Flash sync time

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Flash connection of an SLR camera that can be switched between FP and X synchronization
Leica IIIf - 35mm camera (1951) with flash connection for wired flash synchronization (to the right of the built-in viewfinder)

In photography, flash sync time or sync time (also known as X-Sync ) is the shortest possible exposure time ( shutter speed ) at which the shutter of a camera is briefly fully open and the light from a flash unit ( photo flash ) thus completely covers the entire film image or the image sensor can illuminate. The flash release is synchronized with the shutter so that the shutter is fully open before the flash starts.

The background is that with a focal plane shutter, significantly shorter exposure times can be achieved than its curtains can move - only after a certain minimum, construction-dependent shutter speed is the case that the first curtain is fully open before the second closes (open time ) - only these or slower shutter speeds are suitable for use with a flash.

Typical flash sync times

A focal plane shutter with a shortest X synchronization time of 1/60 s - shading with shorter shutter speeds
Exposure progression for a focal plane shutter with X synchronization.
* A: First shutter curtain opens
* B: Control pulse X-Sync contact
* C: Flash ignites
* D: Flash goes out (much slower)
* E: Second shutter curtain starts
* F: Shutter closed again.

Typical flash sync times of today's 35mm cameras with focal plane shutter are 1/60, 1/125, 1/180 or 1/250 second; Most compact digital cameras are also in this range . Some fast 35mm SLR cameras allow flash sync times of 1/300 of a second.

Many modern cameras use high-speed synchronization (HSS) in conjunction with special flash units to circumvent the limitations of flash sync times and enable flash photography with any adjustable shutter speed, but with reduced flash output and reducing the freezing effect of motion sequences caused by the flash. This technology is implemented through a rapid sequence of several smaller flashes ( stroboscopic effect ) or through a prolonged flash output during the entire shutter speed: uniform lighting must be provided while the shutter slit is being passed through. Most of the light hits the curtains unused. The camera manufacturers have introduced the names FP mode or Super FP mode for this flash mode .

Modern cameras offer a further design tool through long-term synchronization of the flash light to one of the two times after opening the first or before closing the second shutter curtain.

The fastest commercially available digital SLR cameras allow flash synchronization times of 1/500 of a second. Some compact digital cameras also have such short sync times.

For medium format cameras with focal plane shutters, the flash sync time is usually 1/60 of a second or longer due to the larger shutter curtain. However, many medium format cameras use a central shutter (e.g. most products from Hasselblad ) and therefore enable very short synchronization times of 1/500 to 1/2000 of a second.

Analog and digital cameras with focal plane shutter

Modern single lens reflex cameras have typical flash sync times between 1/160 s and 1/320 s. This time (3.2 to 5.5 ms) includes the time the curtains are fully open plus the time the curtains move. The possible time window for triggering and lighting up the 1 ms flash, for example, is therefore shorter.

With studio flashes that burn for a long time (~ 3 ms), these shutter speeds are therefore often too short and have to be extended manually.

Digital cameras with electronic shutter (interline CCDs)

Electronic locks allow slower shutter speeds than those that can be achieved with mechanical locks. This requires other CCD sensors (interline CCDs), which have disadvantages such as reduced full well capacity (therefore usually only start at ISO 200) and significantly increased smearing with short exposure times.

Problems

Fast moving objects in strong ambient light

Regardless of the duration of the flash, the sensor or the film is exposed to ambient light for the duration of the flash synchronization time. If it is sufficiently strong, this causes motion blur and ghosting.

You can reduce this to a certain extent by stopping down or lowering the ISO number or using the full flash output.

Fill-flash with large aperture and strong ambient light

The ambient brightness alone would possibly require a shorter exposure time than the flash sync time. In these cases, overexposure occurs when using a fill-in flash. The primary cause is not the flash, but the ambient light.

This effect can be reduced by

  • Neutral density filter
  • Use of long-burning flashes that again allow short exposure times (HSS flash).

Both lead to a (significant) reduction in the effective flash output, but allow photography with a large aperture with a lot of ambient light and flash use.

Bad flash utilization

For an even exposure of the medium (film, image sensor), one of the two requirements must be met:

  • T flash + T curtain <T exposure
  • T Flash - T Curtain > T Exposure

The first condition (open time) is normally used, in which 100% of the amount of flash light can be used if the camera opening roughly corresponds to the opening of the flash light cone. This is not the case when using telephoto lenses or long focal lengths of a varifocal lens.
With HSS, the second condition is used, in which only the T exposure / T flash part of the light can be used (at 1/8000 s this can only be 2 to 3%).

Often the direct lighting by the flash from (almost) the direction of the camera is perceived as unsuitable: Sharp shadows are created and the plasticity of the face is exaggerated in portraits. Sometimes the flash is then directed against the (light) ceiling of a room, or scatter screens are used (in the studio). This results in a more even illumination, but light is lost and the guide number has to be adjusted. This correction can be omitted with TTL.

Shadowing

Incorrect synchronization or a shutter speed that is too short leads to shadowing when taking flash photos with cameras with a focal plane shutter

Incorrect exposure can occur if the shutter opens too late or closes too early, so that part of the film or the image sensor is shaded by the shutter at the time of the flash . Manufacturers of cameras with focal plane shutters usually specify the shortest possible shutter speed with which correct exposure by a flash unit is still possible. Incorrect exposures are prevented if the camera shutter speed is the same as or longer than the flash sync speed. Many modern cameras prevent the shutter speed from being set too short when a flash is activated.

See also

Web links / individual references

  1. ↑ http://www.fotocommunity.de/info/Blitzsynchronzeit Explanation of open time and HSS