Manual power

The nice thing about setting flash power manually is that it responds to very simple math. Like the inverse square law. Andthat the common shutter speed, aperture and ISO numbers we know are all a stop apart. They lead to tables like this:

SB910/900 or 600/580EX flash. Zoom set to 35mm. Flash held at 2m (6.5’) from subject. Flash not modified.

So if you have a high-end Canon or Nikon flash and you set the Zoom setting to 35mm, when you set your camera to f/16 and ISO value to 100, you get a well exposed picture at about 2 metres (6.5 ft) distance.

A modifier like an umbrella generally takes around 2 stops, so the same table will hold at one metre (half the distance is 4x more light, i.e. two stops more, which would cancel the umbrella’s 2 stops less).

Simple math. And the rest follows simple math, too: increase ISO and you need less power, and open the aperture and you also need less power. As per the table above. A table that can save you a lot of time.


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