I tried my luck over at Robotics but didn't have any luck finding somebody knowledgeable. Somebody therefore suggested to post the question here again.

I understand that there are different types of shutters, both mechanical and electronic, and I can understand how they work. My problem concerns shutter speed. If I use a mechanical shutter, well then the maximum shutter speed depends on that particular element in the assembly, but how does it work for electronic shutters? I have never read "Max shutter speed" in any specs. The only thing I usually see floating around are frames per second. But those do usually not pass a limit of about 250 fps. Depending on how the sensor is built one could think that the maximum shutter speed therefore is 1/250 or 1/500 if it uses half frames.

Can this be right? It seems really slow. I will be faced with the task of recording crisp and clear images of paper which moves at about 17 m/s. That is never possible with shutter speeds that slow. Will I be forced to use a mechanical shutter or am I misunderstanding something?

If so, how should I understand from the sensors specifications which maximum global shutter time it allows? See for example this one: http://www.onsemi.com/PowerSolutions/product.do?id=PYTHON2000

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    Due to the way mechanical shutters record images shorter than the camera's sync speed, you will get rolling shutter artifacts if you use a typical mechanical shutter found in a DSLR. Although each spot on the sensor may only be exposed for a time interval as short as 1/8000 second, it typically still takes around 1/200 second for the slit between the first and second curtain to move across the entire sensor. The same is true of some electronic shutters that read out sequentially (i.e. from top to bottom of the sensor). – Michael C May 17 '15 at 20:44
  • See this link for how a mechanical shutter works at high shutter speed: youtube.com/watch?v=ptfSW4eW25g – Michael C May 17 '15 at 20:45
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    For high speed photography the solution has usually been to not worry about the shutter speed, but rather to control all of the light illuminating the scene in precisely timed short bursts while the shutter remains open a longer period of time. If you have a burst of light only 1/10,000 second in duration and the room is otherwise totally dark, then it doesn't matter if the shutter is open for 1/250 second. The only time light will strike the sensor is in the 1/10,000 second the light is shining. – Michael C May 17 '15 at 20:48
  • The camera will be setup in a fabrication / printing environment and it won't be completely dark. The camera will probably sit in a shadowy position though. – Cyianor May 18 '15 at 8:28
  • @Cyianor Completely dark is not a requirement. Photographic flashes output a lot of energy during a flash burst and the contribution from the ambient light will not matter much. – Hugo May 18 '15 at 8:47

The electronic shutter speed is limited by the rate at which the camera reads the image data from the sensor.

For most CMOS sensors, and therefore most regular DSLRs, the camera reads image data from it progressively, rather than reading all the image data instantaneously. As it reads, it resets the data held by those pixels.

If this process takes, for example, about 1/120 of a second to read the whole frame, this is the hard limit on the speed it can use as an "electronic shutter". For exactly the same reason, this also governs the maximum frame rate that it is able to shoot in video or in live view mode. If a camera is capable of recording video at 60fps, for example, then you know that the image data takes less than 1/60s to read.

A mechanical shutter is able to reach much higher speeds. When using the mechanical shutter, the shutter is closed for the duration it takes for the sensor to undergo a reset, then the shutter will open for a short time and expose the frame (one shutter curtain following the other), then the shutter will end up in a closed state. The data is then read from the sensor while the shutter is in the closed state after exposing. Therefore, the read rate of the sensor has no limitation on the shutter speed.

Mechanical shutters are able to reach high speeds because they involve two independent shutter curtains, and one curtain is able to closely follow the other such that one is opening while the other is closing at the same time. An electronic shutter is unable to do this: it's not possible to be progressively reading and resetting data from the sensor in two places at once.

