I started to write this one up as the next “photography myths” post and had planned some demo shots, however Neil Van Niekerk explained it all brilliantly on his blog here: Tangents back in 2009.   Go there now and read it!  Neil’s blog is full of great articles on using flash, and a whole boatload of other photographic technique.   Here’s my summary, but for more detail – read Neil’s blog!

The next “myth and misdirection” you will come across as a photographer learning how to use flash, especially with natural/ambient light is this phrase:-

“Shutter speed controls ambient, aperture controls flash only”

Which is nonsense.  If you stop down your aperture, it affects all light in the scene – there’s no special “back door” for the ambient light.  The whole picture will get darker.

For manual controlled flash power within normal x-sync range of shutter speeds:-

• Shutter speed controls ambient light
• Aperture controls flash and ambient light
• ISO controls flash and ambient light
• The flash power adjustment buttons on your light, control…. flash!

For TTL automatic flash exposure systems (camera on manual, flash on TTL ) the flash exposure is constant, as long as it is within the bounds of the flash gun to supply enough light, and the camera and subject remain reasonably still in relation to each other.  Close down the aperture?  The flash puts out more light so the flash exposure is the same.  Raise the ISO?  The flashgun reduces the power so the flash exposure remains the same.  Move the light?  The flashgun adjusts and the flash exposure remains constant.  The ambient light doesn’t react to these changes in shutter speed and aperture though, so ambient light exposure continues to change as before.  Stop down the aperture?  The ambient light exposure reduces – flash exposure stays the same.

So for TTL flash with normal x-sync:-

• Shutter speed controls ambient light
• Aperture controls ambient light
• ISO controls ambient light
• Flash exposure compensation controls – flash.

and finally, for “high speed sync” flash where the flash pulses to cover the entire shutter operation, the flash effectively becomes a continuous light so:-

• Shutter speed controls ambient light and “flash”
• Aperture controls ambient light and “flash”
• ISO controls ambient light and “flash”
• Flash power controls – “flash”.