Join Brent Winebrenner for an in-depth discussion in this video Making calculations based on ISO, part of Lighting for Photographers: Flash Exposure.
One of the more challenging things for newcomers to wrap their heads around is the notion that…when you change your ISO, your guide number has to change with the same number of stops…in the same direction.…Increasing the ISO has a same effect as physically increasing the illumination level.…We know this is true because to make an equivalent exposure, we have to adjust our shutter speed…or our aperture to allow less light to reach the sensor.…The same thing is true of our flash.…Since we can't use the shutter to reduce the amount of strobe light that reaches a chip,…we either have to stop down the lens or increase the flash to subject distance when we increase the ISO.…
Increasing our ISO by one stop has a same effect as increasing the power of the flash by one stop.…So, when our ISO changes, our guide number has to change by the same number of stops in the same direction.…Let's take a look at an example and assume our guide number is 110 at ISO 100.…One combination of aperture and flash to subject distance that provides a normal flash exposure…
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