Flash Guide Number Calculator — Manual Flash Power & Distance
Required Aperture
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Max Distance at f/2.8
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Adjusted Guide Number
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How Flash Guide Numbers Work
A flash guide number (GN) is the standard measurement of a flash unit's power output, expressed as the product of the flash-to-subject distance and the f-stop required for correct exposure at ISO 100. According to the ISO 2827 standard, guide numbers are measured at a specified zoom position (usually 35mm or the flash's maximum zoom) in a controlled environment. This calculator uses the fundamental flash exposure relationship to help you determine the correct aperture at any distance, or the maximum distance at any aperture, for manual flash photography.
Guide numbers follow the inverse square law of light: doubling the distance from flash to subject reduces light intensity by 75% (two stops). This is why a flash powerful enough at 3 meters may be completely inadequate at 6 meters. Understanding this relationship is critical for manual flash photography, off-camera flash setups, and evaluating flash purchases. Our exposure calculator can help you balance flash with ambient light for mixed-lighting scenarios.
The Guide Number Formula
The guide number formula is the foundation of manual flash exposure calculation:
Guide Number = Distance x f-stop (at ISO 100)
This can be rearranged to solve for any variable:
- Required aperture: f-stop = GN / Distance
- Maximum distance: Distance = GN / f-stop
- ISO adjustment: Adjusted GN = Base GN x sqrt(ISO / 100)
Worked example: A speedlight with GN 40 (meters at ISO 100), subject at 5 meters, camera set to ISO 400. First, adjust the GN: 40 x sqrt(400/100) = 40 x 2 = 80. Then find the aperture: 80 / 5 = f/16. So at ISO 400 and 5 meters distance, you need f/16 for correct exposure.
Key Flash Photography Terms
- Guide Number (GN): A numeric rating of flash power. Specified in meters or feet at ISO 100. Common values: built-in flash GN 10-12, mid-range speedlight GN 36-42, professional speedlight GN 56-60.
- TTL (Through The Lens): Automatic flash metering that fires a pre-flash, measures reflected light, and adjusts power accordingly. Available as E-TTL (Canon), i-TTL (Nikon), and P-TTL (Pentax).
- Flash Zoom: An adjustable flash head that concentrates the beam to match lens focal length. Zooming to 105mm vs 24mm can increase GN by 40-60%.
- Recycle Time: The time a flash needs to recharge between firings. Ranges from 0.1 seconds at low power to 3-5 seconds at full power. Faster recycling is critical for event photography.
- Inverse Square Law: Light intensity falls off proportionally to the square of the distance. At twice the distance, you receive one-quarter the light (two stops less).
Popular Speedlight Guide Numbers Compared
Guide numbers vary significantly between flash units. The table below compares popular speedlights from major manufacturers, showing GN at both 35mm and maximum zoom positions. Data sourced from manufacturer specifications as of 2025.
| Flash Unit | GN at 35mm (m) | GN at Max Zoom (m) | Max Range at f/2.8, ISO 100 |
|---|---|---|---|
| Built-in Camera Flash | 10-12 | 12 | 4.3 m (14 ft) |
| Canon Speedlite EL-1 | 40 | 60 | 21.4 m (70 ft) |
| Nikon SB-700 | 28 | 39 | 13.9 m (46 ft) |
| Nikon SB-5000 | 34.5 | 55 | 19.6 m (64 ft) |
| Sony HVL-F60RM2 | 38 | 60 | 21.4 m (70 ft) |
| Godox V1 | 32 | 48 | 17.1 m (56 ft) |
Practical Flash Exposure Examples
Scenario 1 -- Indoor Portrait: Using a Godox V1 (GN 48 at 105mm zoom) at ISO 200 for a portrait at 3 meters distance. Adjusted GN = 48 x sqrt(200/100) = 48 x 1.414 = 67.9. Required aperture = 67.9 / 3 = f/22.6, so use f/22. This gives plenty of power for a small softbox modifier, which reduces effective GN by about 1.5 stops.
Scenario 2 -- Event Photography in a Ballroom: Shooting guests at 6-10 meters with a Nikon SB-5000 (GN 55 at 200mm zoom) at ISO 800. Adjusted GN = 55 x sqrt(800/100) = 55 x 2.83 = 155.6. At 8 meters: f-stop = 155.6 / 8 = f/19.4, so f/16 or f/19 works. Bouncing off a white ceiling at 3 meters height reduces effective output by about 2 stops, bringing the practical aperture down to around f/8.
