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Selecting Underwater Flash / Strobes
for D-SLR systems
By Steve Warren
Strobes are an essential item of equipment for most underwater stills photographers. They are used to light up dark places, restore colour and create moods. In this article we'll look at some of the features and benefits that underwater photographers need to consider when purchasing strobes. Because strobes can seem complex, many underwater photographers make poor choices when selecting a strobe and are disappointed and held back when it fails to meet expectations. This is an introduction to help new underwater D- SLR photographers understand strobe features and benefits and avoid purchasing an inappropriate model. It really is only a primer - talk with Mark Koekemoer or myself for more detailed one to one advice.
The main topics we will cover in this feature are strobe power, angle of coverage, triggering, exposure control and powerpacks.
Strobe power is often misunderstood. It's also inseparable from angle of coverage, which we'll look at later. The power of a strobe is usually expressed as a Guide Number or GN. This is an industry standard used by all land and underwater strobe manufacturers, making it easy to compare the power of different strobes. When you look at Guide Numbers for different strobes, the differences may seem huge. For example the flash built into a D - SLR might have a Guide Number of 12 metres and an underwater strobe might have a Guide Number of 24 metres. As with choosing any underwater camera equipment, knowing what subjects you want to shoot, likely underwater conditions and which other camera equipment your strobe must interface with, like lenses, will help you make the right choice. We'll come to that in a moment. Let's start by explaining what a Guide Number is and what it means to us as underwater photographers.
What a Guide Number is not in itself is an indication of how far away you can photograph your subject. Having a GN of 24 metres does not mean
you can light something 24 metres away - especially underwater. We use the GN to establish the F Stop or aperture we need to take a photograph
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with our flash at a given distance. So if our subject is one metre away and our strobe has a GN of 22 we divide distance (one) into the GN (22) and get 22. The 22 is the number of the F Stop or aperture we need to set on our lens to get the correct exposure. If the subject is two metres away we divide the distance (two) into the GN (22) and get 11. This means we need to set an F Stop of F11 on our lens.
To standardise Guide Numbers, most strobe manufacturers state the GN is specified for an ISO or sensitivity of 100. It's important to check this. If the strobe manufacturer chose an ISO of 200 it would slew the results. A strobe with a GN of 22 used to photograph a subject at one metre requires an F Stop of F22 as we've already seen. But if we set the ISO to 200 we find that we would use a smaller F Stop of F32. The strobe has not become more powerful. It's just that a setting of 200 ISO needs less light than an ISO setting of 100. The difference between 100 and 200 ISO is one F Stop, which explains why we needed to change our lens F Stop from F22 to F32. It's also a difference of one F Stop.
Two other issues can confuse how powerful a strobe is. When comparing strobes you need to check you are not confusing metric and imperial Guide Numbers. American strobes may be quoted in feet. So it could be it has a GN of 80, which looks very powerful indeed when ranked next to a Japanese strobe with a GN of just 24. In fact they have identical power. The American strobe has been quoted in ft and the Japanese gun in metres and 80 ft equals 24 metres. The second problem is the effect on power of taking the strobe underwater. The clarity of water varies enormously and scatter, diffusion and absorption all reduce the power of your strobe - and its range. By convention most underwater strobe manufacturers provide an underwater GN in their specifications. Most assume you'll lose half your power by taking the strobe underwater. It's probably an optimistic figure based on unrealistically clear water. So a few strobe manufacturers use an underwater GN of just one third of the surface GN. This can make them look poor by comparison. As an example a strobe with a GN in air of 24 metres has an underwater GN of 12 if we use the half power rule, but only 8 if we use the one third rule. To avoid confusion ignore the underwater Guide Number and only compare the surface GN.
It's also important to understand that there is a difference between the distances a strobe can actually light a subject and how far it can reach to restore colours. Most underwater photographers use strobes principally to restore the colours lost through absorption. Water filters colour both vertically and horizontally. The deeper you dive the less colour there is - reds, oranges and yellows are lost in quite shallow water. But the further you are from your subject the more subdued the colours will also be. Sunlight can just about reveal reds in clear water at 10 metres. It's an awfully lot more powerful than even the largest, most macho underwater strobe. So a realistic maximum range for getting strong vibrant colours, especially reds, using a powerful strobe is about two metres from your camera. This might not seem far, and it isn't. But colour is being absorbed on the strobes way to your subject and then again as the light is reflected back to your camera from your subject. So it has actually got to return colour over a four metre distance. Subjects that are light toned can be shot over much longer distances - like the underside of a manta ray for example. Here the strobe is just providing illumination by reflecting off the manta's pale skin, not really restoring colour. Even then you may the flesh tones take on the hue of the water.
