Water and light

in underwater photography

By J Floor Anthoni (2000-2005) www.seafriends.org.nz/phgraph/water.htm 
Water is a substance which is 800 times denser than air. As soon as light enters the water, it interacts with the water molecules and suspended particles to cause loss of light, colour changes, diffusion, loss of contrast and other effects. A photo taken under water at one metre distance is not unlike a telephoto above water at 800 metres distance, both looking bluish while lacking contrast.
The way light changes under water is responsible for the typical under water 'atmosphere' and it offers creative possibilities not found on land. This chapter shows how light changes as it enters the water. It also discusses techniques to reduce unwanted scatter in photographs and how to restore colour.
surface effects
The shape of the water is decisive on how the light passes through it. Crinkle patterns and cathedral light. Snell's circle.
scatter and diffusion
The way light diffuses as it interacts with matter, depends on the size of the particles. How to minimise ambient scatter.
scatter from strobe light
How does a strobe cause scatter and how can you minimise it? One of the underwater photographer's worst nightmares. What is the ideal strobe?
loss of colour
Colour changes with depth as water filters out the warm colours. How can you restore colour? Mixed light photography and use of filters.
loss of intensity
The intensity of light diminishes rapidly as one goes deeper, depending on circumstances. 
camera angles
Above water your camera angle is not as important as under water. Why? Why do professionals goto coral seas?
For suggestions and corrections, please e-mail the author.
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Surface effects
Water surface effectsThe shape of the water is decisive on how the light passes through it. Coming from an optically less dense medium (air) and entering a denser one (water), the light is partly reflected back while partly entering the water. Depending on the shape of the water, the light forms crinkle patterns or becomes diffused randomly in all directions.

The amount of light that is reflected upward depends strongly on the height of the sun (place on Earth, time of day and season) and the condition of the sea. A rough sea absorbs more light whereas a mirror-like sea reflects more. In the tropics, the sun stands straight overhead at mid-day, resulting in little loss. In temperate seas during winter, the light diminishes by as much as 3 f-stops immediately under the surface.

As a matter of interest, the reflected light is partly polarised (horizontally) and so is the part that enters the water (vertically). Polarisation is maximal in the early morning and late afternoon when the sun stands low in the sky. The vertically polarised light entering the water makes objects less shiny, more colourful, and can be used creatively, for instance to capture the deep colours of shiny fishes in natural light.

Light loss from reflectionThe diagram shows the theoretical loss of light due to reflection. The top left quarter shows sun rays reaching the water's surface. The top right quarter shows the amount of light reflected and the bottom right quarter that of light transmitted. The hours shown are not those of the clock but of the height of the sun. Only at angles less than 30º with the horizon ('four-o-clock') is the light reflectance significant and does loss of light become noticeable under water. However, in practice, and perhaps due to waves and the light diminishing towards sunset/dawn, the light under water diminishes much more quickly. At 'four-o-clock' one loses a complete f-stop (50%).
Note how the light enters the sea at a steeper angle (blue lobe), which means that most of the time, the light comes from almost straight above, which limits natural lighting options. The light reluctantly enters a dive mask for instance or poorly lights subjects from their sides.

Crinkle patterns
When the water is very calm, its undulations resemble weak positive and negative lenses, the negative ones diffusing the light, resulting in dark patches while the positive ones focussing the light into bright patches. This effect also causes the creative 'cathedral' rays, sometimes visible.

A sand flounder enjoying the extra camouflage afforded by the rapidly moving light patches, known as crinkled light. In an environment where everything moves, even its movements are no longer noticeable!

This photo was taken with a warming filter and short time exposure, through a 50mm lens.

Crinkled light hiding a flounder

To increase the crinkle effect:

To decrease the crinkle effect: To increase the sun rays effect, also known as cathedral light:
  • Do as for the crinkle effect but also:
  • Choose a dark background such as a rock wall
  • Choose a dirty water patch with suspended particles
  • Shoot close to the edge of the light
  • Aim the camera towards the light
  • Avoid shooting directly into the sun. Hide the sun behind an object.
  • Shoot during a clear blue sky; avoid clouds
  • Use a wide angle lens
  • Deeply penetrating sun 'rays' are rare, so use your chance well!
  • Use of sun rays to capture atmosphere
    Blue Maomao fish lazing in the mid-day sun in a narrow channel near Goat Island, in the Goat Island marine reserve, New Zealand.

    The example shows how the sun ray effect has been captured by positioning oneself near the edge of the light, close to a vertical wall and shooting towards the sun in a blue sky. A fill flash was used to bring colour to the foreground and to make fish visible in the shade. No colour correction was used in order to accentuate the blue colours of the fish.

    Note that some clouds are visible in the sky. This picture would have been better without them.

    Lens: 15mm

    Diver in sun rays
    A snorkeldiver enjoys herself in a shallow alcove of Ngaio Rock, Poor Knights Islands, New Zealand.

