Slow synchronisation

Generally cameras flashes use shutter speeds of ca. 1/250 to 1/30 seconds. These speeds depend mainly on the flash synchronisation speed   of the camera. The basic idea behind it is that the flash along with the available light illuminates the subject to such an extent that a shutter speed that won't cause motion blurring and can be used without requiring use a tripod. Unfortunately these fast shutter speeds usually end up not using the available light to its full capacity and the light from the flash generally dominates the image unfavourably.  The slow shutter synchronisation is different. Here slower shutter speeds are intentionally chosen in to order to use more available light for the illumination.

When should you use a slow shutter synchronisation?

  • If natural lighting mood is to be captured
  • To avoid a dark background (more on this problem can be found in the article on the law of quadric reciprocity (Background is too dark)
  • To use motion blurring for creative purposes (Attention! Not always desired)
  • For zoom effects

How is the slow shutter synchronisation activated on cameras?
o	Einstellung Nachtporträt – Beispiel Pentax K-m [Abbildung: Pentax] Night portrait
The simplest option to use to take flash photographs with the slow sync is the subject mode "night portrait". Nearly every modern camera (excluding some professional models) offers this function setting. This also indicates the significance manufacturers place on slow sync. Often this setting is on the main dial of the camera. The symbol for this mode is usually a person with a moon or star – see image to the left.

Einstellung Zeitautomatik – Beispiel Pentax K-m [Abbildung: Pentax]Aperture priority (A, AV)
Many cameras automatically use a slow sync while in aperture priority mode. Attention – Some cameras have default setting to a certain shutter speed for flash images. This setting could prevent you from using a slow sync in aperture priority mode. The shutter speed is not changed when the flash is activated.

Einstellung Manueller Modus – Beispiel Pentax K-m [Abbildung: Pentax]Manual mode (M)
In manual mode (M) you can adjust diaphragm and shutter speed according to your requirements. Pay attention to the camera's exposure meter. If you activate the flash additionally, you should choose a time/diaphragm combination that will result in underexposure/precise exposure. On TTL flash devices the flash will ensure sufficient image brightness within the limits of its guide number. The closer your time/diaphragm settings are to those suggested by your camera as optimal exposure, the more the available light will be visible in the image. The further away your time/diaphragm settings are from the suggested settings, the less impact the available light will have and the more the flash's light will dominate the image.

Problems with slow shutter synchronisation
The slower shutter speeds that are used for slow sync increase the risk of blurring and can lead to motion blurring if moving objects are involved. The risk of blurring can be dispelled by using a tripod. The motion blurring can also be used as a creative tool or it can be reduced by adjusting time/diaphragm/ISO combination. Often however, motion blurring is perceived to be disruptive. Therefore at e.g. weddings, take images with faster shutter speeds as well as moody slow sync images.

When using slow sync another camera setting is noticeable: the synchronisation of the first or second shutter curtain.

More information can be found in the article "First shutter curtain / Rear (second) shutter curtain"

More inspiration for slow shutter synchronisation can found here "Photo tip "Slow shutter synchronisation" 

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