Flash guide number
GN = aperture × distance: whether the flash reaches, and what power to set
Parameters
Reach
Max distance: 18 m
✓ reaches
- Effective GN
- 72
- Power for this distance
- ≈1/64
- Bounced (×0.5 light path)
- 1/11
GN applies at ISO 100. At ISO 400 effective GN doubles (√(ISO/100)). Ceiling bounce light path ≈ 2× ceiling height — ~2 stops lost. Manual mode: power = (N×d/GN)².