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Flashguns & artificial light

by Peter Rowlands

Reproduced from in focus 28 (July 1988)

A resumé of the Basic Course talk given in April.

Introduction

The majority of appealing underwater photographs will have been taken and enhanced by the use of some form of artificial light. This form of extra lighting helps to restore the natural colours which have been absorbed by the water and gives most shots more impact.

There are three main types of artificial light for stills photography - continuous light, bulb flash and electronic flash. Of these three, the latter is by far the most widely used.

Continuous lighting

Continuous artificial light can be provided from something as simple as a torch, but is usually in the form of a movie/filming light with a high output bulb of 80 watts or more. They are powered by rechargeable ni-cad batteries and last for twenty minutes and upwards depending on the capacity of the battery pack. They then have to be recharged.

Designed primarily for movie filming, these continuous light sources are rarely used for stills. When they are, a tungsten balanced film must be used for proper colour balance or the light or the lens must be filtered when using daylight film. Since these filters cut the light output, it is best to use tungsten balanced film.

Using continuous light sources with automatic cameras, you can shoot as for available light and the camera's metering system will adjust the exposure. For manual cameras, a separate light meter must be used to establish the light level and the and Shutter speed selected accordingly.

Bulb flash

Bulb flashes are now rarely used as a new bulb is required for each exposure. Although the quality of the light is excellent, the ease of use and reliability of electronic flash has made the bulb flash virtually redundant.

Electronic flash

Electronic flash is very widely used underwater and there are several manufacturers providing units especially designed for underwater use. An electronic circuit transforms a small DC voltage.(around 5-6 volts) into a much higher (around 360 volts), which can trigger a gas-filled tube and cause it to ignite very brightly for a fraction of a second. The circuit then recycles, ready for the next flash.

Electronic flashes (they're called strobes in America) are much more powerful and effective than continuous light as the flash only triggers for an instant when the camera's shutter is fully open. If the camera's speed is set incorrectly, it will be out of synch with the flash and only part of the shot will be lit with flash and the other part with available light. Most modern underwater 35 mm cameras synch at around 1/60th sec, although some modern land cameras have special shutters capable of correct synch up to 1/250th sec. This high synch speed is very useful when capturing fast action such as some fish etc,

Flash exposure

In order to set the correct for a perfect exposure, you must estimate the distance from your flash to the subject. The closer this distance, the smaller the will have to be. To help you choose the correct aperture, all flashguns are given a guide number, which is an indication of its power. The higher the number, the more powerful the flash.

A typical guide number might be 32 in feet with 100 ASA film. You divide the flash to subject distance into the guide number to arrive at the correct aperture, So at 2 feet you would set the to 32/2 = f16. At 4 feet it would be 32/4 = f8.
Underwater, where the water clarity varies so much and the water absorbs light much more, you must take this into account, The simplest and most permanently effective way to establish exact underwater figures is to take a set of exposures underwater.

Using a subject of average reflectance, take a series of flash exposures using apertures from f2.8 to f22. Slide film is best for this as it is more exposure critical. Make sure the flash to subject distance is fixed and known and that you make a note of each used so that when you get the results back from processing you will be able to gauge the which produces the best result.

Once you have established the correct for one distance, repeat the test for another and so build up a set of apertures for various flash to subject distances. Fortunately for us all, manufacturers usually provide an exposure sticker or guide to give these settings, but it is a useful exercise to do, just to make sure.

There are several types of flashgun currently in production. They can be simplified into the following categories.

Manual flashguns

These are standard units which emit the same amount of light each time they are fired. Their circuits are much simpler and should therefore be more reliable. Since the output is consistent, you alter the to reflect the. flash to subject distance.

Multi-powered manual flashguns

These are a variation of the manual units which have the ability to reduce the output to 1/2 power (1 fstop) or 1/4 power (2 fstops). This ability to control the light output gives you more flexibility in your flash to subject positioning/distance.

Auto flashguns

These units have a separate sensor which reacts to the amount of light being reflected back from the subject. If this is too much, it will cut the output of the flash and so ensure no overexposure. This quenched light is then stored for future use, so giving a greater number of flashes.

TTL flashguns

These units are similar to Auto flashguns except that they have a direct link to a sensor in the camera which reads the light falling on the film during the exposure. This link also takes into account the Film speed and the lens aperture. The result is a very neat and accurate system which is not prone to parallax inaccuracies and which reduces the number of linking cords and connections. They make foolproof exposure possible with Your first roll of film.

Slave flashguns

These are usually normal flashguns with an extra switch setting for a sensor which reacts to the light from other flashguns. When another flash is fired, the sensor triggers the slave flash Instantaneously, These sensors react to a rapid change of light rather than a high level of ambient light, so they are more reliable when the triggering flash is as close as possible. If you are buying a flashgun for the first time. this facility is a red-herring (except for your buddy) as you will need another flash to trigger your slave!

Angle of coverage

Most flashguns fall into two main types - standard angle (covering up to 70° for the Nikonos 28 mm) and wide-angle (up to 100° for the Nikonos 15 mm). To make a standard angle flash cover a wider angle. they can be spread with commercially available diffusers (a good old handkerchief wrapped around the flash will do almost as good a job!). Diffusing the flash will reduce the output of the unit so the must be opened up accordingly. The loss of output is a necessary evil if you want even coverage.

If you shoot with wide-angle lenses most of the time, the alternative is to consider a purpose-designed wide-angle flashgun, These usually have a similar output to standard flashguns but, because they cover a wider angle, they are in fact more powerful when compared angle for angle.

Amphibious v Housed flashguns

There are far more land flashguns to choose from and they cost less so a land flash in a housing is worth considering. Housings are available for most normal sizes and should they flood the flash are much easier and cheaper to replace than an amphibious flash. Housings have flat ports which reduce the angle of coverage so you must diffuse them to widen the angle

Batteries

There are two main types of batteries currentlv available. Standard alkalines can be used only once until flat but ni-cads can be used and then recharged for future use. Most modern flashguns will take ni-cads but it is advisable to check your instruction book just in case. The surge of power which ni-cads can provide might damage certain electronic components.

Although more expensive initially, ni-cads offer a long term saving as they can be recharged over 500 times. This major cost advantage is balanced by fewer flashes per charge. Alkaline batteries usually give up to twice as many flashes but will not recycle the flash as quickly.

As your flashgun is such an important part of your equipment, take time to look at all of the options which are available so that you end up with a unit which suits both your needs and your pocket.

Backscatter

When using electronic flash in poor visibility, it will almost certainly light up the suspended particles in the water, The effect is an annoying scatter of light back from these particles which reduces clarity. Keeping the flash as far away from the lens as possible will reduce this problem.

The angle of lighting

The normal positiion for a flash is off camera at about 45°, to the lens /subject axis. This is a good compromise to reduce backscatter and keep the outfit handleable. To improve your artificial light shots, do not always have the flash in the same position. Take the flash off the camera and try side lighting, top light and even back light for much more impact.

Special Effects

Filters can be placed in front of the flash to alter the colour of the light. Red is a very popular addition although overuse of this effect can reduce its impact. Adding filters reduces light output so the exposure will have to be altered.