Why can’t a normal flash exceed X-sync just by staying on during shutter travel?
Asked 4/7/2015
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I understand flash sync like this: with a focal-plane shutter, up to the camera’s X-sync speed there is a brief moment when the whole sensor is uncovered, so a normal flash can fire once and light the entire frame. Above that speed, the second curtain starts closing before the first is fully open, leaving only a moving slit, so standard flash produces banding unless you use high-speed sync (HSS).
My question is: since many flashes have durations around 1/1000 s or longer at full power, why can’t a normal flash simply remain on while the slit travels across the sensor? Is there any effective “steady output” portion of a flash pulse that could illuminate the whole frame at faster shutter speeds, or is the flash output too uneven during the pulse?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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In principle, your rationale is correct. However, there is no usable period during which an ordinary single flash is emitting at constant power.
The power of a typical on-camera flash quickly increases from zero to its maximum value in about 0.1 ms (i.e. 1/10 000th s). Then it exponentially decreases with a half-life of roughly 1 ms; i.e., it decreases to half its maximum value after about 1 ms (i.e. 1/1000th s) and to quarter its maximum value after about 2 ms. (At lower power settings of the flash, the curve may be cut off early.)
That is why a high-speed sync flash setting fires repeatedly at roughly 50 kHz (i.e. 50 000 pulses per second). Thus, the individual pulses overlap and create an approximately constant light source.
Originally by user32543. Source · Licensed CC BY-SA 4.0
user32543
11y ago
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A normal flash pulse is not a flat, steady light source. It rises very quickly, then decays, with most of the light emitted near the start of the burst and a fading “tail” afterward. So if a focal-plane shutter is exposing the sensor through a moving slit, different parts of the frame would receive different amounts of light, causing an exposure gradient or banding.
That’s why ordinary flash is limited to X-sync: the whole sensor must be uncovered at once for a single pulse to illuminate it evenly.
HSS works differently: instead of one decaying pulse, it emits many rapid pulses that overlap closely enough to approximate a continuous light source while the slit moves across the sensor.
There is a related technique often called tail-sync, HyperSync, Hi-Sync, or SuperSync, where triggers time a manual flash or strobe so the tail of the flash is used during shutter travel. It can allow somewhat faster-than-sync shooting, but because the output is still uneven, it often produces gradients and depends heavily on flash characteristics and timing precision.
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