What is flash sync speed?

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

What is HSS and when should I use it?

High Speed Sync lets you use flash above the sync speed by rapidly pulsing it for the duration of the exposure. The tradeoff is lost flash power — roughly one stop for every stop of shutter speed above sync, so about 2-3 stops by 1/1000 s. Use HSS when you need a wide aperture 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 about 0.71 (one stop). Full-power GN 58 becomes GN 41 at 1/2, GN 29 at 1/4, and so on. Working distance range is directly proportional to the 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 — each stop of loss divides the guide number by √2.

Flash Sync Speed Optimizer — Flash Exposure Calculator

Calculate flash exposure settings relative to your camera sync speed.

Camera Sync SpeedYour camera's fastest full-frame flash sync shutter speed

Shutter SpeedThe shutter speed you intend to shoot at (faster than sync triggers HSS)

Flash PowerManual flash output level, from 1/1 full to 1/128

Flash-to-Subject Distance

Distance from the flash to the subject, in metres

ISOCamera ISO sensitivity — higher ISO raises the guide number

Aperture (f-number)

The f-stop you plan to shoot at

Ambient LightingScene brightness (Sunny-16) used for the flash-to-ambient ratio

How We Calculate This

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

f-number at subject = GN ÷ distance

HSS penalty (stops) = log₂(shutter ÷ sync) when the shutter is faster than sync; the guide number then falls by √2 per stop.

The guide number is adjusted for flash power, ISO, zoom and modifier loss, then for any High Speed Sync penalty. The resulting f-number at the subject tells you whether the flash can properly expose at your chosen aperture, and the flash-to-ambient ratio compares it against the Sunny-16 ambient light for your scene.

Frequently Asked Questions

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Last updated: June 2026

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