    This photo was taken with a full daylight correction filter before the lens, shooting upward towards the sun but hiding it behind a steep rock face. This cove was carefully selected as an under water 'studio': a steep wall on both sides to make the sun rays stand out against a dark background; and some weeds in the dark lower right corner to frame the picture. The sky is entirely blue. No flash light was used.

    Lens: 15mm

    Light bending at the surfaceThe water's surface has further consequences for how light continues its path. The diagram shows how light is broken (diffracted) by the surface. A vertically incident ray passes without breaking but as the incident angle (height) becomes less, the light ray is bent to descend more steeply. Finally, light from the horizon passes at an angle of about 45 degrees or more precisely half of Snell's Angle, named after the Dutch astronomer Snell who discovered and described this effect (see separate box). To photograph the full circle from below, one needs a fish-eye lens of focal lens less than 12mm. A 15mm wide angle lens captures a good part of it.

    The consequence of Snell's Window or Snell's Circle is that the light always shines down steeply, even when the sun stands low in the sky. This makes ambient light difficult for lighting subjects. Top lighting is difficult to use creatively, making strobes necessary to bring the light in from the front.
    Snell's window is the circle through which the sky is visible. The area around it is a reflection of the seascape and is usually much darker. This light contrast becomes worse in clouded or semi-overcast conditions, causing problems for wide angle lenses. So treat blue sky weather as wide angle weather, particularly when also the water is calm. Look out for steep rock faces to find cathedral light. Try to hide the sun.

    cathedral light and Snell's window
    Proper use of technique shows cathedral light and Snell's window. The sky is blue without a single cloud. It is 08:00 and the sun comes in at a low angle, the lake's water is perfectly still. The diver obscures the sun as sunrays radiate all around. 13mm fisheye.
    Snells window seen by 13mm fish eye
    A less perfect result as the water is ruffled, creating a bright patch around the canoe that hides the sun. But the subject is perfectly framed inside Snell's window. 13mm fisheye.


    In the example, a mangrove tree has been photographed from below, offering a slightly distorted view of the world above. Snell's Circle runs through the middle of the frame and the horizon is the edge between light and dark. The bottom part of the picture reflects the bottom but about 2 f-stops darker. The photo would have been improved with a graduated grey filter correcting for one f-stop and by applying a half intensity fill flash to the branch in the foreground. At the time this photo was taken, these problems were not apparent. 
    On the right the same image corrected by techniques explained in the Digital Darkroom chapter. When shooting negative film, one can over expose by one f-stop, which gives one the opportunity of applying a grey filter in the Digital Darkroom.
    Looking through Snell's Window same image corrected

    Snell's lawThe Dutch astronomer Willebrord van Roijen Snell (1580-1626) discovered the important law of light diffraction between two media having differing refractive indexes or optical densities or light speeds. The Dutch physicist Christiaan Huyghens later formulated other optical laws in his treatise on light. Snell formulated that light is refracted (bent) towards the optical axis perpendicular to the plane between the media when going from a less dense to a denser media so that
    {sin (a2)}/{sin (a1)}=n1 / n2
    where a1=incident angle,  a2=refracted angle, n1=density of air (=1.00), n2=density of water (=1.33)
    As angle a1 approaches 90 degrees, angle a2 reaches its maximum beyond which total reflection occurs (going from water to air). This critical angle or Snells 'window' is just over 48 degrees to both sides of the vertical:
    {sin (a2)}/{sin(90º)}=1/1.33
    a2=arc sin(3/4)= 48.6º
    Huyghens suggested to look at light beams as travelling fronts of light, like soldiers marching in file can be considered as rows of soldiers. When a light front or row of soldiers meets a denser medium, it is slowed down, causing the front/row to bend and travel/march into a different direction. Note that winds meeting a land mass on an angle, behave similarly.

    Scatter and diffusion
    The way light diffuses as it interacts with matter, depends on the size of the particles. For the ultra small water molecules, blue light is bounced off in all directions equally, while the rest of the light passes through normally. This diffusion was described by the physicist Rayleigh and explains why both the sky and the sea look blue. The diffused blue light appears to come from all directions, particularly deeper down and it has the effect of reducing  contrast while dominating the natural colours.

    Particles as large as phyto plankton but not visible to the naked eye (0.1 to 10 micrometre) act like mist particles, reflecting all colour components of the light back to where it came from. This effect makes driving in the mist an undertaking. This form of diffraction was first described by the physicist Tindall. It causes images to blur but it also offers creative opportunities both above and under water.

    Finally, the snow effect that plagues under water photographers and which is called scatter, is light bounced off visible particles like zoo plankton organisms or even their shed moults. Such 'snow' or 'jelly' or 'snot' often collects close to the surface and should be avoided.

    This orange finger sponge was photographed in a rather dusty environment using an electronic strobe, which accentuates environmental dust particles.
    Orange fingersponge, movie light
    The same orange finger sponge, photographed with continuous movie light and a sufficiently long time exposure. Although the dust particles are still there, causing scatter, they do so while moving across the image, leaving no visible scatter trail. Current and wave action are needed to make them move.

