A flash guide number is a rating of flash power that equals the subject distance multiplied by the aperture f-number for correct exposure at a given ISO, usually 100. A flash with a guide number of 36 meters at ISO 100 gives correct exposure at f/4 when the subject is 9 meters away.
Why It Matters
Flash specifications can be confusing. Knowing the guide number lets you compare units fairly, choose the right flash for a job and calculate settings without trial and error.
It also explains why flash exposure drops quickly with distance. Once you see the guide number formula, you can predict how a change in aperture, ISO or distance affects the picture and set manual power with confidence.
Beginner Guide Number Basics For Flash Photography
The formula is simple: guide number equals distance multiplied by f-number. If you know two values you can find the third. Distance equals guide number divided by aperture, and aperture equals guide number divided by distance. Manufacturers state the guide number for ISO 100, at full power, with the flash head zoomed to a particular setting such as 35mm or 105mm. Always check the unit, because a number in meters is about 3.3 times smaller than the same number in feet.
Key Ideas
- A higher guide number means more power and longer range.
- Guide number depends on ISO: doubling ISO increases usable range by about 1.4 times.
- Zooming the flash head to a narrower angle concentrates light and raises the guide number.
- Reducing flash power to half lowers the guide number by about 0.7 times.
- The guide number is a maximum, and real-world results vary with reflectors and surfaces.
Example Calculations At ISO 100 (Meters)
| Guide Number | Aperture | Distance For Correct Exposure |
|---|---|---|
| GN 30 | f/4 | 7.5 m |
| GN 30 | f/8 | 3.75 m |
| GN 40 | f/5.6 | About 7.1 m |
| GN 40 | f/2.8 | About 14.3 m |
| GN 20 | f/2 | 10 m |
Using The Formula In Practice
- Find the guide number in your flash manual at the zoom setting you will use, in meters or feet.
- Choose the aperture you want for depth of field.
- Divide the guide number by the aperture to find the distance at which full power gives correct exposure.
- If your subject is closer, reduce power by stops using the manual power setting.
- Take a test shot and adjust power by a third or a full stop until the exposure is right.
Common Mistakes
- Comparing a guide number in feet with one in meters.
- Forgetting that the number is for ISO 100 and for a specific zoom position.
- Expecting full range when using a diffuser or bounce, which absorb or redirect light.
- Using full power at close range and blowing out the subject, when a lower power setting would work better.
A Worked Example
You use a flash with a guide number of 36 meters at ISO 100, aperture f/5.6 and ISO 100. Distance equals 36 divided by 5.6, about 6.4 meters, which is the farthest your subject can be for full power exposure. For a person standing 3.2 meters away, which is half the distance, you need about one quarter of the light, so set the flash to 1/4 power, or two stops lower.
Power Settings In Fractions
Manual flash power is expressed in fractions such as 1/1, 1/2, 1/4, 1/8 and so on, with each step representing one stop. Dropping from full power to 1/4 power reduces light by two stops, equal to changing the aperture two stops. This lets you choose between a wide aperture with low power or a narrow aperture with high power. Low power also gives shorter flash duration and faster recycling.
Why The Number Can Mislead
Manufacturers measure under ideal conditions, sometimes with the head zoomed to its most concentrated setting, and the number only describes a spot in the center of the beam. Marketing may quote the highest value at the longest zoom, which suits few real situations. When comparing flashes, check the zoom position, the units and the ISO.
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Advanced Physics, Ambient Light And Efficiency
The Inverse Square Law Behind It
Light from a small source spreads out over a growing area, so doubling the distance reduces the light to a quarter. This is why guide number is divided by distance, and why exposure drops by two stops when distance doubles. See the inverse square law for the full explanation. It also means that moving a flash closer to a subject is the most powerful way to increase its effect.
ISO And Aperture Trade-Offs
Because guide number scales with the square root of ISO sensitivity, quadrupling ISO doubles the usable range. For example, going from ISO 100 to ISO 400 doubles the distance a flash can cover. Review ISO explained and aperture and f-stops for the relationship. Raising ISO adds noise but extends reach, and opening the aperture does the same with a cost in depth of field.
Balancing Flash And Ambient Light
Aperture and ISO affect both flash and ambient light, but shutter speed affects only ambient light within the sync limit. That separation lets you darken a bright background with a faster shutter while keeping the flash exposure constant. The technique is covered in ambient and flash, and the control of flash itself in TTL versus manual flash. Beyond sync speed, see high speed sync, which lowers the effective guide number.
Measuring And Verifying
You can verify a guide number with a flash meter or by test shots. Use a flash meter to read the aperture needed at a given distance and power, and compare it with the calculation. For off camera setups, see off camera flash and bounce flash, where the effective distance includes the path to the ceiling and back. Note that bounce and modifiers reduce light by one to three stops depending on the surface.
Try It
- Find the guide number of your flash in the manual and note the zoom setting.
- Calculate the full power distance for f/4 and f/8 at ISO 100.
- Shoot a test subject at the calculated distance on manual full power and check exposure.
- Reduce the power by two stops and move the subject closer by half the distance to compare.
Common Questions
What is a good guide number?
Small on-camera flashes often have guide numbers of about 30 to 45 meters at ISO 100. Larger units and strobes are higher. Choose according to the distance and aperture you need.
Does guide number work with TTL?
TTL measures and adjusts automatically, so you do not calculate. The guide number still describes the maximum power and range of the flash.
Why is my result darker than the calculation?
Diffusers, bounce surfaces, a wide zoom setting and an aperture or ISO different from the one assumed all reduce the light that reaches the subject.
How does distance affect flash exposure?
Light falls off with the square of distance. Doubling the distance cuts exposure to a quarter, or two stops.
Related Lessons
- Inverse Square Law for why distance matters so much.
- TTL Vs Manual Flash to choose how to set power.
- High Speed Sync to see how it reduces flash range.
- Flash Meter to measure rather than calculate.
- Off Camera Flash to use distance as a creative tool.
Gear That Helps With This
Hand-picked kit that makes this lesson easier to put into practice.
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