The advantages of a powerful strobe include greater range, though this may not be that significant in real terms - the difference between a strobe with a GN of 24 and one of 32 is less than one F Stop. So the distance you can shoot over is not increased by very much at all. But you can use a smaller F Stop. This can let you gain depth of field, especially valuable to macro photographers. You also tend to gain shorter recycling times if the strobe is not being used on maximum power - and high power strobes are not often used on their full power. So you may be able to shoot faster and for more frames than you would with a lower powered strobe. This can be very useful when photographing fast moving fish. Disadvantages of high power strobes can be that they tend to be large and heavy, creating drag in the water, transportation problems when travelling by air and unbalancing your camera system if they are significantly positively or negatively buoyant. They may also need a dedicated rechargeable power pack. You may need a spare, so you can use one underwater while the spare is charging. You may also want to think about a back up charger.
Power isn't the whole story. It's possible to make quite powerful strobes that are compact, lightweight and inexpensive. But these strobes may have an Achilles Heel that will undermine your underwater photography from the outset. To select a strobe you also need to know its angle of coverage and understand why it's important.
Underwater photographers rely very heavily on extreme wide-angle lenses. Many underwater strobes in an effort to cut costs or keep size down handle wide-angle photography very badly. The angle of lenses and strobes are specified in degrees. A typical wide-angle lens used by an underwater photographer will "see" at least 90 degrees and a fisheye will see twice this at 180. Any strobe that covers less than the angle of your lens has the potential to hot spot, like a torch shone on a wall. You may get just part of your image lit by flash, creating a brightly coloured area surrounded by a dull, unlit border. To counter these problems there are equipment and techniques you can use that provide solutions.
If you plan to use wide-angle lenses, it makes sense to start by selecting a wide-angle strobe. A typical wide-angle strobe will have an underwater angle of coverage of 100 degrees or so. If you want reasonable power as well, you need to check the Guide Number. A good starting point is a minimum in air GN of 20 metres. Some strobes, especially lower priced models, which don't offer wide angle coverage can be equipped with a diffuser to spread the beam. However this can significantly reduce the power of the strobe, costing you half the light or more in some cases. You'll have less range; working at higher powers to compensate will slow down your recycling time and slow down your photography. You'll also need to use larger F Stops reducing depth of field.
Using two narrow beam strobes can be a solution. This will increase the coverage. But, especially for someone getting started, there are a lot of disadvantages. Not least of which is a strobe failure leaves you without wide-angle capability (a compelling reason for owning a second strobe is as a back up). In fact two wide-angle strobes are often used by underwater photographers when one can do the job just as well. This is often due to the photographer using poor strobe positioning. In this case the second strobe cancels out shadows caused by the first that could easily be avoided anyway by using different lighting angles. There's certainly a place for dual strobe photography, but this is often most effective for creating mood lighting for close ups rather than lighting a simple wide-angle picture.
Triggering your strobe can be done by optical or hardwired cable. An optical cable is made from fibre optic cable and relies on light from your cameras built in flash to fire your strobe via its slave sensor. An increasing number of D- SLR housings can use this system. Using this system, some strobes will let you use TTL metering, which we'll explain a bit later. If a housing cannot support a fibre optic cable itself, converters may be available to turn the electronic signal from the cameras hot shoe into a pulse of light that will trigger the external strobe. Fibre optic cables can normally be unplugged underwater. So the strobe can be switched between cameras underwater or a strobe taken off the camera altogether and used as a slave. This provides more creative possibilities. Because optical cables are wet connectors, there aren't any leak points, so maintenance is minimal.
Hardwired cables rely on carrying an electrical signal from your camera to your strobe. These have an advantage over commonly used fibre optic systems for TTL use which is explained a little later. Hardwired systems are normally dry connectors. The cable cannot be removed underwater so the strobe must stay attached to your camera system for the entire dive. There are potential leak points where the cable attaches to your housing and usually where the cable attaches to your strobe. So they need more maintenance than fibre optic wet leads.
Both fibre optic and hardwire cables are prone to breakage. For that reason carrying a spare cable is good practice. The longevity of a cable depends not just upon the amount of use and care it receives, but also on the manufacturer. Some brands are better than others.
