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Understanding power ratios

From: Lighting for Photographers: Flash Exposure Fundamentals

Video: Understanding power ratios

At this point in the course, we've discussed how the scales are calculated. We know what a guide number is and how the math allows us to create equivalent flash exposures by making offsetting moves with our ISO, our flash to subject distance, and our aperture. We've seen examples of how it works in a controlled setting. We'll also see how to apply this information during a live model shoot. Troubleshooting lighting problem shouldn't be a trial and error, hair-pulling, time-wasting, iterative process of elimination. Now, we're going to end our primer on small strobes by introducing power ratios.

Understanding power ratios

At this point in the course, we've discussed how the scales are calculated. We know what a guide number is and how the math allows us to create equivalent flash exposures by making offsetting moves with our ISO, our flash to subject distance, and our aperture. We've seen examples of how it works in a controlled setting. We'll also see how to apply this information during a live model shoot. Troubleshooting lighting problem shouldn't be a trial and error, hair-pulling, time-wasting, iterative process of elimination. Now, we're going to end our primer on small strobes by introducing power ratios.

Power ratios are manual settings on the flash that allow you to reduce the guide number by shooting at less than full power. They provide another level of control that you can use to make your manual flash more versatile. Well, small strobes and strobe packs give you the option of shooting at full power or the variety of lower outputs. In the case of most small strobes, you can adjust the flash in third stop increments all the way down to 164th power. This represents a six-stop exposure range that is achieved by decreasing the duration of the flash pop.

This allows you to save batteries, reduce recycle time, use a shallower depth of field, work in smaller spaces, or quickly manage lighting ratios and complex scenes if you're using multiple strobes. You'll see this in action in the next chapter. But now let's tie this back to guide numbers and ISO. When you change the power ratio, you're changing the guide number by an equal number of stops. This concept is simple and familiar. The application of the theory would be just as easy if ISOs and guide numbers shared the same numerical scale.

Unfortunately, they don't. Let's say your flash has a guide number of 100 at an ISO of 100. On the face of it, logic demands that if we reduce the power output by one stop by cutting the power ratio in half, the new guide number should be 50. But it's not. When we review the guide number scale, we see that 50 is two stops below 100. Instead, the new guide number is 71, which is one stop lower than guide number 100. So, you have to do a little mental gymnastics and remember that a one stop change on the guide number scale is based on a factor of 1.4, whereas a one-stop change on the ISO scale is based on a factor of 2.

In actual practice, since I was born lazy, I do my guide number math at my ISO at full power. Then if I decide to adjust the power ratio downward after I've begun shooting, I'll open up my aperture or move my flash closer to match the reduction in power. In the next chapter, you'll see these concepts in a live shoot setting. I'll be making adjustments and calling out details as I go. Hopefully the shoot movies will strongly illustrate the point that once you've got some practice, determining flash exposure can become almost second nature.

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