Guide number from aperture and distance

The browser path is free; successful API calls use the published base of $0.002 per item, and invalid input is not billed.

What you send and what you get back

Send two required positive finite numbers: aperture as the f-number N that produced the correct flash exposure, and distance as the flash-to-subject length in the unit you want the guide number quoted in. Wire the free browser form for field math, then call the identical contract from scripts when you bulk-build lighting tables or lesson answer keys. Aperture aliases include N, f_number, fNumber, f_stop, and fstop, so f/8 can arrive as eight under any of those keys. Distance aliases include d, dist, subject_distance, and flash_distance. Optional iso is the working sensitivity of that frame and defaults to one hundred; aliases are ISO, sensitivity, and working_iso. Optional rated_iso is the reference ISO at which to quote the derived guide number and also defaults to one hundred, matching almost every manufacturer chart; aliases are gn_iso, guide_iso, ratedIso, and reference_iso. Optional precision, with aliases decimals and dp, chooses how many decimal places to expose for numeric fields, defaulting to ten and capped at fifteen so JSON goldens stay stable. Missing aperture or distance, zeros, negatives, non-numeric strings, NaN, Infinity, and magnitudes above the published ceiling fail with invalid_input and are not billed, because a zero f-stop or a zero distance would collapse the product and a non-positive ISO would make the square-root scale undefined. On success the response leads with guide_number at the reference ISO, then effective_guide_number equal to aperture times distance at the working ISO, the echoed aperture, distance, iso, and rated_iso, the iso_scale factor equal to the square root of iso over rated_iso, an f_stop_label such as f/8, a unit_note reminding you that GN and distance share a length unit, the method tag photo_flash_guide_from_distance, the precision used, and the formula string. Classic checks include eight and four point five at default ISO one hundred yielding guide number thirty-six and iso_scale one, or the same pair at ISO four hundred with rated ISO one hundred yielding guide number eighteen and iso_scale two. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance.

Where deriving a guide number shows up in real work

Used-gear shoppers and rental desks meet unmarked or poorly labelled speedlights constantly; a single correct test at a taped distance and a known f-stop is enough to recover a comparable GN before deciding whether the unit matches the listing. Studio assistants who just built a bounce or a softbox often want the effective guide number of that modified source so they can place the next subject without chimping every frame, and quoting that effective GN at ISO one hundred keeps it on the same scale as the bare-flash chart. Prefer this landing when the search intent is to recover a guide number from aperture and distance; use the flash guide number calculator when you already know GN and need the missing f-stop or reach, and use flash distance from guide number when you only want distance equals GN divided by N. Keep optical values inside the published absolute range so products and square roots cannot overflow into non-finite results. Teachers and workshop leaders assign the inverse problem as often as the forward one—given a correct f/5.6 at three metres, what is the GN—and a deterministic API speeds grading without opaque phone apps that silently switch metres and feet. Firmware authors, lighting-planner apps, and camera-simulation games encode the same product when a user types a successful exposure and the interface must display an implied power rating; a pure parse endpoint avoids re-implementing ISO square-root scaling and overflow guards in every client.