As any diver knows, at depth light is absorbed, and one by one the colours of the spectrum disappear. Red is the first to go which disappears at around six metres, followed by orange, yellow… Underwater photographers are constantly battling against the effect water has on colour and light, and use various methods to return good colours to their images.
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Electrical or Optical?
Most of the principles of flash photography are the same on DSLR’s (Digital Single Lens Reflex) and compact cameras. There are however some differences in the way flash units are connected to, and communicate with the cameras.
To fire an external flash unit on a DSLR, an electronic connection runs from the camera hot shoe. through a cable in the housing, and then to a connector called a sync socket. From this socket, a sync cable connects the housing to the external flash unit.
Most digital compact cameras do not have a hot-shoe connector, and even if it does, it may not be accessible through the housing. Because of this, external flash units for compacts generally work as slaves; they are triggered by the built in flash on the camera-via a fibre optic cable, rather than an electronic sync cord. The cable mounts onto the housing in front of the built in flash. When the built in flash fires, light travels up the cable, hits a slave sensor on the flash unit, telling it when to start and stop firing.
When a built in flash is used on a compact, the camera will put out a series of pre-flashes before the main flash fires and the image is recorded. Because of this, when you want to fire an external flash unit, you must use one that has been designed to ignore the pre-flashes, and will wait to fire with the main flash. If you try to use an older strobe (one designed to be used with a film system) the external flash will fire early and so will not have time to recycle and fire on the main flash.
Early strobes which were designed for compact cameras used an ‘auto’ system to control the power output. With an auto system you set the desired aperture on the camera, and set the same aperture on a dial on the flashgun, this gives you the correct exposure.
Newer strobes such as the INON D2000 now provide a ‘TTL’ system. This has been achieved by building a slave sensor that actually copies everything that the built in flash does, including the pre flashes. These guns also feature exposure compensation controls so you can adjust the power output to achieve the desired result.
Other Compatibility Issues
Digital cameras have created a few problems with flash systems on DSLR cameras as well. Tradition film cameras used TTL systems to meter the amount of flash needed to expose the picture properly. DSLR’s use different TTL systems, such as DTTL, iTTL, and ETTL. If you try and use a digital camera with a traditional TTL style strobe, the two will not communicate properly; they speak different languages.
This has presented a problem for manufacturers of underwater strobes, it takes time to develop a flash unit that can properly communicate with a DSLR, and at the rate that new cameras are being released, the strobes are often out of date by the time they’re available!
Many people continue to use older strobes that have been designed for film systems, and simply use the manual power settings to control the power out-put. The instant review on the LCD screen allows you to check the exposure, and adjust if necessary. If you feel this is too much like hard work, there are ways to get a TTL system working.
Using land flash
Some photographers use a normal land flash, and place it inside a custom built housing, this means that the flash is wired directly from the camera hot shoe to the flash, and so communicates properly.
Some small companies are now producing converters that change the signal from the camera into one that the flash unit can understand, the downside with these is that they often mean you have to retro-fit them to your housing, which can be risky.
Even though TTL is now available through various methods, many photographers still choose to work with manual flash, preferring the amount of control it offers the user. It is worth bearing in mind that you can always turn a TTL gun to manual; you can’t turn a manual gun to TTL.
The position that strobes are placed in is critical. Suspended particulate in the water can be illuminated as it reflects light back towards the camera. This is often referred to as ‘back-scatter’. To avoid backscatter place your strobes in a position where they will light the subject only, and not the water in-between the subject and the lens. It’s important to remember that the strobes generally have very wide coverage, and as such they do not need to be pointed directly at your subject.
Do not underestimate the importance of good flashgun arms; it is through them that you control your light source. Good arms will hold their position when you move them, without the need to loosen and tighten the clamps that hold the frames together. You don’t want to be wasting your valuable time underwater adjusting arms, not spent taking underwater pictures.
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