Scenario 3 -- Outdoor Fill Flash: Using fill flash in bright sun at f/11 and ISO 100, with a subject 4 meters away. Required GN = 4 x 11 = 44. A flash with GN 40 at full power falls just short, so you could increase ISO to 200 (adjusted GN = 40 x 1.414 = 56.6) or move 0.5 meters closer. Use our golden hour calculator to find the best natural light times and reduce your fill flash needs.
Tips for Getting the Most from Your Flash
- Bounce flash for softer light: Pointing the flash at a white ceiling or wall spreads the light across a larger surface area, creating soft, flattering illumination. Budget 1.5-2 stops of light loss from bouncing. The effective GN drops by roughly 40-60%.
- Use high-speed sync (HSS) for daylight: HSS allows flash synchronization above the camera's native sync speed (typically 1/200-1/250s), enabling shallow depth of field with flash outdoors. HSS reduces effective GN by approximately 50% compared to normal sync.
- Match flash zoom to lens focal length: Setting the flash zoom head to match or slightly exceed your lens focal length concentrates the beam efficiently. Using 105mm zoom with a 85mm portrait lens wastes less light on areas outside the frame. Check your focal length equivalent when using crop-sensor cameras.
- Use manual flash for consistency: In controlled environments like studios and product photography, manual flash produces identical exposures frame-to-frame. TTL can vary by 1/3 to 1 stop between shots due to changing reflectance in the scene.
- Carry fresh batteries: Flash recycle time and power output degrade as batteries drain. Fresh AA NiMH rechargeable batteries (2500+ mAh) provide the fastest recycle times and most consistent output, lasting approximately 200-300 full-power pops per charge.
Frequently Asked Questions
What is a flash guide number in photography?
A guide number (GN) is the standard measurement of a flash unit's power output, defined as the product of flash-to-subject distance and the f-stop for correct exposure at ISO 100. For example, a GN 40 flash (meters) can properly expose a subject 10 meters away at f/4, or 5 meters away at f/8. Built-in camera flashes have GN 10-12, mid-range speedlights GN 36-40, and professional units GN 56-60.
How does ISO affect flash range and guide number?
Each doubling of ISO increases effective flash range by a factor of 1.414 (square root of 2). The adjusted GN equals the base GN multiplied by sqrt(ISO/100). At ISO 400, a GN 36 flash has an effective GN of 72, doubling its reach. At ISO 1600, the effective GN becomes 144, quadrupling the range. This relationship follows from the inverse square law of light.
What is TTL vs manual flash photography?
TTL (Through The Lens) metering fires a pre-flash, measures reflected light, and automatically adjusts flash power. Manual flash requires calculating exposure using the guide number formula. TTL is faster for dynamic situations like events and street photography, while manual flash gives more consistent results for studios, product photography, and any scenario requiring precise lighting control.
How do I calculate the correct aperture for manual flash?
Divide the guide number by the flash-to-subject distance: f-stop = GN / Distance. For a GN 40 flash at ISO 100 with a subject 5 meters away, the required aperture is f/8 (40 / 5 = 8). At ISO 400, first adjust the GN: 40 x sqrt(400/100) = 80, then divide: 80 / 5 = f/16. Use our exposure calculator for more complex mixed-lighting scenarios.
What guide number do I need for event photography?
For indoor events at 3-8 meters, a speedlight with GN 36-42 (meters at ISO 100) is usually sufficient at ISO 400-800. At ISO 800, a GN 36 flash reaches about 10 meters at f/2.8. Professional event photographers often use GN 56-60 flashes for larger venues. Using a bounce card or diffuser reduces effective GN by roughly 1-2 stops, so factor that into your calculations.
How does flash zoom affect guide number?
Most speedlights have a zoom head that concentrates the beam to match lens focal length. Zooming to a longer focal length increases the GN because light is concentrated into a narrower beam. For example, a Nikon SB-700 has GN 28 at 35mm but GN 39 at 120mm, a 39% increase. Manufacturer-stated GN is typically at maximum zoom, so actual GN at wider positions is lower. Always check the GN table for your specific zoom setting.