When it comes to getting the correct flash exposure, there are three methods. Manual, Auto and TTL. Manual exposure requires you to set the power level of the strobe yourself. This is very easy to do - for most underwater photographers using manual for the first time, there's a surprisingly short learning curve. Initially, photographers learn to get the flash exposure right by watching their LCD screen and using histograms. But very soon they get to know the right settings instinctively.
Auto strobes have a built in light meter. This works completely independently of the D - SLR cameras own meter. This presents some problems that are worth reviewing. The first one is the position of the light meter sensor. On most auto strobes this is built into the front of the gun. It sees a fairly narrow angle. Auto strobes work by bouncing the flash off the subject and measuring the amount of light reflected back into the sensor. When enough light has bounced back, the flash shuts off. So the strobe has to be pointed at the subject at an angle that ensures the flash bounces back into the strobe sensor. If it fires into open water, the light from the flash has nothing to reflect off. With no bounce back, the gun's meter simply assumes the subject is beyond its range and it will fire on full power. This will usually over expose your subject. So you have no option but to point your strobe directly at the subject. This can lead to compromises - at best you may not be able to use an angle that lights the subject the way you want, to bring out texture through side lighting for example. At worst you'll get lots of backscatter.
Because the light meter in the strobe acts independently of your camera settings, you need to tell your strobe what those camera settings are. The old maxim, garbage in, garbage out holds true. So you must set the aperture on your strobe dial to that you have also set on your camera lens. If you set F5.6 on your camera lens, you must also set F5.6 on your auto strobe. If not, the light meter on the strobe will be acting on wrong information and will either under or overexpose the subject. Most auto strobes assume you will be using an ISO of 100. In reality you'll use a range of ISO's and you'll need to mentally compensate for these different settings. You usually do this by altering the aperture on your strobe dial by one F stop for each halving or doubling of your ISO away from its baseline value of 100. Because auto strobes depend upon you telling them exactly what setting you have chosen for your camera, allowing the D- SLR to choose the ISO by putting it on auto ISO or using a programmed exposure mode or shutter priority either of which setting allows the camera to select the F stop for itself won't work. As the camera changes these settings, the strobe won't know what's happening and any decent flash exposure will be purely coincidental.
TTL stands for through the lens and is a flash metering system that began replacing auto strobes for SLR photography in 1975. TTL systems allow fast, fuss free shooting that usually generate acceptable flash exposures most of the time. It's especially valuable with subjects that change their distance rapidly as you can track them without having to manually change power settings. TTL systems use the cameras own metering system to control the flash exposure. The meter sensor is built into the camera body behind the lens (hence TTL). TTL has replaced auto strobes so successfully in land cameras for several reasons. Firstly, the lens will be aimed at your subject. However the strobe can be aimed away from the lens and subject at a ceiling or wall for bounce lighting and the exposure will still be correct. This has value for underwater photographers as we routinely aim our strobes slightly away from our subjects to avoid backscatter. Secondly, because the camera talks to and "commands" the strobe, any changes to aperture or ISO are automatically taken into account, so there's no risk of forgetting to change a setting the way there is with an auto strobe. TTL has become very highly developed. The most sophisticated D - SLR cameras now use distance information from the lenses focusing point to determine where the main subject is located in your photograph, can link to a chosen focusing point, again helping pin point the area that must be given the correct amount of illumination and can work with multi sensor, centre weighted and spot metering patterns. Despite this some conditions can still fool them. So most systems allow you to override the TTL, either by using exposure compensation or setting the flash output manually.
There are several ways to achieve TTL with underwater D- SLR systems. One very effective solution is to use a dedicated land strobe, designed for your camera, and place it in its own housing. The highest performance TTL systems rely on sharing information and using circuits in the camera body, lens and strobe. So a strobe made by the company who built your camera is likely to offer you the greatest degree of exposure accuracy.
Another solution is to use a TTL converter. These were developed to allow a strobe that would not normally work on TTL with a given camera to do so. These are most often used to convert an underwater strobe designed for the TTL system used by Nikonos underwater and Nikon land film cameras, which, before digital, was an industry standard. When Nikon introduced D - SLR cameras, they also introduced new strobe systems, which were incompatible with their earlier film cameras. It's these older circuits, used in nearly all underwater TTL strobes that converters seek to convert to working with modern D - SLR cameras. They are offered by third party suppliers.
