ImageReasonLighting, Flash, and Studio
What aperture or distance does this guide number give me?
Enter a stated ISO-100 guide number, your ISO, and either distance or aperture, to get a direct-flash starting point, with the test condition it depends on shown beside every result.
Answer
Visualize
Calculate to see the source-to-subject line.
| Quantity | Value |
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Compare
| ISO | Guide number | Aperture at this distance |
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Stress-test
Plan
Calculate a result to generate a copyable plan.
The condition behind the number matters
A flash guide number is a compact way to describe a direct-flash starting relationship between aperture and distance. At ISO 100, divide the guide number by the flash-to-subject distance to get the approximate f-number. Raise ISO and the guide number increases by the square root of the ISO ratio. The arithmetic is useful because it turns a published specification into a starting exposure plan before the first test frame.
The condition behind the number matters as much as the number. A manufacturer may quote guide number with a particular reflector, zoom setting, beam coverage, and measurement method. Adding a softbox, umbrella, gel, diffusion, or bounce surface changes the path and loss. This planner therefore keeps the direct-flash condition visible. It does not pretend that a bare-flash guide number can predict a bounced or modified setup.
For example, a guide number of 36 m at ISO 100 gives f/8 at 4.5 m under the stated direct-flash condition. At ISO 400, the equivalent guide number is 72 m, so the same distance corresponds to f/16. That does not mean ISO 400 is automatically preferable. It changes the ambient contribution, noise tradeoff, recycle needs, and working plan. Use the flash-ambient planner when the background exposure matters.
Distance is powerful. If you double subject distance, the f-number required by the guide-number relationship halves, which is two stops more open. A small move can therefore exceed the practical range of a lens or change the visual relationship between subject and background. Use this calculation to set a starting point, then make a test frame. Inspect the subject, background, catchlights, shadows, and clipping separately. Adjust actual flash power or distance from the test, not from a promise embedded in a specification.
FAQ
- Can I use this for a softbox or umbrella?
- Not as an exact answer. A direct-flash guide number does not include modifier loss. Meter or test the modified setup.
- Does a higher ISO make the flash more powerful?
- It changes the camera sensitivity setting and the equivalent guide-number arithmetic. It does not change the flash's stored energy.
- Why do metres and feet give different guide numbers?
- They are different units. Keep the guide number and distance in the same unit system.