by Brian Pitkin

Reproduced from in focus 24 (Oct. 1987)

The greatest obstacle you will have to overcome when taking underwater photographs is the water. Not only is it trying to get into your camera, it distorts the image and filters and absorbs light. The greater your depth, the less light will penetrate and the less colour will be apparent.

Although it is possible to take excellent underwater photographs using natural light, unless you stay shallow your results will lack colour and unless you use fast film, the depth of field will not be very great. The slower the film, the greater the detail recorded, but more light will be required.

Once you have purchased a camera you will need to consider buying a flashgun if you are to put the colour back into your pictures.

Artificial light can be provided in one of two ways, either by a continuous light source, such as an underwater cine light, or by a flash of light from a flash gun. Both sources have advantages and disadvantages.

Continuous or flash

Continuous light has the advantage that you can see what you are Illuminating and you can use a light meter to determine the correct exposure. Moreover, the area of light is usually diffuse. Unfortunately the available cine lights are balanced for tungsten and a blue filter is essential or else a reddish light results. You also need a slower Shutter speed or a larger aperture because the available cine lights are not that powerful. In addition, cine lights are not suitable for light sensitive animals and can be quite bulky.

Flash

Flashguns provide a rapid pulse of very bright light, which freezes notion. You cannot see the effect when you take the picture and unless you are using a dedicated auto flash unit, you need to calculate the exposure beforehand. Unlike cine lights, they can be used for light sensitive animals and are quite compact for their output.

Two types of flash are available, Bulb and Electronic. Early underwater photographers had only bulb flash and although these may occasionally still be used these have largely been superseded by electronic flash. Fortunately land flash bulbs are water and pressure proof so they can be used underwater. The flash contacts are on the outside of the bulb and consequently open to the water. Having to ensure that the contacts were perfectly clean and the bulbs correctly positioned led to a high failure rate. Coupled with this the expense - 36 bulbs cost more than a roll of film - and the need to change bulbs after every Exposure led to their demise in the face of the cheaper and more convenient electronic flashgun, although they can provide a quality and power unmatched by any electronic flash of comparable price.

The majority of underwater photographs are taken with additional lighting, electronic flash being the most widely used form and the simplest method of replacing light and therefore colour.

As with most modern electronic items, flashguns vary from the very basic to the extremely complex, but they all work on the same principle - they convert a small voltage into a much larger one. A battery or batteries charge a capacitor to a high voltage. Once sufficiently charged a 'ready light' comes on. As the shutter release is pressed a charge is sent across a gas filled flash tube. The sudden high voltage ignites the gas producing a bright flash for a split second.

Amphibious or housed?

There is a greater choice of land flashguns and provided a suitable housing is available they will perform perfectly well underwater. Land flashguns are cheaper and, should the housing flood, can be replaced more cheaply than the insides of an underwater flashgun. Most flash housings, however, have a flat port, reducing the angle. So you must either diffuse the land flash or consider an amphibious flashgun with a dome correction port.

Electronic flashguns

Flashguns vary in their angle of coverage and output. They may be powered either by ni-cad rechargeable batteries or dry calls. Depending on the number of batteries, their output, and the output of the flashgun, the number of flashes and the time It takes to recycle can also vary. In addition, flashguns vary from manual single output through manual multi-power to automatic and through the lens (TTL) automatic models, some of which may also be used in slave mode. Furthermore some flashguns incorporate a modeling light, which when switched on enables you to see precisely where you are pointing the flashgun. So how do you decide on which flashgun to buy?

Prices

Prices of amphibious flashguns may be quoted either with or without mounting brackets and arms. They vary from about £130 for the cheapest unit without mounting brackets and arias to over £1000 for the most expensive with mounting brackets and arms.

