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Nikon and Canon’s flash sync is typically 1/250s. This number defines the shortest time that the shutter curtain can maintain fully open. Any use of faster shutter speed will enable the 2nd curtain to close while the 1st curtain is still being opened. This effectively means the shutter is no longer fully open, but only exposes the sensor via a slit. If you apply a normal flash which has a duration of ~1/1000s, only part of the frame will be exposed to the flash and you will get a partial darkened image where the flash has tailed off while the slit is still travelling.
Check out Jeffrey Friedl’s animation on a D3 shutter release in slow motion (originated from Marianne Oelund). In the animation you can see the shutter is fully opened exposing the sensor so the shutter speed must be slower than 1/250s. Should the shutter speed be faster than 1/250s, the 2nd certain will come down while the 1st certain is still opening.
Cameras now days lock the fastest shutter speed to 1/250s when using flash to avoid the above problem and typically, you will get the most benefits of your flash by shooting below the sync speed (1/250s) where you can
i) fully utilise the full flash power
ii) maximise flash range and
iii) achieve faster recycle time.
However, the problem occurs when you are shooting with flash under brightly lit condition and require a shallow DoF or to stop motion where your shutter speed is usually above 1/250s. You will need to enable FP High Speed Sync using an external flash (onboard flash has no FP High Speed Sync). With FP High Speed Sync enabled, the flash will fire for the duration of the shutter curtain’s travel, so it syncs with the camera’s shutter speed (as shown in 2nd half of the below diagram). Because the flash is continuously lit, more flash power is being lost and flash range is significantly reduced.
You can see the shutter and flash sequences between a slow shutter and a fast shutter from the below diagram.
For example: In the below image taken at a recent portrait session, I was shooting into the sun the shutter speed was well above the sync speed, and in fact I also wanted a shallow DoF, the shutter speed was set to 1/8000s. Because I am shooting into the sun, I will need fill-in flash to properly lit the model, otherwise she will be in complete shadow.
This is where I need to turn on FP High Speed Sync, so I can proper lit my subject and shoot @ f/2.8, 1/8000s.
As you can see even with flash power being lost and flash range being reduced, the SB-900 was still able to overpower the shadows when shooting into the sun.
Nikon D700 | Nikon 24-70mm f/2.8 | SB-900 FP
f/2.8 | 1/8000s
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