Many strobes today come with dual connectivity options - that is they can be fired via fiber optic cable, or via an electrical sync cord. These strobes offer the best of both worlds because they will work with the transparent compact housing that you own today, and they will move up with you should you migrate to an SLR housing that does not support optical firing in the future (never say never).
For many camera/housing combinations, there is no choice. Your housing will work with only one type of connection. Some housings however will work with either sync option, leaving the owner to question,
'Which is best?'
Both options have strengths and weaknesses, so let's take a look.
Fiber Optic Connections
First: A little background to help understand how these strobes work. Strobes, including camera flashes do not vary in power or intensity to control exposure. The only variable is in the duration of the flash. It's also important to remember that the entire flash process occurs in microseconds.
In flash mode, most modern cameras fire a 'pre-flash,' a small short duration flash used to determine the proper white balance for the scene and how much light is required for the exposure. Based on what the camera 'sees' in the pre-flash, it calculates the duration of flash to apply to the exposure.
Strobes fired via fiber optic cables generally follow a simple concept - they accurately reproduce the camera flash. Fiber optic cables transmit light from the camera flash to the external strobe. The external strobe simply watches for the camera flash to fire and follows suit. It then watches for the camera flash to quench and again follows suit. Remember that light travels at 186,000 miles/sec., so all of this watching, firing and quenching happens very, very quickly.
Most modern strobes reproduce both the pre-flash and the main exposure flash, so in effect your camera is controlling the external strobe without ever actually knowing about the existence of it.
This pre-flash mimic has another benefit for the camera and housing. The camera 'sees' the bright light from the pre-flash of the external strobe, and calculates (correctly) that less camera flash is required to properly illuminate the scene. The shortened camera flash means less drain on the camera battery, and less internal heat buildup (which can lead to condensation and fogging) within the housing.
Advantages:
- Reliable
- Inexpensive
- Light weight
- No penetration of the housing is required
- Not affected by water
- Allows for “TTL like” exposures in many instances
- Easily repaired if damaged - The most common damage to a fiber optic cable is a kink. In this instance the kinked portion can be simply cut off and the remaining end pressed into service again.
Disadvantages:
- Only works with a housing that allows the camera flash to be visible outside of the housing. If the housing is not transparent, or if the camera has a pop-up flash which cannot be deployed in the housing, then a fiber optic connection will not work.
- Optical triggering does require the camera flash to fire. Although most modern cameras have very robust batteries and are very efficient, firing the camera flash does decrease battery life per charge. As the battery depletes, it can affect the recycle times for the camera flash limiting your ability to shoot 'machine gun' style.
Electrical Sync Connections
An electrical sync connection is just what the name implies. A multi-strand (3-6) wire cable connecting the housing and the external strobe. The camera is in direct communication with, and control of, the external strobe.
Advantages:
- Reliable (when properly maintained and installed)
- Rugged
- Precise strobe control, not limited by the camera flash (more rapid firing at times)
- Works with non-transparent housings and housings where the camera flash cannot be deployed
- Some cables incorporate TTL translation circuitry which negates the need to install such circuitry inside the housing.
Disadvantages:
- Generally more expensive
- Requires more maintenance (more o-rings)
- Will not work if exposed to moisture either on the strobe connection or the housing bulkhead
- Requires a camera with a hotshoe connection
- TTL circuitry options, if desired, make electrical sync considerably more costly
- Requires a breach of the camera housing known as a bulkhead, which is a potential source of leakage
Best of Both Worlds?
The popularity of fiber optics continues to increase due to their ease of use and lightweight, compact form factor. But some shooters want speed and others don’t have a built in flash on their cameras. In the last couple of years, optical flash triggers have started to offer up what could be the perfect solution. The approach varies slightly, such as that taken by Sea & Sea's Internal Optical YS Converter or Nauticam's Flash Trigger for Nikon (NA-D7100).
But the concept is the same: provide a battery powered module that connects to the camera’s hot-shoe and uses low energy draw LEDs to trigger the strobes via fiber optic sync cords. Voilà – all the benefits of both systems with none of the downsides (except perhaps the cost of the flash trigger).
If you ever felt you were giving up something by choosing one system over the other, this could be an option for you. This may be the future of underwater strobe firing and more people are using it every day.
So, there you have it. There is no perfect solution, but thankfully there are affordable, viable solutions for almost every camera and housing combination. If you are lucky enough to have a housing that offers you a choice, consider yourself doubly blessed. You can use the electrical option, and carry a light weight fiber cable as a backup or vice-versa!