Strobe Use for Digital Cameras (for beginners)

Reprinted from the Houston Underwater Photographic Society newsletter.
Do not reproduce without author's written consent.

November 1, 2002 - After a break last month to talk about traveling, we'll get back on track to putting together a digital underwater system. Last month we talked about selecting a camera and housing and looked at some of the costs. This month I'd like to focus on strobe use with today's digital camera setups - a subject of much confusion and misinformation.

Before we talk about strobes, I'll quickly touch on other lighting. When digicams first started going underwater, many people recommended using a constant light source such as a video light. This source is easy to use and they work on digital videocameras, so they should work pretty well with digital camera's right? Well - not exactly. Here are two shots of the same coral, the first taken using a 100 watt video light and the second using dual strobes - both taken with a Coolpix camera:

1/120th @f4
100 watt video light
1/60th @f8
dual strobes

Note that even using a 100w video light source (one of the brightest) the camera metered one stop less light in the first shot than in the second, and the camera is much closer to the subject. The working distance with the video light is really less than a foot. This is because the video light is putting out 100w*1/120*seconds = 1.2 watt seconds - whereas a decent strobe can put out 100 watt*seconds! So, for colors that really pop, a strobe is the way to go.

Learning to take decent digital pictures using a strobe is not easy and just like film photography, it takes patience and trial and error learning. But remember, this is digital, so all of your "film" is free! There are many consumer cameras on the market - and unfortunately, just about all of them treat flash use differently. I'll review each "method" and describe which strobes are compatible and why.

Internal Strobe Only: This is by far the easiest way to get good flash pictures - but remember, you'll have to work within the constraints of the system. Since the camera controls the flash, you are essentially shooting TTL. However, the internal flash is weak, so you will probably have to shoot at the minimum working distance of your lens and wideangle is totally out of the question. Getting close will also help to minimize backscatter.

The strobe you choose will be largely dictated by your camera and housing choice. The next few paragraphs will explain the different systems:

"Hard-Wired" Cameras (TTL and Manual): Some cameras provide a socket for attaching an external strobe and some of the housings for these cameras provide a Nikonos or Ikelite bulkhead for attaching a sync cord. If the camera provides for TTL control, then the system will behave a lot like a Nikonos V or housed SLR. The Nikon Coolpix and Fuji S2 are two examples. If the camera does not provide TTL control of the strobe, it can usually still fire it in manual - but the photographer has to set the strobe power using guide numbers. This is the case for the Tetra housing for the Olympus cameras.

Coolpix 990 hardwired to SB105 (photo Sarah Bernhardt)

Preflash Cameras: Most digital cameras don't do TTL flash metering like film cameras do. Instead, they send out a series of preflashes and basically "read" them as they come back to the camera's sensor. This information is used to set the power and duration of the flash. The majority of consumer digital cameras use this approach and do not have a hotshoe or socket for an external flash, so a slave strobe must be used. The slave strobe can be set to trigger off of the camera's internal flash but of course, the housing must be clear. Unfortunately for us, most traditional slave strobes fire when they detect a flash - so the camera's preflash sets them off. A few progressive companies have modified their strobes to comply with this preflash system.

TTL Slave - Ikelite has converted their TTL slave sensor for digital use. The electronics have been changed so that the slave sensor can detect the preflash coming from the camera's small internal flash and "tell" the strobe to mimic it. The camera reads the light coming back to it (from the strobe) and sets the flash duration. It then fires the camera's main flash and the strobe mimics that too! To avoid backscatter the small flash on the camera is blocked or reflected up towards the TTL slave sensor.

Manual Slave -- Normal slave strobes can be used with digital cameras but they have to be "told" to ignore the small preflash and trigger on the camera's main flash. The Sea and Sea YS90DX is one such strobe. Since it is a manual slave, the strobe designers have added a 12-step power controller for fine adjustments. Guide numbers can/should be used to help set the strobe power, but most digital shooters take a few shots at the start of a dive and review them, then set power according to this "instant feedback."

ROC - The engineers at Light and Motion have actually put the strobe manual power controller IN the housing. They call it the ROC (Remote Optical Controller) and it gives fine control over a wide variety of strobes. Virtually any strobe can be used with this system.


Fuji S2pro and dual strobes. This shot was actually
underexposed by about a stop but was "saved" using Photoshop

Consumer digital cameras don't have the dynamic range of film - so if you are not careful you can easily overexpose a shot. Overexposed or "white" pixels don't contain any information - that's why many digital shooters recommend erring on the side of underexposure. Believe it or not, using Photoshop, you can bring a shot up one, two, or even three stops! The preceding photo is a good example.

Here is the same shot of the moray, before adjusting levels in Photoshop

Hopefully this column has cleared up some of the questions you may have about strobe use. Choosing a strobe and camera/housing setup is certainly a complicated decision. The easiest decision is to go with a hard-wired sync cord and TTL control - afterall that's just about like shooting with a housed SLR. But as you can see from the September column, it's the more expensive choice. Whatever system you choose, with a little forethought and practice, you'll soon be able to get beautiful saturated shots that you are proud to show off to your dive buddies and local Underwater Photo Society friends.

To discuss underwater housings, digital cameras, and strobes with other wetpixel readers, check out our message forums.