Help with guide numbers when using a flash manually

Started Apr 14, 2013 | Discussions thread
Sailor Blue
Sailor Blue Forum Pro • Posts: 12,214
Re: Help with guide numbers when using a flash manually

The "At ISO 100 in meters" means if you set the ISO of your camera to 100 you can figure the aperture by using the guide number at the flash (and lens) zoom setting and the distance in meters if the flash fires at full power.

If you had read the reference I gave you then you should have been able to understand this and how to do the calculations yourself but here is one example just to help you along.

Assuming you are at ISO 100, using a 50mm lens, the flash has zoomed to 50mm, and the subject is at 13' (about 4 meters).

Aperture = GN/Distance = 34 meters/4 meters = 8.25 which is closest to f/9.1 (i.e. f/8.3).

If you increase the ISO to 400 (two stops greater sensitivity) and reduce the flash power to 1/4 (two stops less power) you would still be shooting at f/9.1 but you flash batteries would now last four times as long, the flash would recharge much faster, and the flash would be much less likely to overheat if you took multiple flashes as fast as possible.

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