What the heck is FP High Speed Sync?

The simple question of “how do I set a faster shutter speed whilst using a flash” led me to discover FP High Speed Sync on my camera (just called High Speed Sync with Canon).  Have you ever been looking at a scene, switched on your flash and had your camera dramatically over expose a photo or change your carefully selected aperture?  If the answer is yes, then you need to know about High Speed Sync.

What is FP or High Speed Sync Mode?

Your camera has a maximum flash sync speed.  It’s determined by the fastest speed a camera can open and close the shutter and still fire the flash whilst the shutter is fully open.  For a DSLR it’s typically 1/200 or 1/250s.  Above this speed the shutter is in fact never fully open.  There are two curtains which open and close when you take a photo and above the max sync speed the second closing shutter starts closing before the opening shutter is fully open. When the flash fires only part of the sensor is exposed and you would see a dark band on the resulting photo.  Modern cameras prevent you from setting a shutter speed faster than max sync speed when you turn on the flash.

What if you need to set a higher shutter speed whilst using the flash? High Speed Sync (Canon) or FP (Nikon) modes get round this by firing the flash for the whole duration of the shutter opening and closing, not just a very short burst.  This way, although at any given moment the shutter is not fully open, the scene is fully illuminated the whole time as the sensor is exposed a piece at a time.  The downside is a reduction in flash power and hence maximum range.

Why might you care?

Sync speed comes up as an issue most often in bright light when using the flash to fill in shadows.  In a bright place, you would need a fast shutter speed to correctly expose the scene, particularly if you want a large aperture to get a nice blurry background.  Typically, if you turn on the flash, the camera will restrict the shutter speed to the maximum sync speed e.g. 1/250s.  If the scene calls for a shutter speed faster than the maximum flash sync speed of your camera then you may find you either get an over exposed photo (because the shutter speed is now too long) or you lose your shallow depth of field (because the camera compensates for the slower shutter speed with a smaller aperture which reduces depth of field).  To get round this use FP/HSS mode if you want to use a flash which will allow you to use a faster shutter speed.

How to turn on FP High Speed Sync

On Nikon cameras, FP mode is turned on in the camera.  Helpfully, its the same menu item on all models.  Go into your camera menu, select Custom Settings and navigate to e1.  You should see an option there for ‘Auto FP’.  Now your camera will go faster than the fastest shutter speed with the flash turned on.

On Canon cameras, High Speed Sync is turned on in the flash using the button on the back of the flash unit.

Your camera and flash must support Auto FP:

Supported Nikon Flashes: SB-600, SB-800, SB-900, SB-28DX, SB-80DX

Supported Nikon Cameras: D80, D90, D200, D300, D7000, D700, D3, D3s, D3X, D2H, D2X,

Supported Canon Flashes: 420EX, 430EX, 430EX II, 550EX, 580EX ans 580EX II

Supported Canon Cameras: EOS-1, 10D, 20D, 30D, 40D, 50D, 60D, 350D,400D,450D,500D,550D, 5D, 5D Mk II, 7D

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