Flash travels at the speed of light to freeze the movement of your subject. The challenge with camera manufacturers was to synchronize the movement of the shutter curtains at the exact moment the flash fires. There is a speed limit to every camera brand and model. Canon models have shutter speed sync of either 1/200 or 1/250 seconds. Which means you cannot dial the shutter speed faster than this while firing the flash or else some black bars will appear in the photo representing the not-in-sync shutter curtains.
Now; if you want to shoot outdoor where the sun is shining midday or you want to freeze an action or you would like to blow the background with a wide aperture, then you'll overexpose your image.
Consider the following scenario: you are shooting midday outdoor and you want to achieve a soft, blurred background, which is achieved with an f/1.8 aperture. If you are on 1st curtain sync, your light meter tells you your aperture should be f/16 or f/22. There are six stops of additional light difference. This means that the shutter speed needs to go up to 1/5000s. Try it on 1st curtain or 2nd curtain sync and your photo will be a total burnout.
This is where High-Speed Sync comes into play to solve this problem. High-speed sync is your flash capability to fire at shutter speeds faster than 1/200s or 1/250s.
In sports photography High-Speed Sync is a must. But everything comes at a cost. High-Speed Sync drains your batteries like a monster. It acts as a stroboscopic bursts source of light like if you have a continuous light.
It is worth knowing at this stage how the shutter works. DSLRs have of two curtains that open and close to capture the photo (expose the camera sensor to light). When the shutter button is released, the 1st curtain opens revealing the sensor to light, and then the 2nd curtain closes blocking the light. The exposure time between 1st and 2nd curtain is called the shutter speed. Shutter speed can be 30 seconds, 1/60 of a second or 1/5000 of a second.
With the 1st and 2nd curtain sync top and middle parts of the diagram, the flash fires one time in a near-instantaneous burst of light whereas in a High-Speed Sync mode, the flash fires in a multi bursts of light (bottom part of the diagram).
High-Speed Sync is a function you should explore more. This is why most flash manufacturers have added a quick shortcut button on the back of your flashgun; for quick access. The most important applications of high-speed sync are:
- To stop motion we need a shutter speed of more than 1/200s.
- The second reason is to get enough flash power to balance the sunlight and fill your subject's shadows. Fill flash in direct sunlight is critical to getting good shots in high contrast environment like the sunlight.
- Lets you shoot with wide apertures, which requires less flash power (1/8, 1/16, 3tc) which lets your flash recycle faster thus shooting more frames per second.