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How Guide Number Math Works

A flash's guide number (GN) is a single figure, quoted at ISO 100 in meters, that links three variables of manual flash exposure: GN = distance × f-number. Because it's one equation with three unknowns, you can solve for whichever one you're missing. Know the GN and the subject distance? Rearrange to find the aperture you need: f-number = GN ÷ distance — a GN 36 flash lighting a subject 4.5 m away calls for f/8 (36 ÷ 4.5 = 8). Know the GN and the aperture you want to shoot at? Rearrange the other way to find your maximum range: distance = GN ÷ f-number — that same GN 36 flash at f/2.8 reaches 36 ÷ 2.8 ≈ 12.8 m before it runs out of light. And if you know the correct aperture at a measured distance, multiplying them back out gives you the flash's actual GN, useful for checking a manufacturer's number against reality.

ISO is the variable manufacturers quote GN at but that photographers actually change on set, and it doesn't add linearly. Raising ISO makes the sensor more sensitive, so the same amount of light reaches "correct" exposure at a smaller aperture or greater distance — but the relationship follows the square root of the ISO ratio, not the ISO ratio itself: effective GN = base GN × √(new ISO ÷ base ISO). Doubling ISO from 100 to 200 multiplies the effective GN by √2 ≈ 1.41, not by 2 — so a GN 36 flash effectively becomes GN ≈ 51 at ISO 200, and GN 72 (a full doubling) only arrives at ISO 400.

What to Watch Out For

• Feet vs. meters is the most common mix-up. Many manufacturers quote GN in feet, often at a zoomed-in flash head, to produce a bigger-looking number — GN 60 (ft) is only about GN 18 (m), a genuinely weaker flash than the metric figure suggests.
• Zoom head setting changes GN. A flash's GN is usually quoted at its most zoomed (narrowest, most concentrated) beam angle — spread the beam wider for a bigger area and the effective GN drops.
• GN assumes a bare flash in an average room with some reflective surfaces; bouncing, diffusing, or shooting in a large dark space (like an auditorium) changes the real-world range regardless of the on-paper GN.
• This is manual-exposure math — TTL metering handles the calculation automatically, but understanding GN still matters for off-camera flash, studio strobes, and film cameras without TTL.