    Scatter from strobe light
    Flash scatterStrobelight scatter is caused by brightly lit small objects close to both the strobe light and the lens. As the particle is usually out of focus, it projects the scape of the aperture onto the film. It is a photographer's nightmare because it spoils the photo but more annoyingly because it is never visible when taking the photo.

    In the drawing the red numbers illustrate that at half the distance from strobe to subject, the light is four time stronger. At half that distance again, 16 times! It decreases quadratically with distance, thus increasing dramatically towards to the strobe.

    Fortunately a number of approaches can be followed to reduce its devastating effect:
    • moving the strobe backwardMove the strobe backward. This can often easily be done using the standard strobe arm. Its effect is dramatic, specially for wide angle lenses. At the same time this results in more even lighting. Note that for very wide angle lenses (fisheye, 13mm) the camera will soon be in the way of the light, resulting in an unwanted shadow of the photographer's head. In wide angle photography think of the strobe as your fill light, painting the colours and no more.

    What is the ideal strobe?
    The strobe is an inseparable part of underwater photography and you will buy one as soon as you have decided on what camera to buy. So what are the ideal strobe's characteristics?

    fur seal in 'snowy' sea
    f035222: a mother fur seal photographed under impossible conditions in a 'snowy' sea with less than 10m visibility and a 50mm macro lens. By using a wide aperture F5.6, the background became bright enough to hide most scatter. Also the shutter speed became fast enough. Note how the seal's body fades away, not distracting from the seal's eyes.
    rock bommy with seaweeds
    f034008: rock 'bommy' with varied seaweeds amongst sand ripples, photographed with a fisheye 13mm lens. The strobe was used to bring colour. No colour filter was used because of the short distance to the subject. Scatter disappeared because the strobe arm was long and bent backward, while also the background was kept light. f5.6 1/125s.

    Loss of colour
    Loss of colour with depthWater particles interact with light by absorbing certain wave lengths (see diagram). First the reds and oranges disappear, later the yellows, greens and purples and last the blue. Loss of the colour red is dramatic and is already noticeable at 50cm! At 5 metres depth some 90% has disappeared.  Since the loss of colour varies critically with distance, it is necessary to make corrections by applying colour correction filters. Their use is described later on and their effect is quite substantial. Note that colour filters applied under water, do not taint the blue background, but when they are applied in the computer or in the laboratory, they do.

    The picture shows that it is the total light path that matters. In the case of the strobe, this amounts to about twice the subject distance whereas for ambient light it amounts to depth plus subject distance.

    Light absorption by wavelength
    The diagram on right gives accurate light extinction figures for one metre water. Where clear oceanic water (probably with visibility 50m) gives 40% loss in red light per metre (blue curve), the light remaining (transmission) is thus 60%. For two metres it would be 60% of 60% remaining, = 36%. Clear oceanic water has its least loss in the blue colour, which means that distanct objects would look blue. As a rule of thumb remember that you lose one f-stop (50%) in the reds for one metre light path. So a subject at 50cm distance from the strobe already loses that one f-stop.
    Average coastal water extinction in the reds (green curve) is about 70%, thus transmission is 30%. Over 2 metres the remaining red light is 30% x 30% = 9%. The dip of this curve rests in the green wavelengths, which makes distant objects look green.

    Loss of intensity
    Loss of intensity with depthThis graph shows actual measurements of light intensity (in f-stops) as it decreases with depth (in metres). The measurements were done in northern New Zealand towards the end of February, our summer in the southern hemisphere. Although well on its return to winter, the sun still stands high in the sky, causing minimal loss of light directly under the surface: just under two f-stops. In the tropics only one f-stop is lost, which can be traded off for finer grained film. In temperate seas it is common sense to use faster film like 400ASA. In mid-winter when the sun stands low in the sky, or in early morning or evening, the loss of light can be dramatic, amounting to 4 f-stops!

    The two measurements demonstrate the dramatic loss of light due to poor visibility (red squares), compared to good visibility (blue triangles). At 30m depth in poor visibility, almost no natural light photography can be done on moving objects. Yet, using a tripod combined with fill flash, remarkable results can be obtained, even at -9 f-stops with shutter speeds of 1 second or longer! 

    camera angles
    camera anglesHow to aim your camera is of critical importance underwater. The diagram serves to give you a feel of what the consequences are of your position in the water and the way you aim your camera. Read it with care.
    • a The camera faces down, close to the surface. Although this is a most convenient way of taking photos, you have serious problems:
      • Close to the surface the plankton skeletons and dead animals collect, and there are bubbles from waves - a rich source of scatter.
      • You have the light coming from right behind and this is the worst angle to show up scatter from all three sources: Rayleigh (molecules), mist and snow.
      • Frontal lighting of the underwater world does not show its form. There is also little contrast.
      • The light path from camera to subject to surface is maximal, thus everything looks at its worst - blue
      • You are furthest away from your subject, causing unsharpness
      • You are in the place where the water moves most, inviting for motion blur.

    Why do professionals prefer coral seas?
    Professionals have a job to do, photos to sell and for them time is money and quality is a must. So they all do their work in warm tropical seas. Why?