Getting proper exposure on both the background and subject can be tricky, even when using flashes. Part of being able to do so involves understanding what the terms sync speed and high sync speed refer to and how you can use them to your advantage. Take a look at Michelle Ford‘s quick video below for a primer:
What Exactly Is Flash Sync Speed?
Essentially, your camera uses two curtains every time it is fired. One curtain opens to the reveal the sensor, then the second curtain snaps shut at a predetermined amount of time–your shutter speed setting. If your shutter speed is longer, the entire sensor will be exposed, but when using shorter shutter speeds, only a small band of your sensor is exposed at one time. With a fast shutter speed, because the second curtain is generally already closing before the first curtain has fully opened, the flash may not expose evenly on your camera’s sensor. This can lead to bands of black in your image.
This is where flash sync speed comes into play. Every camera has a maximum shutter speed sync, which generally varies between 1/180 to 1/200 of a second. That number is the fastest shutter speed you can use before the curtains begin to close so quickly that the flash only exposes parts of the image.
Avoiding Black Bands
To avoid the dreaded black bands on your image, you may need to adjust your ISO and/or aperture to allow for a slower shutter speed when possible. Alternatively, some higher end cameras come equipped with a special feature that helps to eliminate black bands when shooting at fast shutter speeds by telling the flash to fire multiple times in rapid succession, creating a strobe-like light. On Canon cameras this is referred to as High Speed Sync–or HSS. Nikon calls this feature Auto FP.
While often a great solution to using flash with fast shutter speeds, remember that HSS and Auto FP require more battery power, so you may want to keep a fresh battery on reserve.
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