Join Brent Winebrenner for an in-depth discussion in this video Understanding why guide number math works, part of Lighting for Photographers: Flash Exposure Fundamentals.
In this movie, I'm going to take a moment to review how the flash to subject distance…and the aperture scales relate to one another.…And then we're going to look at how we can use this information to maintain equivalent…exposures when we're working with small strobes.…Remember, the square root of 2 is an integral part of the Inverse Square Law, which we use…to calculate the change in illumination when the flash to subject distance changes.…It's also an integral part of calculating the size of an aperture.…And an aperture is nothing more than a circle, and you may recall that the formula for the…area of a circle is Pi R squared.…
Because Pi is a constant that doesn't change no matter how big or how small the circle,…we can ignore it while determining the relative change in the size of the aperture.…You probably already know that if we double the area of the circle, we'll create a one…stop increase in the size of the aperture.…And life would really be simple if the formula read area equals Pi times R because each time…
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