine that with a faster than typical shutter speed the interlaced image can start to look over sharp and may have jaggies or color moire not present in the original 50/60p footage.

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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-Fan-Norm2-300x168 NEX-FS700 Significantly reduced shutter when in Super Slow Mo!
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-Fan-SS2-300x168 NEX-FS700 Significantly reduced shutter when in Super Slow Mo!
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-Norm-Flash-300x168 NEX-FS700 Significantly reduced shutter when in Super Slow Mo!
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-SSM-Flash-300x168 NEX-FS700 Significantly reduced shutter when in Super Slow Mo!
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?

shutter3 Shutter, shutter speed and shutter angle.
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.

shutter2 Shutter, shutter speed and shutter angle.
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.

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How I shoot the Northern Lights

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Well I have just returned from Iceland where I held a couple of 3D stereoscopic master classes and a workshop on video for the internet. They went well and we all had fun despite almost a foot of snow fall the morning of the classes. On the last day of my trip I decided to try and get some more Northern Lights footage. As I am often asked how I do this I put together the clip below which explains what settings I use for the Aurora and also gives a brief description of S&Q on an XDCAM EX. Basically what I do is use the EX Slow Shutter at 32 or 64 frames to increase the sensitivity of the camera. For a dim Northern Lights display I use 64 frames but for a bright display I drop down to 32 frames. The slow shutter acts like a long exposure on a stills camera. I then combine this with interval record shooting at 1 frame every second. I did also have a Canon DSLR with me and tried to shoot the Aurora with that. I found I needed a 10 second exposure at 800 asa to get a similar result to that achieved with the EX. The 10 second exposure means that it would take longer to get a decent length video sequence and most of the motion of the Aurora would be lost. Some of the exposure difference was I admit to the slower F4 lens on the Canon compared to the Sony EX’s F1.8, so perhaps with a faster lens you could bring the exposure down to around 5 seconds and this is something I hope to try when I go Aurora chasing next winter.

If you watch the video make sure you stay to the end to check out my attempt to record a piece to camera in 60 mph blowing snow! Don’t know why I even thought it would work. What I will say is that my new Vinten 5AS did a great job of keeping the camera steady in some pretty extreme conditions.

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