If You are fed up with blue underwater photos and have decided to get a strobe to improve the situation… but you are not sure how exactly to use your strobe, then read on as this post will briefly go through the main issues you will need to consider when starting up with underwater photography with a strobe. In this guide, I will only look at issues around exposure. Other things like strobe positioning will be out of scope of this article
1. Make sure your camera has an aperture control.
OK… It might be too late for that but be warned: If your camera does not have a diaphragm-controlled aperture that you can adjust, you will be quite limited in what you can do with your light. How do you know if your camera has this then? Most of the time it is easy. All DSLR and mirrorless camera lenses and most advanced compact cameras have this functionality. So, if your camera has the P, A, S, M exposure selector dial visible you are usually in the clear. There is, however, a dying breed of compact cameras without this functionality. These cameras have now mostly been replaced by smartphones so very little is still available for underwater use. There is at least one outlier in this group; The very popular Olympus TG-5 camera, that seemingly has the manual exposure modes, but, does not have proper user adjustable aperture. It's a great snappy underwater camera but limited if you want to add a strobe.
2. Get close.
Underwater strobes could be roughly divided into two categories. Powerful, larger and more expensive ones that are good for both wide-angle and close-up photography and smaller, less powerful ones that can be mostly used for close-up photography. Regardless of which of these models you get, or how many of each you hang on your camera system, diffused light will not illuminate things far away through the water. Even with the biggest and meanest strobes, you can buy, your maximum distance is realistically under 2 meters in water. This means you will never be able to light big sceneries underwater. You will always need to get close to your target.
3. ISO setting.
The first important setting when using an external underwater strobe is the ISO. With this setting, you can choose how sensitive your camera sensor is to light. Luckily almost all cameras have this setting and it can be easily fixed. In a nutshell, you want to move from an auto ISO to a set number. In 95% of the cases this will be the lowest non-auto setting available to you; Typically, ISO 100 or 200. Your external strobe is only connected (in most cases) to your housing via a fibre optic cable and the strobe unit does not know how sensitive the sensor of your camera is. If left in auto you will often get variable results as the camera tries to guess what kind of setting is needed for each scene. By switching to a fixed setting, you are simply taking out one variable from the equation.
4. What does aperture do?
Aperture is the key for all flash photography. Your aperture is like an iris inside the lens that closes and opens and lets different amount of light into the camera's image sensor. Unlike in topside photography with advanced speedlights, regular underwater strobes do not know the aperture setting of your camera. If you leave your camera on auto aperture setting (typically the P mode) the size will vary wildly from each scene and it is often hard to obtain a decent exposure. You will need to change to a mode where you have to full control of the aperture. These settings are the A and the M on your camera. In the A mode, you will control your aperture while the camera decides the shutter speed. In M mode you can control both the aperture size and the shutter speed. In reality, the shutter speed is usually limited to certain range because you are also firing your internal camera strobe (if you have one).
When you close your aperture down you are limiting how much light will fall onto your camera sensor. But as your underwater flash's pulse is quite powerful (in close range) and can vary in output, it can usually cope even with smaller apertures. Small aperture setting tends to have a more dramatic effect on the ambient light… and this is the key… in underwater flash photography, you are trying to limit how much of the drab coloured ambient light is in your image and replace it with the light from the flash gun. If your aperture is left open, too much ambient light will be in the scene and adding even more light from the flashgun will result to washed out images and blown out surfaces. What is considered a small aperture depends on your camera type but if you start from f/8 or f/11 and work your way down from there you are on the right track.
5. What does shutter speed do?
When talking about exposing images underwater we can't ignore the importance of shutter speed. The shutter speed setting is your second tool for limiting how much of the blue/green ambient light is in your images. When you want to control your shutter speed engage the M setting. (Generally speaking, you ignore the S setting as this won't allow you to set the aperture.) When using a flash gun, the shutter speed setting does not seemingly change the exposure of the foreground targets. The reason for this is that the pulse of the flash is usually faster than the what the time the shutter is open for each exposure. Hence in a fully darkened room, you could have a 1/200s of second or 2-second shutter speed and the exposure would look the same when lit with a flash. A modern underwater flashgun can cope with very fast speeds before the shutter starts to "clip" the max output so you can be quite adventurous with your settings.
By shortening the time your shutter is open you can control the background of your image, while the foreground stays the same as it is lit up by your strobe. All those beautiful dark blue backgrounds you have seen in magazines and books use this trick. The brighter the background or water surface is due to the time of day or your depth the shorter the shutter speed needs to be.
As said earlier there is a limit of what kind of shutter speed you can select with your camera with its flash on. This was traditionally called the x speed but luckily today's cameras don't allow you to exceed this number accidentally. Here compact cameras have the edge as their central -type shutters can cope with very fast sync speeds up to 1/1000 – 1/2000s. The larger leaf shutter cameras like DSLRs and mirrorless cameras have much slower maximum sync speeds. They usually range around 1/200 to 1/250s. In a real-life situation, for the blue beautiful backgrounds, you will be often using the minimum allowed shutter speeds.
6. Flash settings – TTL or Manual.
Ask any seasoned underwater photographer and they will tell you that they only use their underwater strobes in full manual mode. You are of course welcome to do this, but you might have paid lots of money for your strobe to have the automatic functions so maybe you could give them a go. Especially in simple close-up lighting situations, the strobe's automatic (often called S-TTL) modes work reasonably well, as long as you have set your ISO low and fixed your aperture. When you move into wide-angle photography, where you want to control the mix of artificial and natural light you pretty much need to switch both your camera and the strobes into full manual mode to achieve good results. Also, worth remembering that most underwater strobe guns work in their S-TTL mode only when the camera is set to P or A mode. In these modes the camera's built-in flash "pre-fires" for exposure calculation reasons. Most current cameras stop firing their pre-flash when switched to M mode and the automated flash modes do not work anymore. At this point, it's best to turn your strobe unit to manual mode.