Shutter Speed controls how much time the imaging surface is exposed to the incoming light. The way this is done varies a little from camera to camera, but there are 2 main methods in modern cameras.
The most common method involves a pair of blades, or curtains, that move from left to right, or right to left. The first one opens, and then the second one follows it after the chosen period of time. If the shutter is set to 1/100th of a second, curtain one slides across, opening the imaging surface to the light and then curtain 2 slides across closing off that light source. This introduces a challenge for flash photography called sync speed and I will get to that briefly in a minute, and in more detail in a later post.
In the other kind of camera, Hasselblad for instance, the shutter opens and closes more like an iris. In this style of shutter, because of the phyics of moving the leaf blades out and then back in, shutter speeds are lower than in more common digital cameras. This isnt really a problem for the kind of shooting they are designed for, and it has one big advantage in that shutter sync is not a problem, shutter opens, flash, shutter closes. More on that again later.
In both cases though the basics are the same, the shutter defines the amount of time the light is allowed through the camera and onto the imaging surface.
There are 2 end results.
First, this defines how much the motion of the camera, and the motion of your subject, will blur in the final image. The faster the shutter the less blur.
Second, the shutter speed affects the total amount of light in terms of a total photon count that hits the imaging surface. If we view light as a stream of particles, or photons, coming from our subject to our camera, the aperature and lens determine how many photons per second are allowed into the camera, and the shutter controls how long we allow that stream through, resulting in an image. There are lots of ways to think about this process, Im just relating mine.
Now, sync speed. Sync Speed varies a little from camera to camera, but its usually around 1/200th or 1/250th of a second. The shutter curtains take time to move all the way across the imaging surface. Its fast, but it still takes time. If they were instant then sync speed would not exist, but it does. So, at speeds greater than sync speed the first curtain moves across, opening the sensor to the light. Before it arrives at the far side of the sensor the second curtain starts across. The net result is that only a portion of the sensor is going to be exposed at any given time. If a flash fires it will only expose the portion of the sensor that is currently not covered by the first or second curtain of the shutter. So, to compensate, some flash systems, like speedlites, can syncronize with the shutter and fire more than once in rapid succession. It might fire twice, it might fire 4 or 5 times, it depends on the shutter speed. As a trade off though, because the flash is powered by a capacitor, the flash power is reduced. If it needs to fire twice then each pulse will be half strength.
So, to get full power from your flash you need to stay under sync speed. If you recall nothing else, thats the thing to recall. Know your camera’s sync speed and stay under it when using a flash.