![]() The full details explaining use of guide numbers are below the calculator. Guide Number is a tool to determine exposure of direct flash with manual flash power levels. Even if you always only use TTL metering of flash, the calculator can be useful to show sample capabilities of your flash at the same settings. The concept is still very valid and useful, and is still a fundamental for understanding flash. In the old days (including flash bulbs), guide number was all we had. There are also other ways: we might use metered TTL, or a handheld flash meter, or just trial and error works well (for one flash) with the digital LCD preview. ![]() ![]() ![]() The reference base is a known accurate Guide Number for one situation (with specified ISO and flash head zoom mm), from which other situations can be calculated. The distance is the distance from flash to subject. Guide Number = Distance x fstop (values which actually give a proper exposure)į/stop = Guide Number / Distance (aperture for other distances)ĭistance = Guide Number / fstop (distances for other apertures) ![]()
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