This tool helps photographers and videographers calculate the required flash power for proper exposure. It accounts for distance, aperture, ISO, and flash guide number. Use it to plan lighting setups for portraits, events, or product shoots.
📷 Flash Power Calculator
Results
How to Use This Tool
Enter the subject distance, aperture, ISO, and your flash's guide number. Select the distance unit (feet or meters) and set the flash power percentage. Click "Calculate Power" to see the required flash output and exposure details. Use "Reset" to clear all fields.
Formula and Logic
The tool uses the guide number formula: GN = Distance × Aperture. Adjusted for flash power, the effective GN is GN × √(Power%). The required flash power is calculated as (Distance / Effective GN) × Aperture. Exposure Value (EV) is derived from the aperture, ISO, and flash power.
Practical Notes
- Flash power falloff follows the inverse square law; doubling distance reduces light by 75%.
- For portraits, start with f/2.8–f/4 and adjust based on skin tones and background.
- Higher ISO increases sensitivity but may add noise; balance with flash power.
- Consider lens crop factors (e.g., APS-C vs. full-frame) for framing and effective aperture.
- Storage capacity: High-power flashes may require faster recycle times; plan for burst shooting.
Why This Tool Is Useful
This calculator helps photographers quickly determine flash settings without trial and error. It ensures consistent exposure in varied lighting conditions, saving time during shoots. Useful for event photography, studio work, and outdoor portraits where ambient light is unpredictable.
Frequently Asked Questions
What if my flash doesn't have a guide number?
Check the manufacturer's specs or estimate based on similar models. For speedlights, GN is often around 100–120 at ISO 100.
How does aperture affect flash power?
Wider apertures (lower f-numbers) require less flash power for the same exposure. Use this tool to find the balance between aperture and flash output.
Can I use this for video lighting?
Yes, but video lights are continuous; adjust power based on distance and desired brightness. The principles of inverse square law still apply.
Additional Guidance
For advanced setups, combine flash with ambient light using TTL or manual modes. Test with a light meter for precision. Always consider subject distance and background elements to avoid harsh shadows or overexposure.