Flash Synchronisation Speed

Are you familiar with this problem? The photo was taken using a flash and only a small strip of the film is exposed correctly, the rest is black. Why?

A technical detail of flash photography is the flash synchronisation speed. This speed is dependent on the type of camera used and offers information about the maximum speed of the shutter curtain. What has this got to do with the flash?

The flash sync speed is the shortest amount of time that the camera’s shutter is fully open. Classic sync speeds are between 1/60 seconds and 1/250 seconds. Because the speed at which the shutter can open and close fully pushes the technical boundaries, cameras with focal-plane shutters (nearly all SLR cameras – exceptions are the very few cameras with leaf shutters) use a trick to simulate the effect of faster shutter speeds. For speeds that are faster than the flash sync speed the camera only opens the shutter partially and transmit the image via image sensor or film. Combined with exposures with continuous light, it usually gives the same results as a faster shutter speed would. If there are fast moving objects in the image, this can result motion blurring. 

Modern electronic flash devices generate light using a gas filled tube. Light is only emitted for a very short period of time, usually much shorter than the camera’s shutter speed. The result is only part of surface area that should be exposed, actually being exposed. Due to the fact that modern flash devices emit light over a very short period of time, if a shutter speed is faster than the flash sync speed, only part of the image will be illuminated by the flash.

Ultra-modern electronic flash devices and cameras can sometimes emit numerous flashes (stroboscope flash) during a fast shutter speed. This creates a uniform flash illumination. However, the flash’s range is drastically reduced. This technology is relevant for e.g. portrait photographers who would like to use the flash in daylight and at same time use a larger diaphragm diameter for a shallow depth of field. This technique is called High-Speed-Synchronisation.

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