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This question connects with my previous question: Why do cameras that film with high FPS rates cost a lot?
I just read that a normal flash illuminates a scene within a 1/250th of a second. (A flash would keep the scene illuminated for a 1/250th of a second, right? And it's the flash's sync speed, right?) So, is just a 1/250th of illumination enough for freezing motion in a dark room? And(then) why can't we use a 1/250th of shutter speed to freeze motion in a well lit place?
I just read that a normal flash illuminates a scene within a 1/250th of a second. (A flash would keep the scene illuminated for a 1/250th of a second, right?
In general, that's wrong.
Flash duration is flash duration and sync speed is sync speed. Apples and oranges.
The 1/250th of a second is the sync speed of (many) cameras. That's basically the shortest shutter speed at which the shutter is entirely open during the exposure. For shorter shutter speeds, both curtains of the shutter move at the same time, which means that the exposure happens through a moving slit formed by both shutters.
The flash exposes the entire scene. But if there's only a slit of the shutter open during the flash exposure, only that slit of the image will be brightened by the flash - the result is two (or three) horizontal parts of the image with different exposures. (either only ambient light or ambient light + flash)
In summary: To get all the image exposed by the flash, the shutter has to be open entirely when the flash fires. The shortest shutter speed doing that is called the sync speed.
And it's the flash's sync speed, right?
The sync speed is a property of the camera, not the flash
Flash duration on the other hand is the time the flash is on. A flash is not just on or off. The power (light) output of a flash is a curve. flashhavoc.com has a great explanation:
A flash pulse from a studio strobe may look very fast to the human eye, but its actually often a relatively long process, consisting of a bright initial burst, followed by a long slow exponential decline, or long tail of light. (Think of a firework that goes off with a bright burst and then slowly dissipates). So flash duration curves will often appear as shown below, a short peak followed by a long tail of declining light.
This image of theirs nicely illustrates this flash curve and the two different ways to measure the duration of that curve.
The problem is that for many flashes, the flash duration depends on the power setting. If you change power, the flash duration varies.
Andy Gock tested a bunch of flashes (and strobes) in his blog post for their flash durations.
Let me quote the results for the Canon 580EX:
Power μs s 1 4000.0 1/250 2 1088.0 1/919 4 484.0 1/2066 8 266.0 1/3759 16 166.0 1/6024 32 105.6 1/9470 64 71.6 1/13966 128 50.4 1/19841
As you can see, the duration for full power (1) is actually 1/250th of a second. But for all other settings, it's shorter.
The durations become so short because the different power settings are achieved by turning the flash off prematurely. This cuts off the curve and thus results in shorter flash durations. Andy did a great job illustrating all of this in his blog post.
And(then) why can't we use a 1/250th of shutter speed to freeze motion in a well lit place?
A well lit place will not lend itself to freezing motion. That's because the ambient light is strong. The stronger the ambient light is, the more influence it has on the overall exposure.
In order to freeze motion with a flash, the influence of the ambient light (which causes blurryness) is reduced as much as possible, by reducing the shutter speed as much as possible, which usually means to shoot at the sync speed and in a dark environment.
Even with a flash duration = sync speed, the flash still fires in that curve, which means during the exposure, at the time when the curve is at its peak, the flash output is very strong. This period of strong flash will have a bigger influence on the overall exposure and can make a difference in terms of freezing motion.
I just read that a normal flash illuminates a scene within a 1/250th of a second. (A flash would keep the scene illuminated for a 1/250th of a second, right?
Sort of, depending on the specifics of the flash unit and the power setting. For example, a Canon 580EX Speedlite set to full power discharges over 1/250s according to Andy Gock's Actual Measured Flash Durations of Small Speedlight Strobes. But at 1/128 power, the flash duration is close to 1/20000s. The linked site has durations for a number of lights and you can see the same pattern for all the speedlights (but interestingly, not all the moonlights).
Also, and this is important, the distribution of light over the duration of the flash is not constant. Look at the first image in the linked article:
You can see that there's a strong peak right at the start of the flash, followed by an exponential decay. So even if it takes 1/250s for the flash to fire completely, most of the light is delivered in the first 1/1000s.
And it's the flash's sync speed, right?)
No. The flash sync speed is a property of the camera. It's the fastest shutter speed at which the entire sensor is exposed at once. For the camera to benefit from all the light coming from the flash, the flash duration has to be shorter than the camera's flash sync speed.
So, is just a 1/250th of illumination enough for freezing motion in a dark room?
It can be. It all depends on how fast the subject is moving. Physical motion is continuous, so of course you can never truly stop motion with a fast shutter, but you can record an image over such a short time that there's so little motion during the exposure that there's no motion blur. So, if you're shooting a relatively slow-moving object like a falling drop of water, you can use a longer exposure than if you're taking a picture of a bullet piercing an apple.
However, as discussed above, most of the light from a flash is delivered much more quickly than 1/250s. If the most intense part of the flash happens in the first 1/1000s, then you may get a sharp image of a faster-moving object even if 1/250s isn't really fast enough to "stop" your subject's motion.
why can't we use 1/250s to stop motion in a well-lit place?
