A trigger circuit synchronizes camera flash and external flash units to produce evenly lit photos. Two types of circuits exist: synchro output and slave. Slave circuits use sensors to trigger flashes. Different settings can be applied to sync outputs for flexibility. Trigger circuits are essential for professional photographers.
A trigger circuit is an electronic device used in photography to trigger or trigger external or supplementary flash units. When using such supplemental or fill-in flash units, it is critical that the camera flash and external flash units fire at exactly the same time. This is typically accomplished by using a trigger circuit that fires all of the supplementary flashes at the same time as the camera flash. This allows you to shoot clear, shadow-free images in conditions where strong primary lighting casts areas of the subject in deep shadow. There are two basic types of trigger circuits: the synchro output circuit and the slave circuit.
Photography is often said to be the art of successfully capturing light. Unfortunately, this is not an easy task, as the camera is less sensitive than the human eye when it comes to describing subtle differences in light intensity. So while we may clearly see a model in a studio or a wildlife subject, a photo taken of the same subject could be marred by deep, detail-robbing shadows. The best way to overcome this defect is to use fill lighting and flashes to light the subject evenly. Using natural or static studio lighting is easy to manage as the lights are on constantly, but fill flashes must be synchronized with a trigger circuit to be effective.
A basic trigger circuit works on the principle of forming a connection or circuit between the external flash and a trigger source. When a start command is sent from the source, the external flash drives fire. Sync circuits connect to external flash units and directly to a sync port or connector on the camera itself. The camera’s internal circuitry then becomes the firing source, and when it fires the built-in flash, it will also fire the external flash units at the same time.
Slave trigger circuits do not have a direct connection to the camera and derive their trigger source from external sensors. The most common of these is a photosensitive trigger circuit that uses a photo sensor that reacts to light from the main camera’s flash to trigger external flashes. Another type of slave trigger circuit uses a sound sensor to trigger the camera’s shutter release and fire the flashes together and is commonly used in applications such as wildlife photography.
Many high-end professional cameras allow different settings to be applied to their sync outputs, which gives the photographer a lot of flexibility when setting up the lighting. Slave trigger circuitry also ranges from basic single function units to sophisticated professional modules that allow for greater control over how the unit triggers fill flashes. All types, however, are essential pieces of equipment for professional photographers and enable consistent, high-quality results in and out of the studio.
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