Why is flash sync speed slower than flash duration?
Asked 7/22/2019
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My EOS RP has a maximum flash sync speed of 1/180 s, yet a typical speedlight’s flash duration can be around 1/1000 s or shorter at lower power. If flashes are so brief, why can’t the camera use flash normally at shutter speeds faster than 1/180 s? Is the limit caused by flash timing precision, or by how the shutter works?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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Cameras that have a mechanical focal-plane shutter have two curtains, a front and a rear. For longer exposures, the front curtain opens and starts the exposure, then the rear curtain closes to end the exposure.
The mechanical shutter is relatively slow, so to create a quick exposure, the rear curtain must start closing before the front curtain fully opens. So, effectively, a slit travels across the sensor exposing each section for the desired amount of time.
The sync speed (1/180 for your camera) is the fastest speed where the sensor is fully exposed. That is, a slit is not used.
Some speedlights offer a high-speed sync option. In this mode, the flash creates a quick burst of tiny flashes so that the sensor is exposed equally as the slit travels across the sensor.
Originally by user84623. Source · Licensed CC BY-SA 4.0
user84623
6y ago
0
Generated from our catalog & community — verify before relying on it.
Flash sync speed is limited mainly by the camera’s shutter, not by the flash duration.
Most interchangeable-lens cameras use a mechanical focal-plane shutter with two curtains. At slower speeds, the first curtain opens fully, the whole sensor is uncovered, the flash fires, then the second curtain closes. At faster speeds, the second curtain starts closing before the first has fully opened, so only a narrow slit travels across the sensor.
A normal flash burst is very short, so if it fires while that slit is moving, only the part of the sensor currently uncovered gets lit, causing a partially dark frame. The sync speed is therefore the fastest shutter speed at which the entire sensor is fully uncovered at one instant.
The curtain travel itself typically takes a few milliseconds, which corresponds to sync speeds like 1/180, 1/200, or 1/250 s. Faster selected shutter speeds are created by changing the gap between the curtains, not by making the curtains move much faster.
High-speed sync works around this by pulsing the flash repeatedly so light is present during the slit’s travel, but with reduced efficiency.
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