underwater Photo Course :: (2) Equipment Guide :: Underwater Camera Housings :: Underwater Strobes :: Strobe Firing Modes

There are six possible firing modes commonly incorporated by underwater flash units: Manual, Automatic, TTL, Slave, TTLThrough The Lens. A term applied to viewfinders and metering systems that operate behind the lens. As such TTL is accurate because it sees what the lens sees. Slave, and pre-flashMany digicams employ a slightly different system for TTL exposure. First they fire a pre-flash which is measured and they calculate exposure based on that. This happens so quickly that the eye notices only one flash.. This is the first issue to consider for compatibility with your system.

• Manual units give a constant output each time they are fired, and so the correct exposureRecording light onto photo sensitive devices and materials is achieved by the judgement of the photographer relating subject distance to apertureThe variable diameter hole used to control the amount of light passing through a lens. (not as difficult as it sounds). There may also be incorporated a number of different power settings to help the photographer control output.
• Automatic strobes have remote sensors that read the light reflected from the subject and automatically quenchThe process of extinguishing the strobe (flashgun) by the camera. flash output when correct exposure has been achieved. This method of determining exposure can economize on battery use as the flash need not fire at full power every time, but it is prone to error with non-standard subjects and may not, therefore, yield any more successful exposures than manual techniques in many circumstances.
• TTLThrough The Lens. A term applied to viewfinders and metering systems that operate behind the lens. As such TTL is accurate because it sees what the lens sees. (strobes that measure exposure Through The Lens) are the norm. They also respond to the light reflected from your subject, but this is measured by the camera, not by remote sensor, and also takes into account the aperture selected (and any filters). It is therefore more accurate, more versatile, and less prone to errors.
• SlaveA flash unit that is triggered to fire by the light output from another flash unit (the master flash). mode can be incorporated into any of the previously mentioned strobes. The term 'slave' denotes that the strobe can be triggered remotely, simply by being in proximity to another flash going off. Such units are useful to augment your main strobe without the necessity of a connecting lead to your main camera. Usually slave mode is at full output so exposure is again determined by flash to subject distance.
• TTL Slave is a feature that gives you TTL without a connecting lead. In addition to sensing when the main strobe fires a TTL slave also responds to the main strobe shutting down. This simultaneous quench means that, to all intents and purposes, the camera (which makes the exposure decision) treats it as one light source.
• Digital pre-flashMany digicams employ a slightly different system for TTL exposure. First they fire a pre-flash which is measured and they calculate exposure based on that. This happens so quickly that the eye notices only one flash. Many digicams employ a slightly different system for TTL exposure. First they fire a pre-flash, which is measured, and they calculate exposure based on that. This happens so quickly that the eye notices only one flash.

Be aware that older (Nikonos compatible) strobes may not give you TTL with your housed digicam unless there is circuitry built-in that makes them compatible.