Guide Number

The guide number of a flash unit quantifies its maximum light output in a standardised way. At ISO 100, the guide number equals the f-number multiplied by the subject distance in metres (or feet, depending on the rating convention) that produces a correct exposure. A flash with a guide number of 60 (metres, ISO 100) can correctly expose a subject at 10 metres using f/6, or at 6 metres using f/10, or at 5 metres using f/12. This relationship allows photographers to calculate manual flash exposure without a light meter.

Guide numbers are stated at a specific ISO and zoom head position, typically ISO 100 with the flash head zoomed to 105mm. Some manufacturers rate at ISO 200 or at a wider zoom position to produce a more impressive number; comparing guide numbers across brands requires verifying that the rating conditions are the same. A flash rated at GN 60 at 105mm and ISO 100 is not more powerful than one rated GN 60 at 35mm and ISO 100 — the 35mm rating will produce a lower guide number at 105mm.

In M43 TTL flash shooting, the guide number is less directly used than in manual flash, because TTL metering automatically adjusts the flash output to achieve a correct exposure based on what the camera's metering system reads. The guide number becomes relevant when determining maximum working range — how far the flash can reach at a given aperture and ISO — and when diagnosing exposure issues in manual flash setups. More powerful flashes with higher guide numbers can work at greater distances or at smaller apertures, which is useful for large venues, outdoor events, or high-speed sync shooting where flash output is reduced.