What limits a camera’s maximum flash sync speed?
Asked 10/31/2013
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For focal-plane shutters, the maximum flash sync speed is the fastest shutter speed at which the first curtain is fully open before the second curtain starts closing, so the whole sensor is exposed at once. What design or physical factors limit how fast that sync speed can be? Why can’t cameras simply make it much faster without relying on high-speed sync (HSS)?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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The shutter sync is limited simply by how fast the shutter can move in the same way there is a limit to how high a car engine can rev. Increasing these limits increases the demands placed on materials, design and longevity.
Another limit is the distance the shutter must travel (which is determined by the size of the sensor, a full frame shutter has to travel 24mm whereas an APS-C only has to travel 16mm in the same time (this is why some full frame models have slower sync speed).
A way round this limit is to move the shutter from the sensor or film plane to inside the lens. So called "leaf" shutters only have to travel the length of the lens aperture, which is often only a few mm, hence leaf shutters allows substantially faster sync speeds, although sync speed becomes aperture dependant - the wider the aperture the slower the sync.
The final way to break the sync limit would be to move from the mechanical to the electronic domain. Ultra fast electronic shutters already exist, but are expensive and thus reserved for specialist applications.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
12y ago
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The main limit is the focal-plane shutter itself: the curtains can only accelerate, travel, and stop so fast before cost, reliability, vibration, and durability become problems. Higher sync speeds demand stronger materials, tighter tolerances, and more robust mechanisms.
Sensor size also matters. A larger format shutter has to travel farther in the same time, so full-frame cameras often have lower sync speeds than smaller-sensor cameras.
Design trade-offs matter too. Manufacturers may prioritize overall shutter durability, cost, and top shutter speed rather than pushing x-sync higher—especially since HSS provides a workaround, even though it reduces effective flash power.
A different shutter design can avoid some of these limits. Leaf shutters, built into the lens, move over a much shorter distance and can usually sync at much higher speeds. Their sync behavior can vary with aperture.
Electronic shutter options are also limited. Many CMOS sensors are not true global shutters, so they can’t expose the entire frame simultaneously in the way needed for normal flash sync. Specialized global-shutter designs can help, but they are not the norm.
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