A review of Guide Numbers.

1. Speedlight Basics:
Guide Numbers, Calibration and Application
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 1

2. Speedlight Basics:
Shutter speed has no effect on flash exposure.
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 2

3. 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. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 3

4. 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. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 4

5. Speedlight Basics:
This leads us to the first fundamental truth about
flash exposure...
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 5

6. 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. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 6

7. Speedlight Basics:
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.
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 7

8. 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. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 8

9. 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. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 9

10. 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. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 10

11. Speedlight Basics:
GN = Aperture x Flash to Subject Distance
Aperture = GN/Flash to Subject Distance
Flash to Subject Distance = GN/Aperture
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 11

12. Speedlight Basics:
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)
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 12

13. Speedlight Basics:
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’)
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 13

14. Speedlight Basics:
The Set Up
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. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 14

15. Speedlight Basics:
The Set Up
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)
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 15

16. Speedlight Basics:
The Set Up
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.
Michael E. Stern
Photographer Educator Radio Host Author
818-422-0696
Build A Better Photograph Dot Com
Sunday, November 13, 2011 16

17. Speedlight Basics:
FSD Aperture
22 f/5 22’ x f/5 = 110
20 f/5.6
18 f/6.4
16 f/7.1
14 f/8 14’ x f/8 = 112
Equivalent Exposures
12.5 f/9
11 f/10
Guide Number: 110
10 f/11
9 f/12.5
8 f/14
7.1 f/16 7.1’ x f/16 = 113
6.4 f/18
5.6 f/20
5 f/22
Sunday, November 13, 2011 17

18. Speedlight Basics:
FSD Aperture
22 f/5
20 f/5.6
18 f/6.4
16 f/7.1
14 f/8 Notice the relationship
Equivalent Exposures
12.5 f/9 between the FSD and
11 f/10 aperture to maintain
Guide Number: 110
10 f/11 equivalent exposures.
9 f/12.5
8 f/14
7.1 f/16
6.4 f/18
5.6 f/20
5 f/22
Sunday, November 13, 2011 18

19. Speedlight Basics:
FSD Aperture
22 f/5
20 f/5.6
18 f/6.4
16 f/7.1
14 f/8 Notice the relationship
Equivalent Exposures
12.5 f/9 between the FSD and
11 f/10 aperture to maintain
Guide Number: 110
10 f/11 equivalent exposures.
9 f/12.5
8 f/14
7.1 f/16
6.4 f/18
5.6 f/20
5 f/22
Sunday, November 13, 2011 19

20. Speedlight Basics:
FSD Aperture
22 f/5
20 f/5.6
18 f/6.4
16 f/7.1
14 f/8 Notice the relationship
Equivalent Exposures
12.5 f/9 between the FSD and
11 f/10 aperture to maintain
Guide Number: 110
10 f/11 equivalent exposures.
9 f/12.5
8 f/14
7.1 f/16
6.4 f/18
5.6 f/20
5 f/22
Sunday, November 13, 2011 20

21. Speedlight Basics:
FSD Aperture
22 f/5
20 f/5.6
18 f/6.4
16 f/7.1
14 f/8 Notice the relationship
Equivalent Exposures
12.5 f/9 between the FSD and
11 f/10 aperture to maintain
Guide Number: 110
10 f/11 equivalent exposures.
9 f/12.5
8 f/14
7.1 f/16
6.4 f/18
5.6 f/20
5 f/22
Sunday, November 13, 2011 21

22. Speedlight Basics:
ISO GN
100 110
125 125
160 140
200 160
250 180 Notice the relationship
Equivalent Exposures
320 200 between the ISO and GN
400 220 to maintain equivalent
Guide Number: 110
500 250 exposures.
640 290
800 320
1000 360
1250 400
1600 450
2000 500
Sunday, November 13, 2011 22

23. 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.
Sunday, November 13, 2011 23

24. 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)
Sunday, November 13, 2011 24