An alternative route to achieving TTL without using land strobes in housings or conversion accessories is to use an optical TTL system. These rely on seeing the flash from your cameras built in strobe to operate. Your built in camera strobe is used as a "master" to fire and control the exposure from your external "slave" strobe. Advantages include the use of optical cables which means you have a wet connector - so you can disconnect underwater and move your strobe between systems or for off camera creative lighting techniques. There are also no leak points. Exposure will also be very accurate as you can take advantage of your cameras own sophisticated light meter. Disadvantages include the fact that most underwater strobes have higher Guide Numbers than those built into a camera. This means that when your built in strobe reaches maximum output, it will also stop the external strobe from delivering any further power. So the maximum range of the underwater strobe is reduced. You can overcome this by manually selecting a higher power setting on your external strobe. In truth, this is not a problem under most conditions and with most subjects. You also need a D- SLR with a built in strobe and a housing that supports this type of optical firing - many don't.
Powerpacks are something else to consider when choosing a strobe. The powerpack is at the heart of how much power you have, the strobes angle of coverage, speed of recycling and how many shots you can take without changing or recharging batteries. Even a few years ago owning a high power strobe that could cover wide angle lenses, shoot fast and do a couple of hundred frames meant using physically large guns that were very heavy to travel with and depended upon rechargeable batteries.
Today, ultra compact strobes exist which can do what their oversized, overweight ancestors did, only better. The benefits are that travelling, especially by air, is easier - especially if you use more than one strobe. Your system is also lighter for shore diving. Because many modern strobes don't need rechargeable power packs, there are other advantages. Strobes that do use rechargeable power packs normally rely on special dedicated packs. Today, these are normally NI-MH cells and very long lasting. Memory issues have been virtually eliminated, solving one of the main problems of Ni- Cads. But dedicated packs do have problems - they are often heavy and it's likely you will want to have a spare. A spare allows you to use one while charging another. Also, some rechargeable strobes cannot use disposable batteries. So this fallback, useful to cover a failed cell, broken charger or the fact you just don't have time to recharge between dives isn't available. So there's a cost, which can be substantial, and more weight to carry, tied up in owning a back up rechargeable battery. Moreover, the special power packs used in underwater strobes also usually require a unique recharger. And, again, you may want to carry a spare.
Strobes that don't depend upon dedicated rechargeable power packs can, of course, still use them. You can normally use NI- MH rechargeable batteries in any strobe that uses disposable cells. When looking at which power packs best suits your needs, you need to look at the strobes manufacturers specifications. These will tell you how many flashes you'll get from a set of batteries. The number varies with the type of battery you use. Normally you'll be able to see how many flashes you'll get on full power. Keep in mind that a strobe with a maximum Guide Number of 32 might not do as many flashes as a strobe with a maximum power of 20. It has to work much harder. But if you don't use it on full power, you might get as many if not more shots than the gun with a GN of 20 can provide. So there's a fudge factor. You need to also consider how many shots you'll take on a dive and how often you are willing to change batteries. If you are making several dives from a RIB, opening up your strobe to change batteries is not as practical as changing them on a liveaboard or in your hotel room.
Recycling time is something else you'll need to consider. This determines how fast your strobe is ready to fire again after you take a shot. Modern D - SLR's can take several frames per second and their buffers let you shoot a barrage of images before you have to give the camera time to download. This has meant digital photographers can shoot more shots, more quickly than was ever common practice with film cameras. In turn, this has meant a requirement for faster recycling strobes to let us take advantage of this benefit. So, when comparing strobe specifications, we also need to figure in how fast we'll want to shoot. The typical situation in which we'll want to burst shoot underwater with flash is shooting small fish. Modern underwater strobes can often recycle in under two seconds, even when used on maximum power. This is still slower than many D- SLRs can shoot. However by reducing strobe power a little, recycling time is shortened. By shooting on a "low" motordrive setting, rather than "high", you can often shoot with fill flash for a few high speed frames. This will often nail the photograph of a fish that you're looking for. Again, checking the strobe manufacturers spec sheet will provide you with the information to compare competing strobes. If you use an optical triggering system, then also check the speed at which your built in strobe can recycle. It may not be able to fire fast enough itself to keep up with your motor drive and trigger your external strobe.
Underwater photography is a very equipment dependent activity. A strobe is one of the major investments you will have to make. The number of strobes on the market, combined with a lot of misinformation doing the rounds, has made choosing the right one more complex than it really ought to be. This article isn't exhaustive (even if it's exhausting!). For a bit of one to one advice just get in touch.
Steve Warren
INON UK Underwater Photography Instructor - Examiner
For more information check out "The Underwater Photographer" by Martin Edge. This is in its 4th edition and is a classic guide to taking great underwater images. Ocean Optics Mark Koekemoer and Steve Warren are delighted to have been invited by Martin to be contributing authors to the latest edition.
