see also:

• flash technology
• flash x-sync in Olympus digital cameras

• the term flash sync is generally implied to mean the fastest shutter speed on a camera at which a flash can be used and all parts of the frame will be exposed by the light from the flash without the shutter mechanism obscuring part of the frame.
• a camera with a fast flash sync allows more versatility and possibilities for flash in bright sunlit situations
• a fast flash sync potentially allows:
  • greater relative flash output compared to ambient light, and perhaps the ability to under-expose sunlit scenes for added drama if need be.
  • lower flash output levels for faster recycle time and longer battery life
  • wider apertures for shallower depth of field (DOF) (although this could be achieved by using ND or polariser filters)

• most modern flash units (aka speedlights/strobes) have a flash duration of about 1/500th sec at its maximum output and shorter as the output is reduced.
• most cameras have a mechanical shutter mechanism at the film plane or sensor plane (“focal plane” or FP) which effectively consist of 2 curtains which are timed to move across the image frame at a given distance apart depending on the shutter speed.
• at fast shutter speeds, at any one point of the exposure, the gap between the two curtains is less than the width of the image frame and thus a flash firing at these speeds will only expose that part of the image frame exposed between the curtain gaps.
• at the “flash sync” shutter speed for that camera, the distance between the curtains is the whole frame width and thus the flash will expose the whole image without cutting parts off.
• thus cameras with such mechanical shutters can generally only be used at shutter speeds at or slower than the flash sync speed when using a flash.
• old analog SLR film cameras such as Olympus OM system often had a flash sync of 1/60th second
• most digital cameras have a flash sync of 1/160th-1/180th sec
• the Olympus OM-D E-M5 camera and most professional dSLRs have a flash sync of 1/250th sec, but disappointingly, the new Canon dSLRs (Canon 1D X sports dSLR and Canon 5D Mark III dSLR) do not!
• some cameras have a shutter mechanism within the lens itself (“leaf shutters”) and this usually allows flash sync at all available shutter speeds (eg. Hasselblad and Mamiya medium format film cameras)

• the main reason to want a fast flash sync is that the faster the shutter speed one can use, the relatively more powerful your flash output becomes in relationship to ambient light - this is particularly important in bright sunlit situations.
• a secondary advantage is allowing wider apertures in bright sunlight when you do not have access to a ND filter of adequate strength.

• imagine you are using the built-in popup flash with a GN 13m at ISO 100 (remember flash-subject distance = GN / required aperture)
• for a sunlit subject, sunny-16 rule gives a typical exposure of f/16, 1/100th sec at ISO 100, so if the flash sync is 1/160th sec, you would need to use f/13 to maintain the sunlit exposure and thus at full flash output to give the SAME exposure as the sun, the flash would need to be placed 1m from the subject.
• now in reality you would just use the flash to fill in the harsh shadows of the sun, so the flash exposure would only need to equate to perhaps f/8, and thus it could be useful at about 1.5m from the subject which is more like a reasonable camera-subject distance.
• now if the flash sync was 1/250th sec instead, the aperture could be set to f/9.5 and thus the flash could be moved to about 2.5m from the subject for fill in flash, or you could now reduce flash output to increase recycle time or battery duration.
• if you want to use a wider aperture, just add ND or polariser filters to allow wider aperture for same exposure.
• a fast flash sync becomes critical in 2 main sunlit scenarios:
  • when you want to use a soft box for softer fill-in flash which will otherwise decrease your flash output on the subject too much
  • when you want to over-power the sun and under-expose the ambient sunlit scene but use a flash to correctly expose your subject

• this is an obvious option HOWEVER, there is still a limitation - the duration of the flash
  • at highest flash output, many have a duration of ~1/500th sec or longer, thus shutter speeds faster than this may give you less flash exposure at those outputs, for example:
    • Profoto Pro-8a 2400Ws studio flash at full output 1/1600th sec with GN 256m?
    • Lencarta SuperFast 600Ws studio flash at full output, 1/800th sec with GN 75m
    • Canon 580EXII: 1/800th sec (1.2msec) at full output; 1/400th sec (2.3msec) at half output in Quick Flash for short recycle times for firing before ready light ready
    • Olympus FL50, FL36R, FL600R: 1/500th at full output down to 1/20000th sec at lowest output
    • Metz 45CL4: 1/300th at full output down to 1/20000th sec at lowest output
    • Metz 54MZ-4i: 1/200th at full output down to 1/20000th sec at lowest output
    • Metz 76MZ5: 1/150th at full output down to 1/20000th sec at lowest output
• cameras with shutters within the lens (leaf shutters) are usually able to sync at all mechanical shutter speeds, for example:
  • Panasonic LX100 compact camera
  • Sony RX1R II 42mp fixed lens full frame with leaf shutter
  • many medium format film cameras (although most older ones have fastest shutters of only 1/500th sec)
• in the near future, cameras with smaller sensors such as Micro Four Thirds system are more likely to be the first with global electronic sensors which would allow flash sync at all speeds as there would be a mode which does not use a mechanical shutter.
• in the meantime, pro versions of digital cameras often have x-sync of 1/320th sec (eg. Olympus OM-D E-M1)

Thus, one can see that there is a very useful capability of under-exposing the ambient and key shifting, but only up to shutter speed of the flash unit's full output duration, faster shutter speeds allow reduction in flash output and thus reduced recycle time and battery life but no further ability to under-expose the ambient light.

The above table is theoretical and assumes flash duration decreases in a linear manner and that there is full synchronisation with shutter duration. Neither of these are likely to be that found in reality, thus actual testing will be needed.

• most cameras (except those such as Panasonic) allow you to ignore the flash sync and shoot a flash at faster shutter speeds.
• this will of course mean that part of your image will not be lit by the flash, but if you compose your frame so that this part does not have any subject within the flash distance range, then all is OK and one will not notice the issue.
• generally this technique is only possible for 1-2 stops shutter speed faster than the flash sync.

• this mode 1st developed by Olympus in the 1980's for their Olympus OM system cameras is now available on most cameras other than point and shoot cameras.
• whilst this mode allows one to use fast shutter speeds up to 1/4000th sec or faster by pulsing the flash output, unfortunately, the effective GN (output) of the flash declines as the shutter speed increases which reduces much of the advantage of the fast shutter speed, but at least it does allow for wider apertures when you don't have a ND filter with you.

• Pocket Wizard radio TTL flash transmitters and receivers can work with some Canon and Nikon dSLRs to optimise the flash sync to a slightly higher flash sync.

• use more powerful flash units or multiple flash units

• this effectively increases the flash exposure whilst keeping ambient exposure the same

• this only helps one to use a wider aperture for shallow depth of field (DOF), it does not change the ratio between ambient and flash exposures.