Flash Guide Numbers: Calculate Aperture From Distance and ISO

What does a flash guide number mean?

A guide number (GN) is a compact way to describe a flash’s useful output under stated conditions. It connects a direct flash’s subject distance with the aperture that gives a reference exposure. In Nikon’s SB-910 manual, the rating is GN 34 meters (111.5 feet) at ISO 100 with the head at 35 mm, the FX standard illumination pattern and full output. Change the zoom-head setting or output level, and the relevant GN changes.

GN 34 m does not mean you should shoot a person 34 m away. It means that at ISO 100, a direct flash at 1 m gives the mathematical f/34 reference value; at 4.25 m it gives f/8. In practice, your lens may not offer f/34, and the usable range also depends on the scene and flash conditions.

Manufacturers can quote meters or feet. A GN of 34 m is about GN 112 ft, not a stronger flash. The numerical values differ because the units differ. When comparing two speedlights, also check ISO, zoom-head coverage and any modifier or diffusion setting.

The guide number formula for aperture

At full power and ISO 100, the square-root factor is 1, so the formula becomes simply f-number = GN ÷ distance. Rearranging gives distance = GN ÷ f-number. Nikon’s manual states the same relationship and supplies ISO factors and GN tables for different head positions and manual power levels.

The formula assumes the flash’s rated GN applies to your chosen coverage, output and lighting setup. If your manual gives a GN at a specific zoom setting, use that row of its table. A narrow flash beam can show a higher GN than a wide beam because it concentrates light into a smaller area.

Calculate your starting aperture

Enter the guide number at ISO 100 and full power for your flash’s chosen zoom-head setting. The sample starts with Nikon’s published GN 34 m at 35 mm. This calculator models direct manual flash without a modifier or high-speed sync.

For a more detailed setup—including light falloff and multiple lights—open PicDriver’s flash photography calculator.

Worked guide number examples

These examples use a GN 34 m flash at ISO 100, with the same direct-light head coverage as its rating. They show how to adjust one variable at a time; your actual speedlight may have a different GN.

The final two rows illustrate different ways to change light at the subject. Quarter power cuts the light energy to one quarter and the f-number to half. Moving the flash to half its distance increases idealized illumination about fourfold and doubles the f-number for the same exposure.

A feet-based example

If your flash is rated GN 100 feet at ISO 100 and the flash is 12.5 feet from the subject, 100 ÷ 12.5 = f/8. Do not divide a GN in feet by a distance in meters. Convert one unit first, or use the calculator’s unit switch.

Why ISO and manual power use square roots

For the same flash energy and distance, moving from ISO 100 to ISO 400 is a two-stop increase in sensitivity. You can close the lens two stops—from f/8 to f/16—while keeping a similar flash exposure. Because f-numbers scale with the square root of light ratio, the effective GN doubles: 34 × √(400 ÷ 100) = 68 m.

Changing the speedlight from full power to 1/4 power reduces its emitted light to about one quarter. The effective GN is halved: 34 × √¼ = 17 m. At the same 4.25 m, the matching aperture is f/4. Nikon’s SB-910 manual includes separate GN rows by manual output, which is preferable when your flash’s actual behavior differs from ideal scaling.

When guide-number math needs a reality check

Bounce flash and modifiers

A ceiling bounce, umbrella, softbox, diffuser or gel changes the light path and often absorbs or spreads light. The direct-flash GN no longer predicts the required aperture accurately. Take a test image, use a flash meter, or make a new effective GN from your measured result. Nikon’s flash basics describes bouncing as a way to change light direction and quality.

Flash head zoom and beam coverage

A speedlight’s GN can rise as you zoom the head tighter, but the beam then covers less of your scene. The SB-910’s manual shows different GN values at different zoom positions—for example, GN 34 m at 35 mm and GN 46 m at 85 mm in its specified full-power FX table. Use the number for the coverage you actually set, and watch the edges of a wide frame.

High-speed sync, TTL and ambient light

High-speed sync (HSS) uses a different flash delivery pattern; its effective GN can depend on shutter speed and flash model. The regular single-burst GN formula is not a reliable HSS exposure prediction. TTL also varies the flash output automatically, so a fixed full-power GN does not describe every shot. Ambient light may add exposure to the subject, while shutter speed strongly affects how bright the ambient background appears within sync limits. Consult your camera and flash manuals for the supported sync mode and take a test frame.

At very short distances, in reflective rooms or with uneven beam patterns, a single GN cannot describe every pixel’s exposure. Treat a calculated f-stop as a fast starting estimate rather than a promise of perfect brightness.

A quick manual-flash workflow

1. Find the right GN. Read your flash manual’s ISO 100 guide-number row for the zoom-head position, illumination pattern and manual output you will use.
2. Measure flash-to-subject distance. If the light is off camera, ignore the camera-to-subject distance. Keep the GN and distance in the same units.
3. Choose ISO and calculate an f-stop. Use the formula or the calculator above. Select the nearest available camera setting as a starting point.
4. Set a supported sync shutter speed. Choose a shutter speed within your camera’s normal flash sync range unless you deliberately enable a supported HSS mode.
5. Take a test photograph. Check the subject, background and highlights. Adjust flash power, aperture, ISO or placement according to the look you want.

If you need more depth of field for a group, you may want a smaller aperture. You can move the flash closer, increase ISO, raise flash power or use a stronger light—each has practical limits. PicDriver’s depth-of-field calculator helps you judge the focusing side of that tradeoff.

Common questions

Sources and further reading

• Nikon SB-910 reference manual: guide numbers, ISO factors and power tables
• Nikon: The Basics of Flash Photography
• Nikon: High-Speed Sync technique

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