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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. Here’s how you do it:

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.
On a full frame sensor (FX) a 16-mm fisheye lens will capture the majority of the circle, but not all. I was once under the impression that the deeper you went the more could be included. This is incorrect! To photograph the full circle, you need a fisheye lens equivalent to a 12-mm lens on a full frame FX sensor.

The best conditions for shooting Snell’s window are when the surface of the sea is glass calm and the sky is blue with (to be very persnickety) white fluffy clouds.

Nikon D200, 10.5mm Fisheye, Subal housing, f 11 at 1/125th sec, natural light. Snell’s window is at its best when the surface of the water is glass calm. Puffy white clouds in a clear blue sky emphasise the contrast. The trick is finding a subject which complements the effects of the window. I’m always looking to place circles within circles and hard coral formations work well. To have illuminated the coral with flash would have (in my opinion) diminished the effect so I’m inclined to use natural light whenever I can.

black and white Snell's window underwater photograph

Nikon D200, 10.5mm Fisheye, Subal housing, f 11 at 1/500th sec, natural light. This image is all about the contrast and a number of layers between

1.    The silhouetted diver
2.    The view through the window (the inner circle)
3.    The reflected circle of light (the outer circle)
4.    The portion of reef at bottom left

An underwater photograph of a lion fish in Snell's window

Nikon D200, 10.5mm Fisheye, Subal housing, f 13 at 1/250th sec, natural light. My angle of view is not directly upwards, that’s why only a portion of snell is visible but it’s just enough to provide more depth perspective, which creates interest and something else to ‘jazz up’ the picture. My strobe did not recharge in time to fire but I prefer the position of the lionfish, which is in perfect silhouette. Also notice the strong diagonal of the edge of snell curving at top left and running down to bottom right

Nikon D200, Subal housing, f 5.6 at 1/90th sec, natural light. No more than one meter deep, the surface was again flat which emphasises the circle. I used a 12mm – 24mm lens and shot hundreds of shallow water upward angles of the pier at Nuweiba in the northern Red Sea.

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