Photography
Flash Exposure Calculator
Enter guide number, distance and ISO to see a suggested f-number.
What aperture for flash?
Manual flash starts from guide number (GN): a manufacturer rating that links power to distance and aperture. A common form is aperture ≈ (GN ÷ distance) × √(ISO ÷ 100), with distance in the same units the GN was measured (metres or feet). A GN 36 flash at 3 m and ISO 200 suggests roughly f/8.5 before modifiers. Softboxes, bounce and high-speed sync change the real stop — use this as a meterless starting point, then chimp or meter. Depth of field context sits in depth of field; shutter starting points in shutter speed.
Worked example
GN 36 ÷ 3 m = 12; at ISO 200 multiply by √(200/100) ≈ 1.414 → about f/17. At ISO 100 the same distance is f/12.
Limits and assumptions
Direct flash, rated GN, matching distance units. No TTL automation or modifier loss factors.
Frequently asked questions
What is a guide number (GN)?
Guide number is a flash power rating. In simple terms, GN ≈ aperture × distance (in the rated units) at ISO 100 — rearrange to solve for f-stop.
Metres or feet?
Match whatever the flash manual uses for that GN. Mixing units ruins the result.
Why the ISO square root?
Each stop of ISO multiplies sensitivity; the √(ISO/100) term scales the ISO 100 GN formula.
TTL vs manual?
TTL meters for you. This page is for manual power and learning how GN relates to distance.