1. Speedlight Basics:
Guide Numbers, Calibration and Application
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 1
2. Speedlight Basics:
Guide Number
A guide number is the numerical expression of the power of a
speedlight at any given ISO.
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 2
3. Speedlight Basics:
Guide Number
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......
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 3
4. Speedlight Basics:
Guide Number
Therefore, manufacturers tend to overstate their GN’s to help
convince you to buy what they are selling.
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 4
5. Speedlight Basics:
Guide Number
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 5
6. Speedlight Basics:
Guide Number
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.
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 6
7. Speedlight Basics:
Guide Number
Be a Rock Star in the flash world, perform a guide number test.
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 7
8. Speedlight Basics:
Guide Number
The guide number formula is the basis for the test you will perform.
GN = Flash to Subject Distance x Aperture
FSD = GN/Aperture Aperture = GN/FSD
You will determine the guide number by making a series of bracketed
aperture exposures while maintaining a constant flash to subject distance.
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 8
9. Speedlight Basics:
Guide Number
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.
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 9
10. Speedlight Basics:
Guide Number
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.
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 10
11. Speedlight Basics:
Guide Number
Why does the GN matter when you shoot in ttl mode?
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.
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 11
12. Speedlight Basics:
Guide Number
Why does the GN matter when you shoot in ttl mode?
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.
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 12
13. Speedlight Basics:
Guide Number
Why does the GN matter when you shoot in ttl mode?
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.
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 13
14. Speedlight Basics:
Summary
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
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 14
15. Speedlight Basics:
Summary
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.
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 15
16. Speedlight Basics:
The Set Up
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 16
17. Speedlight Basics:
The Set Up
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 17
18. Speedlight Basics:
The Set Up
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 18
19. Speedlight Basics:
The Set Up
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 19
20. Speedlight Basics:
The Set Up
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 20
21. Speedlight Basics:
The Set Up
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 21
22. Speedlight Basics:
The Set Up
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 22
23. Speedlight Basics:
The Set Up
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 23
24. Speedlight Basics:
The Set Up
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 24
25. Speedlight Basics:
Guide Number
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.
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 25
26. Speedlight Basics:
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 26
27. Speedlight Basics:
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 27
28. Speedlight Basics:
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 28