Do I need a trigger to use a flash off the camera?

You need something to close the circuit at the moment of exposure: a radio trigger, a cable, or an optical slave that sees another flash fire. The central hot shoe contact is what all three ultimately operate.

Off-camera flash is one electrical problem — closing a circuit at the right instant — with three common solutions.

The three

A cable, which is exact and short. A radio trigger, a transmitter on the shoe and a receiver at the flash, with a published range in metres. An optical slave, which fires when it sees another flash, needing line of sight.

What the shoe supplies

One contact, common to all makers, that closes at exposure. Everything past it is maker-specific — which is why a flash from one system fires on another body and loses its automatic modes there.

The constraint that governs all three

The sync speed the camera publishes. Above it the shutter never opens fully, so a single burst lights only part of the frame. One product here states a high-speed ceiling of 1/8000 s; the mode spreads the burst over the whole travel of the slit and costs output in exchange.

What is published, and what is not

Published: the trigger’s range, the modes it carries, the flash’s own output figure — three titles print a guide number, two an energy in watt-seconds. Not published: how reliably a given pairing triggers at the edge of its range, which is why the stated metres are a maximum rather than a working distance.

Where this page and the maker’s own manual disagree, the manual decides. Every focal length, aperture, guide number, stop of stabilisation, load rating and sealing claim on this page is reported from the manufacturer’s own documentation, with its unit written as published, and named as such — none of it is measured here. An engraved figure is reported as the maker engraves it, and is never restated in another convention, because the two are not the same number. And where two light sources are set beside each other: a guide number and an energy in watt-seconds describe different quantities and cannot be compared with one another. A guide number depends on the sensitivity and the reference coverage the maker chose before printing it; a watt-second figure describes the energy a capacitor holds before the tube fires. Neither is the light that reaches a subject, which depends on the modifier, the distance and the room.