Angle of coverage

The angle of coverage can vary from one amphibious flashgun to another by between about 60° to 110°. You will probably went a flashgun which will cover the angle of the lens you are using, particularly If you wish to Illuminate all of the frame. The standard 35 mm lens covers an angle of 46°underwater; a 28 mm lens covers and angle of 59°, and a 15 mm lens covers 94°.

If you are taking close-up or macro shots, all of the currently available amphibious flashguns will cover the subject area more than adequately. The wider the angle of a flashgun, the greater the amount of water Illuminated by the flash. If you use a wide-angle flashgun to take close-ups you may illuminate all of the water between the lens and the subject. If the water is not gin clear, you will also light up the sediment held in suspension in the water. This will, at the very least, make your pictures less sharp. So there is a slight trade off between angle of coverage and sharpness in less clear waters.

Fortunately it is possible to increase the angle of a narrow angle flashgun to cover a wider lens by placing a diffuser In front of it. Although this any give a more natural, softer appearance to your pictures it will also reduce the flashgun's output by up to one or two fstops, depending on the thickness of the diffuser.

Alternatively, it is possible to reduce the angle of cover of a wide-angle flashgun by using a snout. This is merely a cylinder of light tight, usually plastic, material which fits snugly onto the front of the flashgun and extends a short distance In front, preventing the light from spreading to such a wide-angle. This is something you will have to make for yourself, however, as they are not supplied with any amphibious flashgun currently on the market.

Batteries or Nicads?

Dry cell batteries are easy to use, but expensive when compared with rechargeable Ni-cad batteries. As dry cell batteries lose their power the recycle time of the flashgun increases. Infuriating though this can be at times you do get some indication that they are running flat. A further advantage is that wherever you go, you do not need to rely on a power supply to recharge. Of course you may find the Increased weight of two or three weeks supply takes you over the baggage allowance.

Ni-cad batteries give full power until shortly before they fail, recycle time remains constant but you get virtually no warning that they are about to run flat. This is not a problem if you know when the ni-cads were recharged and how much use they have had since. Unfortunately, however, ni-cads need to be *trained'. If you recharge ni-cads before they are completely flat they will 'remember' and become conditioned to a short charge and a short working life before they require charging again. Always fully discharge ni-cads before recharging them, preferably In a torch or by running the modeling light if included. If your flashgun has integral ni-cads you will need to check that a suitable power supply is available whenever you travel abroad.

Output

Obviously the more powerful a flashgun is the more light it will produce. The brighter the flash, the smaller the needs to be, consequently, a greater the depth of field that can be achieved. However, most camera lenses give better definition at an Intermediate aperture, so you do not necessarily need the most powerful flashgun to achieve the sharpest image.

Generally speaking, the wider the angle of cover of a flashgun the more powerful that flashgun has to be. The output of a flashgun Is usually quoted either In terms of a guide number or an at a set distance, with a particular Film speed (usually 100 ASA), although Ikelite also specify the output of their models in watt/seconds.

The Guide number is most frequently quoted in metres, although feet are used. Many manufacturers quote a land guide number as opposed to an underwater guide number, although many give both. The guide number denotes the relationship between distance and aperture. If you divide the guide number by the distance between subject and lens, the result is the at which you should set your lens. So, for example, a guide number of 32 at 100 ASA and a subject to lens distance of 2 metres, would mean an of f16 (f 32 divided by 2),

The land guide number is approximately four times the underwater guide number. Thus a land guide number of 32 at 100 ASA would give an underwater of f8 at 1 metre (f32 divided by 4, and then divided by 1) or f4 at 2 metres (f32 divided by 4, and then divided by 2). But remember, whether land or underwater numbers they are only a guide. You will need to shoot one test roll of subjects at known distances and apertures to be certain.

Many flashguns, particularly the more powerful ones, have the ability to reduce output down to 1/2 (1 fstop) or 1/4 (2 fstops). This ability allows greater flexibility in flash to subject distance and flashgun positioning.

