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.