What is flash sync speed?

Flash sync speed is the fastest shutter speed at which your camera can fully expose the sensor during a flash burst. Typical values are 1/160 to 1/250 s. Using faster speeds requires High Speed Sync (HSS), which reduces flash power.

What is HSS and when should I use it?

High Speed Sync allows flash use above the sync speed by pulsing the flash. The tradeoff is a 2-3 stop loss in flash power. Use HSS when you need wide apertures in bright conditions (e.g. outdoor portraits at f/1.4).

How does flash power affect guide number?

Each halving of flash power reduces the guide number by a factor of 0.71 (one stop). Full power GN 58 becomes GN 41 at 1/2, GN 29 at 1/4, and so on. Distance range is directly proportional to guide number.

How do light modifiers affect flash output?

Softboxes typically lose 1-2 stops, umbrellas lose 1-1.5 stops, and grids lose 0.5-1 stop. Always account for modifier loss when calculating flash exposure.

PhotoCalcs

Flash Sync Speed Optimizer — Flash Exposure Calculator

Calculate flash exposure settings relative to your camera sync speed.

Camera Sync Speed

Your camera's max flash sync speed

Flash Power

Flash output power level

Flash-to-Subject Distance

Distance from camera to subject

ISO

Camera ISO sensitivity setting

Aperture (f-number)

Lens aperture (f-stop number)

How We Calculate This

Guide Number = Base GN × √(power fraction) × √(ISO/100)

f-number at subject = GN / distance

The guide number is adjusted for flash power, ISO, zoom setting and modifier loss. The resulting f-number at the subject distance tells you whether the flash can properly expose at your chosen aperture.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.