Does a long exposure in DSLR camera have a similar effect to a global shutter?

Question

Regular DSLRs make use of mechanical shutters and progressive electrical readouts, and hence images are captured in a rolling shutter.

But, when used to capture pictures with a long exposure, where the entire sensor is open for an extended duration, are all pixels of a sensor exposed simultaneously? The readout will definitely be sequential rows of pixels, but will the data acquired in every pixel correspond to a simultaneous event?

The reason to ask this question is to understand whether a DSLR camera may be suitable for an academic research purpose.

Answer 1

I think you've slightly misunderstood how the shutter works. Initially, the first shutter covers the sensor and the second does not. The first shutter then slides away to expose the sensor, and the second shutter follows it to cover the sensor again. The shutters always move at the same speed, regardless of the exposure time.

For a short exposure, the second shutter will start to close off the sensor before the first shutter has finished opening it. The effect is that an open slit moves across the sensor, with the width depending on the desired exposure time. For longer exposures, though, there are three phases: the first shutter opens all the way, the sensor spends some time completely uncovered, and then the second shutter closes. In this case, all pixels are indeed exposed simultaneously, except briefly at the start and end of the exposure.

Answer 2

Most DSLRs have shutter curtains that transit the face of the sensor in approximately 2-4 milliseconds. For any particular camera model, barring a mechanical malfunction, the transit time for each shutter curtain will always be the same, regardless of the exposure time. This is the case with shutter times anywhere from 1/8000 second, where the second curtain is 'chasing' the first curtain across the sensor a small fraction of a millisecond behind it, to Bulb mode, where the offset between when exposure begins and ends on one side of the sensor to the other is still that same 2-4 milliseconds.

For a 1/8000 second exposure, a 4 millisecond offset is roughly 32X the length of the exposure.

For an 8 second exposure, the same 4 millisecond offset is a miniscule 0.0005X of the total exposure time.

How readout is done is inconsequential for still images, since it is performed when no light is reaching the sensor after the shutter curtains are closed.

Reason to ask this question is to understand whether a DSLR camera be suitable for an academic research purpose.

Even top level professional cameras are not what would be considered 'lab grade'. The precision required for creative/historical/documentary photography is relatively low. Anything within one-sixth stop is considered acceptable for a camera with minimum adjustments of 1/3 stop for aperture (Av), exposure time (Tv), and sensitivity (ISO). In terms of exposure time measured in seconds or fractions thereof, one-sixth stop is roughly 12%. So if a camera is set to take a photo using a shutter time of 1/250 second (4ms), anything between 1/285 second (3.52ms) to 1/225 second (4.48ms) is considered acceptable.

The other thing to remember is that the numbers we use in the "standard" shutter speed scale are not the actual target numbers. They're rounded versions of the powers of two that were selected at a time when cameras were even much less precise than they are today. For example, what we call 1/250 second is actually 1/256 second (which means we give or take that allowable 12% time variation from 1/256, rather than from 1/250)! So the allowable window for our named 1/250 shutter time is actually +/- 12% of 1/256 seconds, which figures out to between 1/229 seconds (rounded to the nearest whole denominator) and 1/291 seconds (rounded to the nearest whole denominator). But since we like to round numbers to the nearest "5" in photography, we'll call them 1/230 and 1/290 seconds, respectively.

For more about marked numbers versus target numbers versus actual numbers used and why it doesn't really matter for creative/documentary/historical photography, please see: Is there a sane reason why ¹⁄₁₂₅ is not, instead, exactly half of ¹⁄₆₀?

My application involves using an extremely short pulse laser to freeze motion(as short a duration as 9ns). So I was worried whether rolling shutter will kick in even at long exposures and make my regular camera unsuitable for the experiment.

Yes, as long as the first curtain is already fully open when the light pulse begins, and the second curtain has not yet begun to close when the light pulse ends, the entire sensor will be illuminated by the entire duration of the light pulse. For still images the sensor is only read out once following the end of the exposure. There is no 'scanning' going on while the shutter is open.

As long as your laser pulse is significantly shorter in duration than the camera's flash sync speed (typically around 1/200-1/250 seconds or 5-4 milliseconds), you might consider using the camera's flash sync capability to trigger the laser. (How to do that would probably need to be a separate question.)

We also have some existing questions about high speed photography using very short pulses of light, rather than short shutter times, to freeze the motion of high speed objects. Among them:

What problems might there be with using a laser as a flash source for high speed photography?
How can I capture the movement of a bullet?
How do I trigger the flash in a way that takes luck out of high-speed photography?
Why doesn't a shutter speed of ¹⁄₂₅₀th freeze motion when a flash of that duration does?

Answer 3

It is still the same shutter, even in long exposure and pixels are not exposed simultaneously.

However, the time difference (in relation to the total exposure time) becomes smaller as the exposure time increases. It might become insignificant depending on your requirements.

If you cannot tolerate rolling shutter at all, you still have 2 options:

• use external global shutter
• use flash (the shutter opens and closes in darkness, flash illuminates the whole scene simultaneously)

Answer 4

CMOS pixels are reset and then gather the cumulative charge from the light they receive until they are read & reset again. The CMOS is usually reset, the shutter process takes place (front curtain, delay, then rear curtain) and then values are read.

Because of the mechanical process, there will always be a short period during the shutter opening/closing events where the sensor is partially obscured.

If this affects you will depend on what type of event you are attempting to capture with a long exposure. If you're hoping to capture a bright but unpredictable event like lightning then it's possible that a shutter curtain could be half-closed during the event. If you're looking for something under more constant lighting conditions then the 1/125 or 1/250th sec that are needed would be inconsequential in an exposure of 1 sec or more.

Answer 5

For the purpose you describe: Yes, all pixels are exposed simultaneously; they work independently, in parallel. Even for the short periods of time at start and finish, they're still all working together - but the shutter will need some time to get out of the way completely.

So if a short event were to happen during the long exposure, what you will get is a simultaneous read across the surface of the sensor.

Answer 6

I think a lot of the people here are mistaken about the sensor. At your speeds, it doesn't matter what the physical shutter does (if it even has a physical shutter) Most sensors send their information one pixel at a time (or in clumps) rather than all at once, as if it were scanning - like reading a book. It starts on the upper left, reads across, goes down a line and repeats. This allows for a single stream of information to be recorded and requires far less wiring. This is done quickly, but if your need is 9 nanoseconds, then this will be very noticeable. For that, you will need a "global shutter" sensor that reads the sensor all at once. Currently, these are relatively rare and expensive because each sensor has to have its own stream instead of every sensor sharing the same stream. A lot more wiring.

I STAND CORRECTED: It starts the scan one line at a time, and reads the scan one line at a time, but if you have a longer exposure, those sensors are continuing to collect light the entire time. So long as your 9 ns flash happens between the time of every line being activated and the first line being read, you should be ok. (For example, if you have a one-second exposure and you do your 9 ns flash at the 1/2 second mark.