Speedlight Basics:

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

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Guide Numbers, Calibration and Application

1Sunday, November 13, 2011

Speedlight Basics:

Shutter speed has no effect on flash exposure.

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

2Sunday, November 13, 2011

Speedlight Basics:

Aperture does affect flash exposure.

All things being equal, the larger the lens opening, a greater amount of light reaches the chip.

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

3Sunday, November 13, 2011

Speedlight Basics:

Flash to subject distance does affect flash exposure.

All things being equal, the closer the flash is to the subject, the greater the amount of light that falls on the subject. If more light falls on the subject, then more light will be reflected through the lens and onto the chip.

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

4Sunday, November 13, 2011

Speedlight Basics:

This leads us to the first fundamental truth about flash exposure...

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

5Sunday, November 13, 2011

Speedlight Basics:

Equivalent flash exposures are made by adjusting flash to subject distance and aperture.

1) When you move the flash closer to the subject (increasing the amount of light falling on the subject), you must compensate by closing down the aperture.

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

6Sunday, November 13, 2011

Equivalent flash exposures are made by adjusting flash to subject distance and aperture.

2) When you move the flash further from the subject (decreasing the amount of light falling on the subject), you must compensate by opening up the aperture.

Speedlight Basics:

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

7Sunday, November 13, 2011

Speedlight Basics:

Apertures and flash to subject distance changes behave predictably. Therefore, informed photographers can quickly calculate how to achieve a desired flash exposure.

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

8Sunday, November 13, 2011

Speedlight Basics:1) Assume the output from a strobe is constant.

If the lens is opened up one stop, the flash to subject distance must increase until one stop less light falls on the subject if you want to maintain an equivalent flash exposure.

We can predict how far to move the flash to achieve a one stop change.

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

9Sunday, November 13, 2011

Speedlight Basics:

Now that we can mathematically describe two of the three variables - flash power (GN) and aperture that control flash exposure, a simple equation allows us to solve for the third variable, the flash to subject distance.

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

10Sunday, November 13, 2011

Speedlight Basics:

GN = Aperture x Flash to Subject Distance

Aperture = GN/Flash to Subject Distance

Flash to Subject Distance = GN/Aperture

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

11Sunday, November 13, 2011

Speedlight Basics:

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

Assume your flash has a GN of 100 and you want to shoot at f/8. What FSD will provide normal exposure?

FSD = GN/Aperture

12.5’ = 100/(f/8)

12Sunday, November 13, 2011

Speedlight Basics:

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

Assume your flash has a GN of 100. Now you want to place your flash 8’ from the subject. What aperture will provide normal exposure?

Aperture = GN/FSD

f/12.5 = 100/(8’)

13Sunday, November 13, 2011

Speedlight Basics:

Guide numbers are affected by FSD changes.

When the flash is moved closer the GN increases (the flash becomes more powerful) and when the flash is moved further away the GN decreases. (the flash becomes less powerful)

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

The Set Up

14Sunday, November 13, 2011

Speedlight Basics:

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

Guide numbers are affected by ISO changes.

When the ISO increases the GN increases (the flash becomes more powerful) and when the ISO decreases the GN decreases. (the flash becomes less powerful)

The Set Up

15Sunday, November 13, 2011

Speedlight Basics:

Michael E. SternPhotographer Educator Radio Host Author

818-422-0696

Build A Better Photograph Dot Com

Guide number changes track the aperture scale.

Memorize the aperture scale and think of f/stops as feet and you’ll become a genius at working with your flash in manual mode and understand how it’s range is limited in iTTL mode.

The Set Up

16Sunday, November 13, 2011

Speedlight Basics:

Equivalent Exposures

Guide Number: 110

22 f/5

20 f/5.6

18 f/6.4

16 f/7.1

14 f/8

12.5 f/9

11 f/10

10 f/11

9 f/12.5

8 f/14

7.1 f/16

6.4 f/18

5.6 f/20

5 f/22

FSD Aperture22’ x f/5 = 110

7.1’ x f/16 = 113

14’ x f/8 = 112

17Sunday, November 13, 2011

Speedlight Basics:

Equivalent Exposures

Guide Number: 110

22 f/5

20 f/5.6

18 f/6.4

16 f/7.1

14 f/8

12.5 f/9

11 f/10

10 f/11

9 f/12.5

8 f/14

7.1 f/16

6.4 f/18

5.6 f/20

5 f/22

FSD Aperture

Notice the relationship between the FSD and aperture to maintain equivalent exposures.

18Sunday, November 13, 2011

22 f/5

20 f/5.6

18 f/6.4

16 f/7.1

14 f/8

12.5 f/9

11 f/10

10 f/11

9 f/12.5

8 f/14

7.1 f/16

6.4 f/18

5.6 f/20

5 f/22

Speedlight Basics:

Equivalent Exposures

Guide Number: 110

FSD Aperture

Notice the relationship between the FSD and aperture to maintain equivalent exposures.

19Sunday, November 13, 2011

22 f/5

20 f/5.6

18 f/6.4

16 f/7.1

14 f/8

12.5 f/9

11 f/10

10 f/11

9 f/12.5

8 f/14

7.1 f/16

6.4 f/18

5.6 f/20

5 f/22

Speedlight Basics:

Equivalent Exposures

Guide Number: 110

FSD Aperture

Notice the relationship between the FSD and aperture to maintain equivalent exposures.

20Sunday, November 13, 2011

22 f/5

20 f/5.6

18 f/6.4

16 f/7.1

14 f/8

12.5 f/9

11 f/10

10 f/11

9 f/12.5

8 f/14

7.1 f/16

6.4 f/18

5.6 f/20

5 f/22

Speedlight Basics:

Equivalent Exposures

Guide Number: 110

FSD Aperture

Notice the relationship between the FSD and aperture to maintain equivalent exposures.

21Sunday, November 13, 2011

100 110

125 125

160 140

200 160

250 180

320 200

400 220

500 250

640 290

800 320

1000 360

1250 400

1600 450

2000 500

Speedlight Basics:

Equivalent Exposures

Guide Number: 110

ISO GN

Notice the relationship between the ISO and GN to maintain equivalent exposures.

22Sunday, November 13, 2011

Speedlight Basics:The ISO Scale

3 4 5 6 8 10 12 16 20 25 32 40 50 64 80 100 125 160 200 250 320 400 500 640 800 1000 1250 1600 2000 2500 3200 4000 5000

ISO changes have a profound affect on guide numbers.

23Sunday, November 13, 2011

Speedlight Basics:The Aperture Scale

1.0 1.1 1.3 1.4 1.6 1.8 2.0 2.2 2.5 2.8 3.2 3.6 4.0 4.5 5.0 5.6 6.4 7.1 8.0 9.0 10 11 12.5 14 16 18 20 22 25 29 32 36 40 45 50 56 64 72 80 90

Think of this as a ruler. (in feet)

24Sunday, November 13, 2011