Flash Sync Speed
When you use a flash the flash sync speed will come into play. First the camera will open the shutter. Then the flash fires. Finally the shutter closes. While the flash fires, it’s duration is very short. It is about 1/1000th second long. This burst of light must occur when the shutter is fully open. If it is not fully open part of the sensor will be covered while the light is on. If the flash fires before the shutter is open or after it closes the light from the flash will not hit the camera’s sensor at all. So the flash must synchronise with the shutter.
Specified Flash Sync Speed
To ensure the flash and the shutter synchronise, every DSLR has a specified flash sync speed. It is the fastest shutter speed that your camera can synchronise with a flash. On most DSLRs this is between 1/160sec and 1/250sec. Your flash will work perfectly well at any shutter speed that is slower than your flash sync speed. It will not function fully at faster shutter speeds.
Off-camera lights and Flash Sync Speed
Normally a camera will not allow you to increase the shutter speed above your flash sync speed. The exception is when an off-camera flash is attached to your DSLR. Using off-camera flash or studio lights with a radio trigger lets you to use a shutter speed that’s faster than your flash sync speed. If you do use a faster speed than your Flash Sync Speed the shutter will interfere with the shot. You will see a black section in the resulting photo. This is where the shutter covers part of the sensor while the flash fires. It is simple to prevent the error. Always use a shutter speed below your flash sync speed when using off-camera flash or studio lights.
High Speed Sync or FP Flash
Modern flash units have a mode called high-speed sync (HSS on Canon cameras). On Nikon cameras it may be known as FP sync which stands for “Auto FP flash” (Focal Plane mode). This allows any shutter speed with your flash. It reduces the flash’s maximum output. Instead it extends the flash duration.
The HSS flash (FP flash) needs to be longer to cover the entire time that the shutter is open. The long flash will ensure light throughout the shutter opening. These high-speed sync modes only work with on-camera flash. A connection over radio triggers is not intelligent enough to transmit the instructions for HSS or FP Flash. Using off-camera light units means you have to rely on Flash Sync Speed. Another way is to use a long tether to connect the camera to the light.
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