A BIG BLUE WOBBLY THING that mermaids live in!” That was Baldrick’s attempt to define the sea in the classic BBC comedy Blackadder. Balders wasn’t exactly the sharpest tool in the box, but the fact that even he had spotted that the sea is blue is a reminder of the importance of us getting this colour right in our pictures.
The sea is blue because, as most divers know, water absorbs the other colours of light faster, leaving just blue light at depth. We use our flashes to restore foreground colour, and for successful images we need to set these against beautiful background blues.
However, despite blues being so important in our images, most photographers take them for granted and are far more focused on their subjects. As regular readers know, this column constantly champions backgrounds, and this month due diligence is given to techniques for getting the best blues.

CONDITIONS ARE crucial. The richest and most vibrant blues occur when the water is clear and the sun is high and bright. Digital cameras are certainly more forgiving and adjustable with regards to conditions than film.
But it remains very important to seek out the best conditions.
The direction in which we aim the camera relative to the sun is a major factor. Shooting with the ambient light, rather than against it, will always produce the best blues.
This means that we should look up, spot the sun and approach the reef with it behind us, shining over our shoulder onto the scene.
Blue water lacks its sparkle late in the day or early in the morning, but these are ideal times for more atmospheric images. Instead of chasing blues, we can shoot against the light and incorporate pleasing sunbeams into our images.
Exposure is also very important. All the cameras that we use under water were originally designed for land photography and their metering systems are programmed for that environment.
As a result our camera’s automatic metering will overexpose the background water colour, making it looked washed out.
If you shoot using auto-exposure modes, you should dial in some under-exposure to hold a nice rich blue.
As a starting point I would suggest an under-exposure of two-thirds of a stop (-0.7), although this varies from camera to camera depending on the metering system and the composition of the picture.
If you shoot using manual exposure, you don’t really need to worry about this. Simply shoot, check your LCD screen and adjust your shutter speed. Change the shutter speed rather than the aperture or the ISO setting, because those two will also affect your foreground flash exposure. Remember, it is a faster shutter speed for darker blues and a slower one for lighter blues.
While there are incorrect exposures, there is no correct blue for underwater images. There is room for some personal taste because the one that looks best to you, with that subject, is correct.
Many photographers favour deep, dark blues, which add drama, impact and moodiness to underwater images. These deep blues look particularly good next to warm-coloured, brightly lit foreground subjects.
My own taste is often for slightly lighter, brighter blues (bright does not mean washed-out).
About 10 years ago, Martin Edge coined the term “Mustard Blue” to describe it, although many more people chase these attractive brighter blues too.
The best bright blues come in shallower water (shallower than 14m) especially when diving over reefs surrounded by bright white sand.
These blues give a cleaner and more vibrant look and are underexposed less than moody blues.

THE FINAL FACTOR that impacts on the blues in our images is our strobes.
I realise that this statement makes no sense at face value, but I will explain below. It comes down to the colour of light that they produce and how our digital cameras process our pictures.
But before I get into detail (and some of you doze off) the take-home message is that warmer strobes give richer blues.
This effect is related to something we all see when we take bad pictures. When we shoot a scene without enough strobe we have to make big adjustments in post-processing to restore the colours. These white-balance corrections are applied to the whole image, so also affect the background, usually making it look washed out.
So the first rule for strobes and blues is always to light the subject properly, so that we don’t suck blue out of the background with the white balance.
The second, more subtle factor is to be aware of the role played by the colour of light produced by our strobes.
When we shoot any photo on auto white balance, the camera will look at the foreground, check for any colour cast and process the picture accordingly, to give us pleasingly correct colours. Usually we think no more about it.
Underwater flashes all produce white light, but not all white light is the same. Some is bluer, called cooler, and some is more orangey, known as warmer.
You can find out which yours is with a quick Internet search, but as an approximation US and European strobes are warmer and Japanese strobes are cooler-coloured.
This difference matters because the small difference between the colour of a warm strobe and a cool strobe is enough to affect our background.
When the camera takes a photo it selects a white balance for the foreground based on the flash illumination. When that is with a cooler, blue light the camera compensates by warming up the image to produce correct colours in the foreground.
This auto white balance adjustment, which is needed for a correct foreground, also affects the background. And warming up blue water like this changes it from a rich blue to a less attractive greeny, muddy blue, like the example above left.
When we use warm strobes, the camera has to cool down the picture slightly to give correct colours in the foreground. This also means cooling down the colour of the background, or making the blue more of a rich, pleasing blue! The effect is subtle, but it is there in every picture we take.

Poor exposures are the main reason for poor blue backgrounds. Remember that shutter-speed controls the blue, and be prepared to take several frames of the same scene, varying by just one click until you get the perfect blue exposure.
If in doubt, shoot a series and decide later when viewing on the computer.

Water colour, brightness and saturation can be adjusted in post processing, using the Hue, Saturation and Luminance colour specific sliders in Adobe Lightroom or the Selective Colour adjustment in Adobe Photoshop.
However, both tools take time and will affect other areas of the image with similar colours. It is better to get it right in camera.

Strobes can easily be modified to give us the look we want. A few years ago, I persuaded Inon to make strobe diffusers with a warming gel built in, and these now come with all new strobes.
Or you can just buy some lighting gels, such as Lee Filters’ 444 recipe, which is inexpensive. Use a single sheet for a weak effect and a double sheet for a strong effect, held in place behind the standard diffuser.