5 minute read
Underwater Photography
from Scuba Diver ANZ #41
SNELL’S - A WINDOW TO THE SKY
If you want to get an impressive wide-angle photo, there are a few compositional elements you can always rely on. The Snell’s window is one of them, but what is Snell’s window? Mario Vitalini explains
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Photographs by Mario Vitalini
Snell’s window is an effect caused by light from
the sun being refracted when it passes through the waterline and the underwater scene being reflected on the surface.
It is named after the Dutch mathematician Willebrord Snellius, who discovered the refraction law we know today as Snell’s window. When underwater, if we look upwards, we will see a circle of light with darker sections on the sides. Effectively what our eyes see is a 180 degrees image of the surface, the brighter area. The darker edges are the reflection of the bottom.
How to achieve a Snell’s window
The most-important tool you need to achieve a perfect Snell’s window is a fisheye lens. In order to get that circle of light, your lens needs to cover a very large angle of view. A 15mm fisheye lens on a full-frame or an 8mm on a micro 4/3 (mirrorless) camera will do a perfect job.
If you point the camera directly to the surface you will get almost a complete circle, however you can get partial windows by shooting at a lesser angle. The closer to the vertical axis you aim your camera the greater the section of the circle of light you will get.
Exposure considerations
The surface conditions are also important. When there are a lot of waves, most of the sunlight will bounce on the surface limiting the available light underwater. If the conditions are calm and the seas are flat, the results are great. Technically you want the sea to be as flat as possible however I do prefer when there are a few ripples to add some texture in the background.
You will be shooting upwards and will see the sky, for this reason, I prefer to shoot Snell’s windows when the sky is clear and there are some fluffy clouds.
Diving considerations
When framing your shot bear in mind you will have to look to the sky, and this creates a series of problems. A well-fitted mask will solve the first one. By tilting your head up, you will force any water in your mask into your nose and eyes. Make sure you clear it properly before you compose.
Is also a good idea to think about your breathing. Shooting upwards will get your bubbles on the frame. Always time your breathing so your bubbles have reached the surface or are out of the picture.
Technical considerations
Another issue you will have to deal with is the fact you are likely to be shutting towards the sun and therefore will need to deal with an incredibly bright background.
To reduce the risk of overexposure you can use a very high shutter speed and a small aperture but you may end up making the rest of your shot very dark. I do prefer to cover the sun with a subject, it can be a fish, a boat, or a diver, to create a striking silhouette. Just ensure the subject is in the middle of the Snell’s window and no parts go over the edge of the circle.
Composing for Snell’s window shots
You do not have to point the camera straight up in order to benefit from a Snell’s window in your composition. An angle of at least 45 degrees to the surface will be enough to get a fraction of the circle in your frame.
I particularly like to use the Snell’s window when shooting table corals. If you plan to give this a go, please do pay attention to how you position yourself, and under no circumstances try it if you can’t get the shot without damaging other corals around.
Once you have tried this, why not try to get your dive buddy to pose next to the coral? The result can be an interesting play of curves. When trying to compose shots with multiple subjects you need to be very careful, if you do not position them properly in your frame you may end up with a slightly busy and cluttered picture.
Lighting your Snell’s window picture
To get a good Snell’s window you will be shooting upwards and that means you will have a huge amount of ambient light getting into your lens. Controlling the exposure is key and the situation normally calls for a fast shutter speed and a small aperture to expose for the bright background. If you decide to shoot using only ambient light then you can push the shutter speed, if on the other hand, you want to use strobes, you need to pay close attention to the subject distance. Because you will be using a small aperture you will have to crank up the power on your flashguns, but if you are too close to your subject, you may overexpose it.
Next time you find yourself on a dive with a fisheye lens on your camera have a look at the sky and start practising some Snell’s windows shots, I’m sure you won’t be disappointed. n
