Flash Exposure Value (EV) Calculator
Calculate flash exposure value and correct flash power for a given aperture, ISO, and subject distance using the guide number method.
Flash Exposure and Guide Numbers
Getting correct flash exposure manually requires understanding the guide number (GN) system. A flash’s guide number tells you the relationship between aperture, distance, and power at a given ISO.
The Guide Number Formula
GN = Aperture (f-stop) × Distance (meters or feet)
Or rearranged:
- Required aperture = GN ÷ Distance
- Effective distance = GN ÷ Aperture
The guide number is always stated at a specific ISO (usually ISO 100). When you change ISO, adjust the effective GN:
GN_adjusted = GN_ISO100 × √(ISO / 100)
Common Flash Guide Numbers (ISO 100)
| Flash Type | Guide Number (m, ISO 100) |
|---|---|
| Small on-camera flash | 12–20 |
| Mid-range speedlight | 30–40 |
| Pro speedlight | 45–60 |
| Large studio strobe | 80–160 |
Inverse Square Law
Light follows the inverse square law: when you double the distance, the light intensity drops to one-quarter.
New exposure = Original exposure × (D1 / D2)²
This means a subject at 2 m receives 4× more light than one at 4 m. For portraiture, keeping subjects close to the flash dramatically improves exposure and softness.
EV (Exposure Value)
Exposure Value is a logarithmic scale combining aperture and shutter speed. For flash photography, the relevant EV is the aperture-based EV at a given ISO:
EV = log₂(N² / t)
Where N = f-number, t = shutter speed in seconds. Higher EV = brighter conditions — requires less flash power or narrower aperture.
Practical Tips
- TTL mode automates flash exposure — use manual for full creative control.
- Flash sync speed (usually 1/200 or 1/250 s) is the fastest shutter speed you can use with flash.
- High-speed sync (HSS) bypasses the sync limit but reduces effective guide number significantly.
- Bounce flash off a ceiling reduces effective GN by approximately 1.5–2 stops.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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