Join Brent Winebrenner for an in-depth discussion in this video Understanding how the guide number is used, part of Lighting for Photographers: Flash Exposure Fundamentals.
In this movie, we are going to look at how the guide number is used.…The test we performed and analyzed in the previous movies gives us a guide number that…is unique to our small strobe.…The guide number is similar to exposure value because it considers how much light is available…based on the power output of our strobe and how much light is required by the sensor as…determined by our camera's ISO setting.…Remember, when we did the Guide Number test, the formula that we used was guide number…equals Flash to Subject Distance times Aperture.…
The guide number is constant until you change your ISO.…So, once you know your guide number, you can plug it in to the guide number Formula to…solve for either Flash to Subject Distance or for Aperture.…So, Flash to Subject Distance equals Guide Number divided by Aperture,…or Aperture equals Guide Number divided by Flash to Subject Distance.…When I'm out shooting, I'm either going to select my aperture for depth-of-field reasons…which then dictates how far I put my flash from the subject, or instead I may choose to…
Even with today's automatic flash systems, there are good reasons to understand how flash exposure really works. Brent details these concepts in this course. The course describes how to calculate the true power of your flash and how to modify its output to match your needs, a technique that can extend battery life, reduce recycle time, and provide exposure control that is more predictable than fully automatic modes. The course concludes with several shooting scenarios during which Brent explores the creative use of gels, reflectors, and other light modifiers.
- Understanding exposure basics
- Reviewing aperture, ISO, and shutter speed
- Adjusting strobe power and flash to subject distance
- Finding the guide number of a strobe
- Making calculations based on ISO
- Understanding power ratios