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Flash Guide Number

Flash guide number is a value used to describe the maximum illumination that a flash unit is able to deliver effectively for any given combination of ISO and Aperture. In the digital age, photographers seldom bother about flash guide numbers. Modern day digital cameras and flash units are so intelligent that they make use of all kinds of metering functions and also communicate with each other. The lens communicates aperture, angle and distance values from camera to the subject, the camera then matches it with that of flash unit taking into account the presence or absence of ambient light, metering mode used, etc and then automatically sets the desired flash power. How very convenient.

Flash Guide Number
Flash Guide Number

However this was not the case not too long ago, there was a time when photographers needed to manually calculate the flash power needed for each and every shot depending upon the guide number of the flash and the distance from the flash to the subject. Even today when needed to set values manually under special conditions a good understanding of how the system works could prove in handy. Moreover if you ever come across a situation in which you will be using flashbulbs then this information becomes invaluable.

When considering the purchase of a flash, there are three very important aspects which a photographer need to consider, they are;

a. Guide Number - Power of the flash unit for a given ISO
b. Recycling Time - Time needed for the flash to fully recharge between flashes.
c. Coverage - Coverage measures the angle of the flash beam. It states how wide or narrow the beam will be.

Traditionally Guide Numbers for flashes are given for ISO 100 and flash to subject distance will be given in either Feet or Meters. To convert values given in meters to feet - Multiply by 3.3 and to convert values given in feet to meters - Multiply by 0.3.

Exposure Calculation

Guide number = Aperture x Distance

If for a given situation you are unsure about which aperture value to use; first measure the distance from the subject to the camera (this information is available on your lens scale) . Then divide the guide number (will be supplied by the flash manufacturer) by the distance.

Flash Guide Number
Flash Guide Number

To Calculate Aperture

Imagine the distance on your lens scale is 8 meters. And guide number for your flash unit is 45 then the aperture value should be.

Guide number (45) = A * 8


A = Guide number (45) by Distance (8)

A = 45/8 = 5.6


Aperture is f/5.6

To Calculate Flash Distance

Using the equation Guide number = Aperture x Distance


Flash Distance = Guide number / Aperture

Inputting the above values of guide number 45 and aperture f/5.6 we get

FD = 45/5.6 = 8

therefore flash distance is 8 meters.

To Calculate Guide Number

If you are using a flash unit for which you are unsure of the guide number set the flash unit to manual and put the flash unit at a distance of one meter from the subject. Take a reading ideally from a flash meter with the meter set for ISO 100. Fire a test shot; at one meter distance the aperture reading that is registered on your flash meter will be your flash units guide number.

if for example say we got a reading of f/11 at one meter. Applying the equation Guide Number = Aperture * Flash Distance

Guide Number = 11/1 = 11 i.e Guide Number = Aperture.

In the next article we will discuss about Photography - Lighting - Flash - Flash Synchronization

Related Reading

  1. Flash
  2. Flash Bulb
  3. Continuous Light
  4. Gig Photography
  5. Neon Light

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