Flash Synchronization

Flash synchronisation, commonly abbreviated as flash sync, refers to the precise coordination between the firing of a photographic flash and the opening of a camera shutter so that light from the flash reaches the photographic film or electronic image sensor at the correct moment. Proper synchronisation is essential to ensure even illumination of the image and to avoid artefacts such as partially exposed frames. Flash sync has evolved alongside camera shutter technologies, from early mechanical systems designed for flashbulbs to sophisticated electronic timing circuits used in modern digital cameras.
Flash synchronisation plays both a technical and creative role in photography. It allows photographers to balance artificial flash with ambient light, freeze motion, control depth of field, and deliberately combine sharp flash-lit subjects with motion blur from longer ambient exposures.

Principles of Flash Synchronisation

The fundamental requirement of flash synchronisation is that the flash must fire when the shutter is admitting light to the film or sensor. In cameras with mechanical shutters, this is traditionally achieved using an electrical contact within the shutter mechanism. This contact closes a circuit at a specific point in the shutter’s travel, triggering the flash unit.
In electronic digital cameras, synchronisation is typically managed by a programmable electronic timing circuit. In some designs, this electronic control still receives input from a mechanical shutter contact, while in others it operates entirely electronically. The flash unit is connected to the camera either by a cable using a standardised coaxial connector, such as the Prontor-Compur connector defined in ISO 519, or via electrical contacts in a hot shoe accessory mount.

Shutter Types and Synchronisation Constraints

The type of shutter used in a camera has a significant influence on flash synchronisation capabilities. Leaf shutters, which open from the centre outward and then close again, typically allow flash synchronisation at all shutter speeds because the entire frame is exposed simultaneously.
By contrast, focal plane shutters, commonly used in single-lens reflex (SLR) and digital SLR cameras, consist of two curtains that travel across the film or sensor. At slower shutter speeds, the first curtain fully opens before the second curtain begins to close, allowing the entire frame to be exposed at once. At faster shutter speeds, however, the second curtain begins closing before the first has fully opened, resulting in a narrow slit that moves across the frame. Under these conditions, a single brief flash would only illuminate part of the image.
Because of this limitation, focal plane shutters impose a maximum shutter speed at which conventional flash can be used. This speed is known as the X-sync speed and is typically around 1/60 to 1/250 second, although some modern cameras achieve higher values.

X Synchronisation (X Sync)

X sync, short for xenon synchronisation, is the simplest and most widely used flash sync mode. In this mode, the flash fires at the exact moment when the shutter is fully open. Xenon electronic flash units produce an extremely brief burst of light and respond almost instantaneously, making them ideally suited to this timing.
X sync is designed specifically for electronic flash systems and remains the standard synchronisation method in modern photography. In cameras with focal plane shutters, X sync limits the maximum usable shutter speed for normal flash operation. Typical X-sync speeds are around 1/60 second, though some advanced cameras reach 1/500 second or higher. Electronic shutters used in certain digital cameras may avoid this limitation altogether, allowing very high X-sync speeds.

Flash Bulb Synchronisation Modes

Before the widespread adoption of electronic flash, photographers relied on single-use flashbulbs, which required a short delay to reach peak brightness after ignition. To accommodate these characteristics, cameras were equipped with various bulb synchronisation modes that closed the shutter contacts slightly before the shutter fully opened.
Common bulb synchronisation modes included:

• S sync (slow), used for bulbs with a relatively long rise time
• M sync (medium), designed for class M bulbs that reached peak brightness approximately 20 milliseconds after ignition
• F sync (fast), intended for class F bulbs that peaked around 5 milliseconds after ignition

These modes ensured that the bulb reached maximum brightness at the moment the shutter exposed the film.

FP Synchronisation

FP sync, also known as flat-peak synchronisation, was developed for use with FP flashbulbs in conjunction with focal plane shutters. FP bulbs burned at near-constant brightness for a relatively long duration, allowing them to illuminate the entire frame as the narrow slit of the focal plane shutter travelled across the film.
Although each portion of the film was exposed for a very short time, the overall exposure of the frame occurred over a longer interval determined by the shutter curtains’ travel time. FP sync made it possible to use flash at shutter speeds far faster than conventional X sync, albeit with bulky and inefficient flashbulbs. Classic cameras such as the Nikon F offered FP, M, and ME bulb synchronisation options alongside X sync.

V Synchronisation

Some leaf-shutter cameras, such as those equipped with the Friedrich Deckel Synchro-Compur shutter, included a V sync mode. The letter V derives from the German term Vorlauf, meaning delay. This mode was typically used in conjunction with a self-timer, allowing the flash to fire after a predefined delay suitable for certain flashbulbs or photographic techniques.

High-Speed Synchronisation (HSS)

High-speed synchronisation (HSS) is a modern solution that allows electronic flash to be used at shutter speeds faster than the camera’s normal X-sync limit. Instead of emitting a single, brief burst of light, an HSS-capable flash unit produces a rapid series of lower-power pulses over a short time interval.
This extended light output effectively acts as a continuous light source while the focal plane shutter slit travels across the sensor. As a result, the entire image area receives illumination even though the shutter is never fully open at any one moment. HSS enables photographers to use wide apertures in bright conditions, control depth of field, and freeze action while retaining the benefits of flash fill. The trade-off is reduced flash power and shorter effective range.

Rear-Curtain Synchronisation

Rear-curtain synchronisation, also known as second-curtain sync, is a creative flash mode available on many modern digital SLRs and mirrorless cameras. In this mode, the flash fires just before the shutter closes rather than immediately after it opens.
When used with a slow shutter speed and a moving subject, rear-curtain sync produces motion blur that appears behind the subject, followed by a sharp, flash-lit image at the end of the exposure. This effect creates a more natural sense of movement, making it particularly useful for conveying speed or direction in dynamic scenes.

Creative and Practical Applications

Flash synchronisation is not only a technical requirement but also a creative tool. Faster shutter speeds are often used in situations with significant ambient light to provide flash fill for backlit subjects, reduce motion blur, or allow the use of smaller apertures to increase depth of field.
Conversely, deliberately slow shutter speeds combined with flash can record ambient light motion blur while preserving a sharp subject illuminated by the flash. This technique is widely used in sports, dance, and night photography to create dynamic and expressive images.