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Understanding how the guide number is used

From: Lighting for Photographers: Flash Exposure Fundamentals

Video: Understanding how the guide number is used

In this movie, we are going to look at how the guide number is used. The test we performed and analyzed in the previous movies gives us a guide number that is unique to our small strobe. The guide number is similar to exposure value because it considers how much light is available based on the power output of our strobe and how much light is required by the sensor as determined by our camera's ISO setting. Remember, when we did the Guide Number test, the formula that we used was guide number equals Flash to Subject Distance times Aperture.

Understanding how the guide number is used

In this movie, we are going to look at how the guide number is used. The test we performed and analyzed in the previous movies gives us a guide number that is unique to our small strobe. The guide number is similar to exposure value because it considers how much light is available based on the power output of our strobe and how much light is required by the sensor as determined by our camera's ISO setting. Remember, when we did the Guide Number test, the formula that we used was guide number equals Flash to Subject Distance times Aperture.

The guide number is constant until you change your ISO. So, once you know your guide number, you can plug it in to the guide number Formula to solve for either Flash to Subject Distance or for Aperture. So, Flash to Subject Distance equals Guide Number divided by Aperture, or Aperture equals Guide Number divided by Flash to Subject Distance. When I'm out shooting, I'm either going to select my aperture for depth-of-field reasons which then dictates how far I put my flash from the subject, or instead I may choose to place my flash a certain distance from the subject because of space constraints or for creative reasons.

That then determines my aperture. So, you know your guide number. Then you're going to choose either your aperture or your flash to subject distance. Once you've done that, solving for the missing variable is easy. To help remember how these three variables relate to one another, we can think of them as a triangle. For example, let's say our guide number is 100. If we're shooting at f/8, we simply divide 100 by 8 giving us a flash to subject distance of 12.5 feet. If, on the other hand, we decide to put our flash 16 feet from the subject, we'll end up shooting at f/6.3.

With practice and a thorough working knowledge of the aperture and flash to subject distance scales, this process becomes second nature. It's helpful to remember that the numerical structure of the flash to subject distance and the aperture scales are identical because they're based on the square root of two. So, now let's try a practical application to see how this all fits together. As you can see, we're just about set up to take a quick head and shoulder portrait shot of Gabby, our model here.

But what we're going to do right now is illustrate the interchangeability of the aperture and flash to subject distance. And we're going to start by dividing my guide number of 90 on this flash by the aperture that I want to start shooting at, and that's f/22. So, when I do that division, I know that I have to place my flash 4 feet from the subject. So, I do that. I make sure it's aimed in the right direction. Confirm that I'm shooting at 22.

Give me a big smile, that's nice! And as expected, we get a beautiful perfectly-exposed image. But I can tell that I blew her away with that full power flash pop from 4 feet away. So, I could address that simply by reducing the illumination on the flash or the power on the flash and opening up the lens. But because the purpose of this exercise is to illustrate the interchangeability of flash to subject distance and aperture, we're going to take that approach to doing this.

So, what I want to do is reduce the amount of illumination on Gabby by 3 stops. And to do that, I'm simply going to walk the flash back from 4 feet to 5.6 feet to 8 feet, and finally to 11 feet. I'm going to go back to my camera, and I'm going to open up the lens from f/22 to f/16 to f/11 to f/8, and I'm going to fire off another exposure. And indeed, I get another perfectly illuminated picture.

The skin tones are identical. What you're going to notice is that the background tonality is changed. You may have also noticed that I didn't do any division after I placed my flash initially. And the reason that I am able to get away with that is because I've become so familiar with the flash to subject distance scale, and the aperture scale, and I know how they interrelate. When I started out, I did indeed do the long- hand math, and that's probably the way that you're going to get started. But no matter which approach you take, as long as you do it correctly, you're going to get good exposures.

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