The number of flashes you can expect from a single set of batteries or ni-cads in optimum conditions are generally specified by the manufacturer. The more powerful, wide-angle flashguns generally give more flashes per set of batteries or ni-cads. Remember, however, that you will probably not shoot 36 pictures one immediately after the other. The longer your flashgun is turned on the fewer flashes it is likely to give per full charge.

The recycle time varies from one flashgun to another. It can be as little as 1 second. Without a fast recycle time it is virtually impossible to take a series of photographs of the same fast moving subject. If you are using dry cells, many manufacturers suggest you change the batteries when the recycle time exceeds 15 seconds. This can seem like an eternity when you are waiting to take the next photo.

Automatic versus TTL flashguns

Most serious photographers use their flashguns in manual mode, so that they have full control over lighting. There are occasions, however, when you may not have time to set up a shot and rather than miss an opportunity to capture a spontaneous event, you might find automatic or TTL (Through The Lens) Exposure control useful.

All flashguns can be used in the manual mode. Some flashguns can also be used in automatic mode and some can be used in manual, automatic and TTL mode.

Flashguns with automatic exposure control are equipped with a flash sensor, either mounted within the flash head or in a separate optional unit fitted to the cameras accessory shoe. They monitor the light reflected to the sensor from the subject, as the shutter opens, and cause the flashgun to hold back if sufficient light for correct exposure has been detected, thus preventing overexposure.

Flashguns with TTL exposure control are more advanced as they have a direct link, via the flash lead and plug, to a sensor within the camera. They operate in a similar way but monitor the light reflected by the subject which passes through the camera's lens to the camera's internal flash sensor as the shutter opens. In theory the TTL system should give more accurate exposure control, but both automatic and TTL flashguns give acceptable results.

Slave flashguns

A slave flashgun Is triggered by the flash from another flashgun. In order to do this the slave incorporates a flash sensor, which reacts to a sudden but bright flash of light. Some flashguns are designed specifically as slaves, but many of the available amphibious flashguns will also function in slave mode.

A slave used in tandem with another flashgun can be useful for illuminating subjects from more than one angle or for lighting two different yet remote subjects, which could not be illuminated from a single flashgun. They can also be used to simulate an underwater torch if held by a model or dive buddy.

Most of the available slaves or flashguns with slave mode do not work over very great distances underwater as they rely on a significantly bright flash to trigger them. The closer the two flash units are the more reliable the slave. If using two flashguns of equal power at a similar distance from the subject and camera the amount of light Is doubled and the Exposure Increases by one fstop.

Modeling lights

Some flashguns incorporate a modeling light which enables you to see precisely where you are pointing the flash. In my experience, there have limited use in clear bright waters except at very short subject to camera distances but can come in extremely handy in dim or dark underwater conditions.

A modeling light runs off the same set of batteries as the flashgun, so Its use will reduce the number of flashes that you get from a full charge. It is better to switch it on only when needed and to switch It off Immediately afterwards.

If the flashgun you choose does not incorporate a modeling light then you can always purchase one of the large range, of mini-torches and attach It to your flashgun or flash arm. Some mini torches are available specifically to be used as modeling lights and mounting brackets are available to fix them in place.

Conclusions

You cannot take macro and close-up shots easily without using artificial light and unless you are in shallow, brightly lit waters you will need artificial light to put the colour back into your pictures whatever lens you are using. The simplest and most widely used way of providing artificial light is with an electronic flashgun.

>There is no particular flashgun which will suit all your needs underwater, although some of the more powerful and expensive models with multiple power output, slave mode, manual and TTL Exposure control fulfill most, especially if used with a diffuser and a snout as appropriate. Many of the cheaper less powerful flashguns are perfectly adequate for close-up and macro photography and all will have a sufficient angle of cover for use with the Nikonos 35 mm and 28 mm lenses. Moreover they can be diffused to give a wider angle of coverage, but with a consequent reduction in output.

Reproduced from in focus 24 (Oct. 1987)

Google Ads