Ok, for this post I am going to assume that you have a) an underwater camera set-up that allows you some manual control, and b) a single or pair of underwater flashguns that you can also take control of.
If you have got a recent high end compact camera, DSLR or one of the many new Mirrorless cameras this should be the case.
And if you have bought a strobe/flashgun from a reputable dealer they will have had the conversation that should have resulted in you getting a controllable setup.
This is only going to look at how to balance your exposure using your strobes in underwater photography.
Flash positioning will be covered at a later date.

A strobe or flashgun is considered by many to be an absolute necessity for serious underwater shooting, however I wouldn’t quite go so far as to say that, as lot’s of picture taking opportunities are well within the grasp of shooting available light only, however you will quickly find that there are many circumstances in underwater photography where you will most definitely need some extra light over and above what the sun dishes out.
This is particularly relevant if you shoot a lot in less than ideal visibility, or you dive in non tropical dive spots.

So the purpose of this post is to get you up and running with a starter set of instructions that you can utilise and build on to expand your strobe knowledge.
It should give you a little bit of understanding in simple strobe technique, and how you can use the combination of your camera and available light and also the addition of strobes to the equation.

I will use a set of real world examples, shot at the same time of day, and of the same subject so you can see how manipulating shutter speed, aperture and flash output can give you a number of different picture choices.

Flash Exposure Demo 1
Flash Exposure Demo 1

Above is the shot straight from the camera, and with no settings changed than from how I had been shooting a few minutes earlier, which just shows how the light can alter dramatically as you change position.To set the scene I was having a gentle bimble on the last dive of the day, the light was getting low, and the vis wasn’t fantastic. However what I could see with my eyes differed enormously from the shot on my screen. The background was a lot lighter and bluer for a start, and the foreground was being lit, albeit dimly, by gentle late afternoon sunlight.

So how to make the scene on the screen resemble more my experience?
At this point when in the water, it’s sometimes a good idea to switch off your strobes and adjust your shutter speeds and aperture to get a decent available light shot. During the height of the day at this depth (around 10m) that may have been possible, but in the gloom of the late afternoon it becomes trickier, and your shutter speeds would be slow, your apertures wide, and your ISO’s needlessly high and grainy.

And taking all that into consideration weighing up my options, I decided to opt for a mix of daylight and strobe. I actually purpose shot this scenario just to make this demo and these circumstances proved perfect. Its not the most exciting of subject matter, but it wasn’t going anywhere so served this end well!

I am shooting with my fisheye lens so am pretty close, within arms reach of the corals. My aperture of f10 was small enough with this lens to give me reasonable depth of field ( the amount from front to back which is considered in focus) but the shutter speed, whilst adequately fast enough to freeze any ambient lit motion, was not letting enough light in to make the background light enough, it looked like a night dive.
The flash is lighting the foreground ok(ish), so I need to make the background lighter by slowing the shutter speed down thus letting more light to fall on the sensor making it brighter.

Making the background lighter by increasing or lengthening the shutter speed.
Making the background lighter by increasing or lengthening the shutter speed.

Here by lengthening the shutter speed to a 1/25th of a second I have let more light fall on the sensor, rendering the background a few shades lighter, to a deep blue colour. I have left the strobes alone for now, and kept the aperture the same so there is little change in the foreground. I am still not happy with the colour of the background and would prefer it lighter still, however I am in danger of introducing camera shake if I let the shutter speed drop further. My judgement call in this instance is to chance a slower speed, as the subject is stationary, and the image stabiliser in this camera is good so I can probably get away with more than you would think. Be careful though when doing this as if it was a moving subject like a turtle or a moray eel then you risk a double image caused by subject moving whilst the shutter is open a relatively long time, it should be good for stationary reefscapes though.

Lengthening the shutter speed to a 1/15th of a second, has made the background a nice blue.
Lengthening the shutter speed to a 1/15th of a second, has made the background a nice blue.

A 1/15th of a second has been just the right shutter speed in this situation to make the background light enough. So now to set to work on adjusting the foreground exposure.

Leaving everything else as is I have now turned up the left strobe to match the rights output.
Leaving everything else as is I have now turned up the left strobe to match the rights output.

So now we have a brighter foreground by increasing the left strobes output, this has also boosted the overall foreground exposure in total, and a tiny bit too bright in my opinion now, background looks fine still though. So just minor tweaks to either the flash output or the aperture to fine tune the foreground look, and the background exposure altered with the shutter speed. So onto our final shot.

Just a minor tweak to reduce the overall flash exposure on the foreground.

Generally satisfied with the exposure, I just turned down the strobes a little as I thought I had overdone it a little with the strobes and it looked less natural. What you choose to do and how you want the end result is entirely up to you, I have tried to replicate what the actual conditions looked like, but who says that that is the “shot”.
Bear in mind that being able to control these three simple things gives you ultimate control over the end result in all sorts of conditions, so its worth learning how to do. Spend a couple of dives just practicing these simple skills, so you have it down pat for any situation that arises.

This was in a low light situation and using really low light can force you to use very long shutter speeds, which you can turn to your creative advantage. As here with these shots I have emphasised the background blur to lend a sense of speed or motion to the picture.This is called “dragging” the shutter and is a useful tool in your box of skills.

A slow shutter speed drag shot.
A slow shutter speed drag shot.


You can take it to extremes for dramatic effect and use it to produce twirls like this shot of my videographer friend GeGe. Some planning helps here, and we discussed the concept beforehand on the boat. So underwater there was no confusion and it only took two tries to get it right. All I did was instruct GeGe to adopt the agreed pose and then I set a slow speed of around a 1/4 of a second and when taking the shot I rotated the camera around its axis. This meant that the flash would freeze the foreground, and the long shutter speed combined with the movement “dragged” the image into a circular pattern.

A dragged shutter shot combining a circular motion and long shutter speed with a flash exposing the foreground has given this outcome.
A dragged shutter shot combining a circular motion and long shutter speed with a flash exposing the foreground has given this outcome.

Getting flash like this allows you to have some fun and at the same time your “getting flash” ie understanding it more.
This is only covering the exposure side of underwater photography.
I will be covering the fine art of flash positioning in a future blog post.
Don’t forget that you can also try out all these techniques when you’re  on land too.
Speak to you all soon.