If 1/250s is fast enough to stop motion for your subject, then you can certainly do that if there's enough ambient light to shoot at that speed. The ambient light has constant intensity, though, so unlike the flash you'll get significant exposure through the entire 1/250s and motion blur is more likely to be an issue at that speed.
The flash sync speed will generally be a little slower than the camera's shutter transit time, i.e. the time it takes for both shutter curtains to move move from one edge of the sensor to the other. You can shoot at a much higher speed than that in ambient light, but you need to know that the duration of the entire exposure cannot be less than the shutter transit time. At higher shutter speeds, any one portion of the sensor will see only that short exposure but the times at which two parts of the sensor are exposed will be different. If the subject is moving, that can lead to image distortion like this:
This is called the rolling shutter effect, and it's only a problem if you're shooting fast-moving objects and need an image that's free of any temporal distortion. Most of the time our subjects aren't moving so fast that there's a lot of distortion, and we're happy with the sharp, evenly-lit images that a focal plane shutter can give us. Other times, like when you're using photography to actually measure something, the distortion may be more noticeable and problematic.
Shooting in a dark room reduces motion blur made visible by ambient light, but perhaps more importantly it also lets you use a lower power setting on your flash. It's easy to completely overpower room lighting with a flash set to full power, but as seen above, full power also has the longest flash duration. The lowest power setting on the flash gives you the shortest flash duration and therefore the greatest ability to stop motion, but you can't compete with ambient light at 1/128 power, so instead you turn off the lights.
The question seems to make an incorrect assumption: that at a sync speed of 1/250 second the entire sensor is uncovered at the same time for 1/250 second. This is not the case. Most of that time is consumed by the first curtain opening and the second curtain closing. There is only a shorter moment between the transit of the two curtains when the entire sensor is simultaneously uncovered. It is at this moment that the flash must fire and release at least the major portion of its energy. With "slower" flashes such as some studio strobes, a shutter time slower than sync speed must be used.
With a focal plane shutter each part of the sensor is exposed for the same amount of time. But each part of the sensor is not exposed during the same time period. Whether at a shutter speed of 30 seconds or 1/8000 second, a typical camera with a sync speed of 1/200 second will begin to expose the beginning of one side of the sensor (normally the top, which is the bottom of the inverted image) about 1/250-1/300 second before the last of the other side. Likewise, that same side will be covered by the second curtain the same amount of time before the other side is. It is the difference between the beginning of the first curtain's movement to uncover the sensor and the beginning of the second curtain's movement to cover it back up that determines exposure time.
So how does this apply to flash? It means that even with an exposure time at sync speed, the entire sensor is uncovered at the same time for only a fraction of the total exposure time.
The distinction between different types of flash should be mentioned as well. Most hot shoe mounted portable flash units (often called speedlights) regulate power by controlling the duration of the flash. The lower the selected power, the shorter the flash time. Many studio strobes control power, at least partially, using flash duration. But many also vary power via the amount of energy loaded into the capacitors prior to the flash firing. Speedlights at full power generally have shorter discharge times than studio lights at full power. Part of this is attributable to the greater illumination power most studio flashes can output compared to most speedlights.
These differences in discharge times mean all flashes may not necessarily work properly at a particular camera's sync speed. For some flashes a much slower shutter time than the camera's sync speed may be required to allow the full duration of the flash to illuminate the entire sensor.
With high speed photography that is used to "freeze" the motion of high speed objects, very specialized flash units and even arrays of flash units are used. These types of flashes are capable of releasing all of their energy in a very short time interval. For the fastest subjects, such as bullets in flight, they are normally employed in environments with no ambient lighting at the time of exposure. These are highly sophisticated (and generally very expensive) lights that use all kinds of technologies not employed by run of the mill everyday studio flashes or speedlights.
1/250 is the time that it typically takes for a mechanical shutter curtain to travel from one end position to another. So for any faster shutter speeds, the closing curtain has to start moving before opening curtain has finished its travel, and 1/250 is the fastest shutter speed where, for just a moment, the whole sensor (or film frame) is exposed at once. Flash can then be popped for that moment, and illumination from the whole scene is recorded in a much shorter time than the 1/250. For example, flash duration of Nikon SB-900 is 1/880 at full power, and even shorter at lower power levels.
So 1/250 in a well-lit place is weaker at freezing motion because in this case, each part of image is actually illuminated for the whole 1/250, not only the short pop.
Your understanding of how a flash works is wrong.
The flash duration is more in the range of 1/1000 to 1/8000. Due to physics and the design,the shutter needs to be open longer 1/1000 in order for the flash to "Sync".
Flash photography usually involves both ambient light, and light from the flash. Usually we like to use faster shutter speeds to avoid letting in the ambient light and in most cases the image will only be illuminated by the very brief 1/1000 to 1/8000 flash from the Speedlite.
Most cameras are limited to 1/200 or 1/250 shutter speed before you run into sync problems. This is because most DSLR cameras use a focal plane Vertical-travel shutter which progressively opens and closes to allow light to reach the image sensor.
Sometimes we use a slow shutter speed to intentionally allow ambient light into the scene in addition to the brief burst of light from the flash. This is called "Dragging the Shutter".
