Diaphragm, Exposure and ISO rate in flash photography

In flash exposure, the flash’s illumination is determined by flash performance, diaphragm and ISO rate. For the flash sync speed, the exposure has no influence on the illumination caused by the flash because modern electronic flashes only emit light for a very brief period and the lengths of time are well within the camera’s exposure time. However, exposure time has an impact on the overall exposure.

In flash exposure, the flash’s illumination is determined by flash performance, diaphragm and ISO rate. For the flash sync speed, the exposure has no influence on the illumination caused by the flash because modern electronic flashes only emit light for a very brief period and the lengths of time are well within the camera’s exposure time. However, exposure time has an impact on the overall exposure. The exposure time (combined with diaphragm and ISO rate) determines the amount of available light and its effect. The closer a combination of exposure, diaphragm and ISO rate is to a setting that would result in the correct exposure if a flash were not involved, the more the available light comes to the forefront and the light from the flash is pushed to the background. The keyword here is slow shutter synchronisation – more information in the article “Slow shutter synchronisation“.

The relevance of ISO rate, diaphragm and shutter speed in flash photography

ISO rate

  • By increasing the ISO rate the flash’s range is extended
  • Additionally the impact of available light can be increased (comparable to slow sync)

Diaphragm

  • The diaphragm’s diameter also has the effect of extending the flash’s range 
  • The diaphragm setting can also influence how the available light is perceived
  • Influence on flash’s range – see formula for guide number – link article

Exposure (time) / Shutter speed

  • Exposure has an impact on how the available light is affected (see slow sync also) if the shutter speeds are slower than the flash sync speed
  • The background on flash photographs is often relatively dark as the illumination from the flash loses intensity the further away the objects are (see also „law of quadric reciprocity“). A slow sync with fast shutter speeds can remedy this as it increases the impact of available light
  • If the shutter speeds fall below the flash sync speed, generally (exception is fast shutter synchronisation) only part of the image is illuminated by the flash. It usually results in undesirable partial exposure where only a strip of the image is correctly exposed.
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