When photographing an impressive landscape, you often want to have almost everything in focus, the tree that in just a few feet away, as well as the mountain range in the distance – so you dial-in a very small aperture like f/22.
The small aperture lets only very little light enter the camera, which will lead to a much longer exposure time, meaning you may have to use a tripod or a lens featuring image-stabilization. Using the narrowest lens opening (like f/22 or f/32) also allows for the “star-effect”, in case the sun happens to be in the picture.
However, sometimes you want to do just the opposite and have your main subject separated, i.e. blur the background. To do that you would open the aperture, e.g. to f/2.0, like in this photo.
Using a wide open aperture allows a lot of light to enter the camera and the exposure time now will be very short, sometimes too short. Here is what I mean. Using a flash may allow you to isolate your subject even more, but common small flashes fire for only a very brief moment, like 1/250s or maybe only a 1/20,000s.
Once you drop the exposure time on your camera, below the so-called “sync speed” however, the camera won’t expose the whole sensor for your dialed in exposure time, but a narrow slit will travel over the sensor exposing only a part of it at a time.
So if the sync speed of your camera is slower (e.g 1/160s) than the longest time your flash is on, you will find a part of your image under exposed.
High-speed Sync to the rescue. Small high-speed sync flashes, which should rather be called longer flash time flashes, will continuously fire pulses and illuminate the scene for as long as it takes for the small slit to travel across the whole sensor, thereby ensuring that every part of the images get the same exposure.
This photo was exposed for 1/250s, which is shorter than the sync-speed time of the camera (1/160s) meaning the flash needed to fire for 1/160 of a second, not just the exposure time of 1/250s. Again the small slit traveling across the sensor exposed every part of it for just 1/250s, but its travel-time was 1/160s.