Guide Number

A guide number rates the light output of a flash. It equals the flash-to-subject distance multiplied by the f-number that produces a correct exposure, measured at a stated ISO, usually ISO 100. A larger guide number means a stronger flash. The rating lets photographers compare flash units and calculate manual flash exposure with simple division.

Divide the guide number by the distance to the subject to get the aperture. A flash rated 32 in meters lighting a subject 4 meters away needs f/8. Dividing by the f-number instead gives the working distance: the same flash reaches 8 meters at f/4. The math assumes direct flash at the rated ISO.

Pop-up flashes built into cameras sit around 12 in meters at ISO 100. Mid-size speedlights land between 20 and 40, and flagship models reach 60 at their longest zoom setting. Bigger numbers help with distant subjects, small apertures, and modifiers that absorb light. For close work and bounce techniques, a mid-size unit covers most situations.

Both appear on spec sheets, so the unit matters. A guide number of 60 in feet equals about 18 in meters, since one meter is about 3.3 feet. Using a feet-based rating in a meters-based calculation, or the reverse, throws the exposure off by more than three stops. Spec sheets normally state the unit next to the number.

Yes. Ratings are quoted at a stated ISO, usually 100. Each doubling of ISO multiplies the effective guide number by about 1.4, so a flash rated 32 at ISO 100 acts like 45 at ISO 200 and 64 at ISO 400. Raising ISO is the standard way to stretch flash range when a subject sits beyond the rated reach.

A zoom head concentrates the flash burst to match the lens. At a 105mm or 200mm setting the beam is narrow and intense, which produces the biggest possible number for the spec sheet. At a 24mm setting the same flash spreads its light wide and the real guide number drops sharply. Comparing two flashes only works when both ratings use the same zoom setting, ISO, and distance unit.

No, as long as the shutter stays at or below the camera's flash sync speed. The flash burst lasts a fraction of a millisecond, far shorter than the shutter stays open, so the whole burst lands on the sensor. Shutter speed controls how much ambient light registers instead. Only aperture, ISO, distance, and flash power change the flash exposure itself.

Not directly. The rating assumes the flash points straight at the subject. Bouncing off a ceiling or wall lengthens the light path, and the surface absorbs part of the burst, so the simple division no longer predicts exposure. TTL metering or a test shot works better for bounced light. The same applies to softboxes, diffusers, and other modifiers.

Not always. More power adds size, weight, cost, and longer recycle times, and close-up or indoor work often runs a flash at a small fraction of its power anyway. A higher rating helps outdoors, with light-absorbing modifiers, at small apertures, or at long distances. For a first speedlight, a tilting and swiveling head often matters more than raw power.

The formula works because flash light follows the inverse square law: doubling the distance spreads the light over four times the area, cutting it by two stops. F-numbers follow the same doubling pattern, so multiplying distance by f-number gives a constant for a given flash. That constant is the guide number.