Guide Number Test Reveals Your Flash's True Power
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My primer on Guide Numbers and how to calibrate a speedlight.
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Guide Number Test Reveals Your Flash's True Power
- 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