Feb 132012

Diver in Kilsby's Sinkhole

About the site

Kilsby’s Sinkhole is a huge, crystal clear freshwater cave formation in Mt Gambier. From most areas in the cave you can look back and see daylight, although past 40m you can get down between the rocks. A classic sinkhole formation created by a roof collapse thousands of years ago, Kilsby’s has a rockpile in the middle coming up to 15m depth or less. One side descends to 25m or so, and the other down to 60m+. Visibility is usually in excess of 40m – you can watch divers swim along the opposite wall and see their trailing bubbles ascend to the surface above.

Divers in the shallow end of Kilsby's Sinkhole

About the dive

The clear water, natural light and sheer size of the space leads to some interesting photographic opportunities. I was last here over the New Year’s weekend, taking advantage of the summer sun angled right into the water. This photo was taken two weekends ago and while it was still summer, clouds were scudding across the sky. Underwater the changing light was obvious as the cave went from bright to dark and back again.

This was a Sunday afternoon dive and there had been a bunch of divers through on Saturday and that morning. While the water was still ten times clearer than anything I’ve seen in the ocean, exhaled bubbles create a certain milkiness. When combined with the overcast skies this dive had a very different feeling to the brilliant sunlight of a month or so prior.

About the shot

With less light penetrating the water, my options for capturing what I could clearly see were a little more limited. This shot is definitely a compromise between noise and ISO, f stops with fuzzy edges and shutter speeds to freeze diver movement. I rarely change my ISO on a dive, but as the sun went in after the first few minutes I took the time to switch from my standard 400 to 800 (with thanks to the 5DII’s low noise). This gave me a little more flexibility with my other settings.

As I talked about the first time I took photographs in here, from this vantage point the floor is a very long way away. Unless you’re taking 30m of scaffolding with you on the dive a tripod is not an option. However, as I backed up to frame this shot I found myself under a flat ledge at exactly the right height. After adding some air to my Armadillo and wedging myself comfortably against the underside of the ledge I turned the strobes off and rearranged the strobe arms to create a flat platform on top of the camera.

From here I could use my buoyancy to brace the camera against the ceiling. Using my 14mm lens I can usually hand hold shots down to 1/25th or 1/30th of a second. With a tripod much more is possible, and a roof is almost as good. As I extended the exposure time though, I discovered on the review screen that I was getting blurry diver movements. Lucas was pretty close to motionless, but his bubbles were obeying the laws of physics and creating smeary grey marks up the photos. Moving back through my options, this shot was taken at 1/15th of a second, and was way too dark.

That left me with the last variable, aperture. Using a wide-angle rectilinear lens behind a dome port means fuzzy edges at wide open apertures due to the combination of curvature of the virtual image and shallow depth of field. I habitually shoot at f8 or above. To get this shot I gradually moved down through the f stops until this was taken at f3.2. The soft edges are noticeable on the rocks at the bottom of the photo and the bubbles caught on the roof above, but the focal point of the shot – the diver – is clear and sharp.

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