Flash Photography Distance Calculator

Enter Calculation

Number of flash units 1 2 3

Guide Number (GN1)

Guide Number (GN2)

Guide Number (GN3)

Aperture (f-number / F-value)

Film/Sensor sensitivity (ISO) 100 200 400 800 1600 3200

Results

Distance to subject (d)

13.33

meters (m) for correct flash exposure

Effective Guide Number (GN')	32
ISO multiplier (sqrt(ISO/100))	1

What this calculator does

This tool computes the maximum flash-to-subject distance at which a camera flash delivers correct exposure. It uses the flash unit's Guide Number (GN), the lens aperture (f-number) and the film or sensor ISO sensitivity. It also lets you combine up to three flash units fired from the same position and direction (multi-strobe), which raises the effective output. This is universal photographic physics and has no region-specific rules.

Flash-to-subject distance is the key value this calculator finds.

Key terms

Guide Number (GN) expresses flash power. By convention it is quoted for ISO 100 in meters: \( \text{GN} = \text{f-number} \times \text{distance} \) for correct exposure. f-number is the lens aperture; the smaller the value, the more light is admitted. ISO is sensitivity; doubling ISO is one stop of extra light. ISO 100 is the reference at which a 1 m projection distance gives correct exposure for a given f-number.

How to use it

Pick how many flash units you are firing (1-3). Enter each unit's Guide Number, your chosen aperture, and the ISO from the dropdown. The result is the distance in meters at which the flash exposes correctly. Shooting closer than this is fine (you can stop down); shooting farther will under-expose the subject.

The formula explained

The base exposure relation is \( F = (\text{GN} / d) \times \sqrt{\text{ISO}/100} \). Solving for distance gives $$ d = \frac{\text{GN}}{F} \times \sqrt{\frac{\text{ISO}}{100}} $$ The square-root term scales the Guide Number for sensitivity: ISO 400 gives a factor of 2, ISO 1600 a factor of 4. For multiple flashes, the effective Guide Number is the root-sum-of-squares $$ \text{GN}' = \sqrt{\text{GN1}^2 + \text{GN2}^2 + \ldots} $$

Distance grows with Guide Number and shrinks as the f-number increases.

Worked example

Two flashes, GN1 = 32 and GN2 = 24, at f/8 and ISO 100: $$ \text{GN}' = \sqrt{32^2 + 24^2} = \sqrt{1600} = 40 $$ Then $$ d = \frac{40}{8} \times \sqrt{1} = 5.0 \text{ m} $$

Combining multiple strobes increases effective Guide Number and reach.

FAQ

Why root-sum-of-squares for multiple flashes? Light intensity adds linearly, but Guide Number scales with the square root of intensity, so combined GNs add in quadrature.

What if my GN is quoted in feet? The output will then be in feet; this tool assumes the meter convention. Convert GN to meters first if needed.

Does a higher ISO let me shoot farther? Yes. Each doubling of ISO multiplies the usable distance by about 1.41 (\( \sqrt{2} \)).