# Guide number and exposure [duplicate]

I am having a bit of a difficulty calculating this.

Guide#= 32 feet at ISO 100

X sync speed= 1/250 Min ISO= 100 Max ISO= 800

What exposure values to expose to capture subject that's far away? What exposure values for closest possible subject? What are max and min distances of subjects that can be properly exposed?

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## marked as duplicate by mattdm, John Cavan♦, Paul Cezanne, MikeW♦, NickMMar 18 '13 at 16:11

Ignore shutter speed. As long as it's at or under the sync speed, it will be fine, and changing it won't affect the flash exposure.

Assume ISO 100 for a start.

Guide number of 32 divided by f/stop gives you the distance to your subject.

• f/8: 32 ÷ 8 = 4 feet
• f/4: 32 ÷ 4 = 8 feet
• f/2: 32 ÷ 2 = 16 feet

If you bump up ISO, you can extend the range. ISO 400 will double your range, so at f/2 you could reach 32 feet.

As for minimum distance, you can reduce power of the flash. The guide number of 32 is at full power. So lower it if you have subjects very close.

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