The ISO in Flash Photography (shown in a Live Shoot)

This is the 2nd episode of a series of 3 where I explain you the role of each Exposure Triangle factor in Flash Photography. Here we are looking at the ISO where the Flash is the only light source for our shot. If you have not seen the previous episode I invite you to do so:

• The Shutter in Flash Photography (shown in a Live Shoot)

If you do not know how the ISO affects your photography (without flash) I invite you to check the episode: The ISOthat I did in early 2012. In flash photography, ISO could be considered as the master switch for your exposure. Let's see what I mean with a few illustrating photographs. In the photo below you can see the subject is correctly exposed and the ambient light is not recorded by your camera. This was achieved with the Canon 5D III, 24-105mm lens, Flash power 1/2, Flash Zoom 28mm, Aperture f6.3, Shutter speed 1/200sec, ISO 400.

If we then only increase the ISO by three stops we get ISO 3200. You can see the resulting photograph has now more details from the overall scene (the ambient light was more recorded by the camera) and our subject is now over-exposed:

If we then decrease the ISO value down to 100, change nothing else and take an exposure the subject will then become un-exposed and the ambient light not recorded:

Conclusion

I hope you now understand why I compared the ISO as a master switch for your Flash Photography exposure. By changing its value, you affect the entire exposure: the part of the scene lit by your flash as well as the rest of the scene lit by the ambient light.

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