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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 is f/11 at a distance of 10 feet. We know that if we change the ISO to 200 and do nothing else, we will overexpose the flash by one stop.
Therefore, if we choose to leave the flash at 10 feet, we must close down the aperture by one stop to f/16 to maintain an equivalent exposure. Therefore, the new guide number at ISO 200 is 160, f/16 times 10 feet equals 160. There's a one-stop difference between ISO 100 and 200. There's a one-stop difference between f/11 and f/16. There's a one-stop difference between 11 feet and 16 feet, and there's a one-stop difference between guide number 110 and guide number 160. Now, let's prove this a different way.
Once again, assuming our guide number is 110 at ISO 100. Remember, one combination of aperture and flash to subject distance that will provide a normal exposure is f/11 at a distance of 10 feet. If we change the ISO to 200 and choose to shoot at f/11, we have to move the flash back one stop from 10 feet to 14 feet to maintain an equivalent exposure. Once again, the new guide number at ISO 200 is 160, f/11 time 14 feet equals 154, rounded to 160.
So just to wrap things up, here's the guide number scale on one-third stop increments. Notice how it compares so neatly to the aperture scale. As you can see, the values are very interrelated. Making adjustments to one requires an adjustment to another. But the guide number scale helps you to make these adjustments without having to think about all of the underlying math. As you'll see later in the shoot chapter, working knowledge of this relationship allows me to nail my exposures very quickly and to work with confidence while making adjustments on the fly.
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