Join Brent Winebrenner for an in-depth discussion in this video Using strobe power and the flash-to-subject distance, part of Lighting for Photographers: Flash Exposure.
The amount of illumination falling on a subject is a function of two variables, the power…of the flash, and the flash to subject distance.…We can control the amount of light falling on the subject by adjusting either one of these variables.…We're going to talk about the power output of small strobes in great detail when we discuss…guide numbers and power ratios, but first let's tackle the Inverse Square Law and learn…how to predict the effect that changing our flash to distance has on illumination.…At first this may seem like a lot of math, but stick with me and you'll see how this…pays off once you understand how it works.…
Intuitively, we know that the closer we move the light to our subject, the more illuminated the subject becomes.…Unfortunately, this is not a linear progression.…On the face of it, it seems that if you cut the flash to subject distance in half, you…would double the amount of light falling on the subject.…Unfortunately, this is not true.…Instead the change in illumination is inversely proportional to the change in the flash to subject distance.…
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