Guide number

Definition

A guide number is the manufacturer’s shortcut for flash reach: GN (metres, ISO 100) ≈ f-number × subject distance. GN 32 at f/8 is 4 m; at ISO 400 the same head behaves like GN 64. Bounce, zoom head, modifier and TTL ceiling all change the real number. It is a bare-bulb, on-axis estimate, not a studio plot. Never point a laser meter or high-power strobe at eyes to “check” GN.

Calculators that use this

• Exposure & light

  Flash Guide Number Calculator

  Turn a flash guide number, f-number and ISO into maximum subject distance, with GN scaled from the ISO 100 rating.

• Studio, storage & utilities

  Inverse Square Law Calculator

  See how light intensity falls when you move a point source: ratio of (d1/d2)² and the stop change of 2 log2(d2/d1).