Flash Exposure Calculator
Guide number based flash exposure, aperture, and range.
Results
- Guide number at your ISO
- 40.0 m
- Required aperture at subject distance
- f/8.0
- Max flash distance at your aperture
- 5.0 m
Your flash's effective guide number once your chosen ISO is factored in.
The aperture that gives correct flash exposure at the subject distance you entered.
The farthest a subject can be from the flash and still be correctly exposed at this aperture.
How it works
A guide number describes how much light a flash puts out, expressed so that dividing it by the subject distance gives the aperture for correct exposure. It is the foundation of shooting flash in manual mode, where the camera does no metering for you.
Guide numbers are quoted at ISO 100 and in metres unless stated otherwise, and they assume the flash head is at a particular zoom setting. Raising ISO extends effective range by the square root of the ISO increase, so four times the ISO doubles the distance the flash reaches.
Two caveats worth remembering. Manufacturers quote guide numbers at the most flattering zoom setting, so real-world output is often lower than the box claims. And the inverse-square relationship assumes direct flash: bounce it off a ceiling and the effective distance becomes the whole path from flash to ceiling to subject, which is why bounced flash eats so much power.
The formula
N = GN / distance, GN(ISO) = GN₁₀₀ × √(ISO / 100)
N is the f-number for correct exposure, GN the guide number in metres, and distance the flash-to-subject distance, not the camera-to-subject distance.
Frequently asked questions
- Does shutter speed affect flash exposure?
- Not for the flash itself, as long as you stay at or below sync speed. The flash burst is far shorter than the exposure, so aperture, ISO and distance control it. Shutter speed controls how much ambient light mixes in, which is how you balance flash against a background.
- Is the distance from the camera or the flash?
- From the flash. If the flash is off-camera on a stand, measure from the stand to the subject, not from where you are standing.
- How do I account for reduced flash power?
- Each halving of power costs one stop, which divides the guide number by the square root of two. At 1/4 power, a guide number of 40 behaves like 20.