Flash and fast shutter speeds

Started Feb 14, 2011 | Discussions thread
Lemming51 Forum Pro • Posts: 14,642
Re: Flash and fast shutter speeds

mSum wrote:

I'm a total newbie when it comes to flash. Is it possible to have the flash fire when using shutter speeds faster than 1/250 on my 60D? I've been shooting little song birds in the shadows of my backyard and I really just want to use a flash to put a highlight in their eye.

I assume because the sync speed of the flash is 1/250 you should shoot at 1/2500 or the shutter could open and close before the flash fires? Is that the limitation?

Not quite. The flash fires immediately when the shutter opens, and the duration of the flash is fairly short (1/1000 to 1/10,000 sec.). The limitation comes from how a focal plane shutter works. At the x-sync and longer shutter speeds the curtains open to expose the entire sensor at once. At shutter speeds faster than the x-sync the curtains do not open and close faster. Instead the 2nd curtain follows shortly behind the 1st, forming a slit that travels across the sensor. At 1/2000 shutter speed, each portion of the sensor is exposed for 1/2000 of a second, but it still takes 1/250 of a second for the curtains to go across the entire sensor. So the light from a single flash burst will be apparent only on the portion of the sensor uncovered by the two curtains at the time the flash fires.

The 60D's built-in flash can only do the single burst for flash. To use flash at higher shutter speeds requires an accessory speedlite capable of FP High Speed Sync, like Canon's EX series (eg. 430EX II). Set for high speed sync, the speedlite pulses while the shutter curtains are crossing the sensor.

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