Snells 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

fig. 6.29 I took this image below the diveboat at about 20 m. The visibility was sandy and I knew that a decent flash-lit shot of this Napoleon would be a challenge so I specifically went for a silhouette. The bonus was the position of another photographer in the same composition, which was pure luck. F8 at

1/350th sec, ISO 200, flashguns turned off, 14mm end of a Tokina 10—17 mm fisheye lens.

fig. 6.29 I took this image below the diveboat at about 20 m. The visibility was sandy and I knew that a decent flash-lit shot of this Napoleon would be a challenge so I specifically went for a silhouette. The bonus was the position of another photographer in the same composition, which was pure luck. F8 at

1/350th sec, ISO 200, flashguns turned off, 14mm end of a Tokina 10—17 mm fisheye lens.

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 are when the surface of the sea is glass calm and the sky is blue with (to be very pernickety) white fluffy clouds.

Snell Window

fig. 6.30 This Snell's window study was taken in the hot tub at Mimpi Menjangan dive resort in Bali. I was taking a break, housing in hand when I spotted the scenery directly above my head. I guessed the settings and held my housing parallel to the under-surface in no more than 30 cm of very hot water. I took a shot, checked the LCD, corrected the settings and played around for 10 minutes with this idea. F8 at 1/125th sec, ISO 100,10.5 mm fisheye. Notice the settings! Remember, if in doubt try f8 at 125th.

fig. 6.30 This Snell's window study was taken in the hot tub at Mimpi Menjangan dive resort in Bali. I was taking a break, housing in hand when I spotted the scenery directly above my head. I guessed the settings and held my housing parallel to the under-surface in no more than 30 cm of very hot water. I took a shot, checked the LCD, corrected the settings and played around for 10 minutes with this idea. F8 at 1/125th sec, ISO 100,10.5 mm fisheye. Notice the settings! Remember, if in doubt try f8 at 125th.

Snell Window Image Surface

fig. 6.31 Nikon D200, Nikon 10.5 mm fisheye lens, f11 at 250th sec,

ISO 100, flash fill on Jellyfish in Palau. Looking directly up from a depth of 2 m. The blue is the colour of the sky and overhanging trees are visible towards the right-hand side.

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