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  • That's only true for non global shutter CMOS sensors. CCDs or CMOS sensors aren't constrained by how fast data can be read out (in a global shutter architecture charge is transferred from the pixel to a buffer until the camera is ready to read it out). The first generation digital cameras with CCDs would often shoot at 1/1600s, faster than the fastest mechanical shutters available. They could have been made faster if the manufacturers desired (they're only limited by the switching speed of the sensor). – Matt Grum May 19 '15 at 10:34
  • Are there CMOS sensors with global shutters? I'm aware that CCDs do this but wasn't aware of CMOS sensors doing it. – thomasrutter May 19 '15 at 12:12
  • Yes there are CMOS sensors with global shutters, e.g. the one in the Blackmagic Production camera. It's more difficult to implement than in a CCD but it's perfectly possible. – Matt Grum May 19 '15 at 12:54
  • @MattGrum So there are definitely electronic shutters with faster shutter times than 1/120? I finally got a data sheet from a camera manufacturer which stated the possible shutter speed. The fastest shutter speed (CMOS global shutter) they state is 13 microseconds, which should be about 1 / 77 000. They didn't state this under chip information but underneath camera information. Could it be that the fastest shutter speed is more determined by the camera manufacturer than by the sensor? – Cyianor May 24 '15 at 21:32
  • It will be determined by the technology used. Live view mode is traditionally quite limited due to the reasons given above, but as Matt Grum pointed out some sensors have a global shutter feature which is why you get that really fast speed. No global shutter = very slow, global shutter = very fast. With the rise of electronic viewfinders, live view mode as default, PDAF that can work in live view mode, mirrorless, and so on, global shutters are a logical step to avoid the double-closing shutter action (or indeed the need for a physical shutter). – thomasrutter May 25 '15 at 0:20

I don't really know what you're asking about. If you want to capture sharp and clear images of moving paper, its speed isn't really relevant. What's important is rather the angular velocity of the details you want to capture. If the paper is far away it would be no problem to take a picture.

A more detailed shot of the paper could end up blurry (the angular velocity is higher) but you can raise the shutter speed but even with high shutter speed you will experience a rolling shutter effect.

To avoid this effect you can keep the shutter open completely open for a short amount of time and use a flash to freeze the action instead. Photographic flashes usually release the flash in a very short period of time and if the flash is the only source of light that will really affect the exposure it will most certainly freeze the moving paper.

Unless you're very specific in what you're trying to achieve it's very hard to help. Have you tried taking the photo already? What are your problems and what are your goals?

Regarding the part of the question about shutters (mechanical and electrical) and their maximum speeds I fail to see how they connect to the part about the photo of the paper. The fastest mechanical shutters around for DSLRs can achieve a speed of about 1/8000 s. Electronic shutters are not that common among DSLRs but the old EOS-1D used one and it could achieve a speed of 1/16000 s. There can get even faster than that and maximum frame rates of video cameras has little to do with it.

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  • The reason why I posted this on robotics first is that this is a machine vision task and I won't be using a DSLR or something similar. I need a tiny or small camera and resolutions of 2 Megapixels would be absolutely acceptable. The only thing is that there is going to be the web from a printing press running at about 17 m/s under the camera and the camera will be about max 10 cm away from the paper. I only want to capture little features and it is possible to predict their position. What's important though is to capture them as crisp and clear as possible. – Cyianor May 18 '15 at 8:27
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    @Cyianor If you're using a flash with a short flash duration say 1/20000th second that would correspond to movement of the paper of about 0.85 mm during that flash. The flash duration will put an upper limit on how sharp images you'll get. This blurriness will be oriented in the same way as the paper is moving though. If the detail of interest is perpendicular to the movement the resolution will be high. If you're going taking a large amount of images you'll burn throuh the flash quite quickly. Unfortunately the requirements sharp images and fast movement are hard to satisfy at the same time. – Hugo May 18 '15 at 8:43
  • Is this even possible if it is not completely dark? I will be working in a printing / fabrication environment and even if there will probably be hardly any daylight there will still be artifical light. Could I still have "long" exposure times of 1/200 for example and get a crisp image with a short flash like 1/20000th of a second? – Cyianor May 18 '15 at 8:48
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    @Cyianor It depends on how bright the ambient light is and how powerul flash you're using. Look at this video to get the idea: https://www.youtube.com/watch?v=91gU93J2Q8k – Hugo May 18 '15 at 8:55
  • Thanks! This actually helps a lot. But it still doesn't answer one of my original questions. If I look at image sensors, like the one I posted above, how do I find out the maximum shutter speed? – Cyianor May 18 '15 at 9:11

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