How to use curves to good effect

If you are into your underwater photography and intent on improving your pictures then looking to better your compositional skills and ideas is the way forward. Last month I discussed dramatic diagonal lines and the dynamic effect they have on the viewer. This month I want to introduce you to a similar concept but more to do with circles, curves and one of my own favourite underwater techniques – shooting through ‘Snell’s window’.

Lines that curve through a composition lead the eye in a similar way that a shoreline, river or winding road do. The circle is made up of a continuous curve, and its movement keeps the eye in the picture frame whilst leading it around the composition. An excellent example of this is ‘Snell’s window’, which is a powerful tool in wide-angle. The circular curve of Snell also makes a soft frame in which to include the main centre of interest.

What is Snell's window?
‘Snell’s window’ was named after the Dutch mathematician, Willebrod Van Roigen Snell, who lived between 1580 and 1662. A professor of mathematics at Leiden University, he discovered the law of refraction known today as ‘Snell’s law’. In underwater photography, Snell’s window is portrayed as an arc or half-circle through which the sky is visible. The area around the circle is a reflection of the seascape, and as such is much darker than the sky. Many newcomers often claim never to have noticed it.

What technique do you use?
As you descend, look back towards the surface through a wide-angle lens; it’s important to keep your vision directed in this way. You will see an arc or, depending on your depth, a half-circle. This window, arc or circle, call it what you may, is your only visual access to the world above. If the visibility is good and the surface is flat, you will clearly see the sky through the water. You don’t need to be in the sea to see Snell’s window; it’s clearly visible through the surface of a swimming pool.

What are the best conditions?
The best conditions are when the water surface is glass-calm and the sky is blue. The trick is remembering to look directly above you – up through the surface. These ideas are best illustrated in pictures.

What camera equipment do you need?
You don’t need the latest and most expensive camera to copy these ideas. All are possible with a basic compact camera with or without a wide angle add-on adaptor. Wide angle provides (almost but not quite) a complete view of Snells circular window so next time you are photo diving look directly above you and shoot through Snell.

Main image:
Diver against Snell. Taken from a depth of 10m looking up towards my buddy in silhouette directly below the window. The shade of light blue is the colour of the sky through Snell. The darker blue circle is the reflected light beneath the surface. F11 at 1/500th second.