01 gn test

Speedlight Basics:

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

Guide Numbers, Calibration and Application

1Sunday, November 13, 2011

Speedlight Basics:Guide Number

A guide number is the numerical expression of the power of a speedlight at any given ISO.


818-422-0696




The higher the guide number, the more powerful the flash. The more powerful the flash, the “better” it is. The “better” it is, the more you want it. The more you want it, the more you’ll pay......


818-422-0696



Speedlight Basics:

Therefore, manufacturers tend to overstate their GN’s to help convince you to buy what they are selling.


818-422-0696


Guide Number


Speedlight Basics:


818-422-0696


Guide Number


Speedlight Basics:

If the manufacturer’s GN’s are overstated and the flashes aren’t as powerful as they claim, you’ll get underexposed flash images if you rely on their numbers.


818-422-0696


Guide Number


Speedlight Basics:

Be a Rock Star in the flash world, perform a guide number test.


818-422-0696


Guide Number


Guide Number

Speedlight Basics:

The guide number formula is the basis for the test you will perform.

GN = Flash to Subject Distance x Aperture

You will determine the guide number by making a series of bracketed aperture exposures while maintaining a constant flash to subject distance.

FSD = GN/Aperture Aperture = GN/FSD


818-422-0696



Speedlight Basics:

Perform the test using your flash attached to the camera.

Remember, your camera’s EI (chip speed) may be different from the manufacturer’s ISO. Because the actual chip speed determines exposure, your GN test results are only valid for the camera with which the test is performed.


818-422-0696


Guide Number


Speedlight Basics:

Place your flash ten feet from your subject and make a series of flash exposures. Begin at f/20 and open up, in 1/3 stop increments, to f/8. This will result in a total of nine images. Somewhere in this series of images, you’ll find an exposure that is technically correct and esthetically pleasing.

Guide Number


818-422-0696





818-422-0696


Flash exposure is a function of ISO, the strength of the flash and the combination of aperture and flash to subject distance.

Assuming the ISO and the GN remain the same, photographers must maintain an equilibrium between aperture and flash to subject distance.

When the flash to subject distance increases and less light falls on the subject, the photographer must open up the lens to compensate.

Why does the GN matter when you shoot in ttl mode?




818-422-0696


Knowing your guide number and understanding the guide number formulas allow you to trouble shoot your flash results and help prevent you from blowing assignments.





818-422-0696



If your ttl flash exposures are underexposed, the first place to look is your combination of flash to subject distance and aperture, relative to your GN.

Also, if you are using filters on your lens, especially something that absorbs a stop or more of light (a polarizing filter for example) you need to consider the effect of the filter when analyzing your results.


Speedlight Basics:Summary


818-422-0696


A guide number is a mathematical expression of the strength of a flash at any given ISO. If the ISO increases, the GN will increase by the same number of stops.

The higher the GN, the stronger the flash.

Manufacturer’s tend to overstate their GN’s.

The same flash, used on two different cameras, will have two different GN’s if the effective chip speeds of the cameras are different.

The formula for determining a GN is:

GN = Flash to Subject Distance x Aperture


Speedlight Basics:Summary


818-422-0696


You can determine your GN by multiplying the flash to subject distance by the aperture that gives you the “perfect” exposure.

Preserving the detail in the highlights while maintaining shadow detail trumps the mid-tone exposure values when determining the GN.

Knowing your GN and having the ability to apply the GN formula forms a foundation for troubleshooting and analyzing your flash exposures.


Speedlight Basics:The Set Up


818-422-0696





818-422-0696



Speedlight Basics:

This test can also be performed with a flash meter....

Place the camera exactly ten feet from the meter Fire the flash and read the f/stop on the meter. Taking into account the ISO and zoom head setting, the f/stop readout multiplied by ten will provide the calibrated guide number.

Guide Number


818-422-0696



Speedlight Basics:


818-422-0696



01 gn test

Education

guide number michael

speedlight basics

guide numbera guide

guide number test

guide numbers

guide numberbe

guide numberif

guide numberplace