On the market for a couple of years, the Sea and Sea 12mm Fisheye lens for Nikonos models I to V has been a steady seller, according to its makers. I decided to test it during a three-week trip around Australia's Great Barrier Reef.

Ruggedness and ease of handling count for a lot when you're a dive photographer - this lens seems well constructed with a sturdy, black, anodised aluminium casing. It has a chunky aperture and focus knobs for a good grip. The viewfinder, which slots into the top of a Nikonos, is large, sturdy and bright. A large knurled wheel locks it snugly into place. A pool test showed excellent edge-to-edge sharpness and the full effect of its wide angle of coverage.

My great faith in Murphy's Law (if something can go wrong it will and at the worst possible time), as it pertains to underwater photography, led me to the venerable Swiss technician Peter Brawand at the firm of Poraday in Sydney. Having repaired flooded equipment for 15 years, he can give a flood prognosis on anything.

Peter pointed out that if the 12mm flooded, disassembly meant undoing just two small Allen key-style grub screws. A good soak in distilled water, and another in an alcohol bath, would keep it safe until a competent technician (as opposed to some degenerate photographer) could reassemble it relatively cheaply. With this, Peter accepted his consultancy fee of a six-pack of beer, ceremoniously handed me a tiny Allen key and told me to go away and stop bothering him.

The catalogue details say the lens 'offers a remarkable 167-degree angle of coverage, nearly twice the coverage of the 15mm lens' and 'permits full-frame images of the sea's biggest subjects just 13cm (5.1in) from the film plane'. What it doesn't do is give you the circular image on film that you get from most conventional fish-eyes - the 12mm lens produces a flat image, with no such distortion.

During my three weeks' experimenting with the lens on the Barrier Reef, I discovered that I could shoot two divers and a dining-table-sized coral bombie at a distance of 1ft and still get them in. I also discovered that at f16 and focused to 1ft, the 12mm gave infinite depth of field back to the plane of focus and half of infinity forward of the plane of focus. In practical terms, this means that a clownfish biting the front of the lens will be in focus, and so will everything else back to infinity.

Being able to get so close means reduced backscatter. Stopping down to a small aperture means that the flash range will be only a foot or two. If you also use a slow shutter speed, such as 1/60 or 1/30 second, the background will be well illuminated by ambient (natural) light, which doesn't show backscatter to anything like the extent that flashlight does. Thus, you can shoot in poor visibility and make it look like average visibility, and shoot in average visibility and make it look like excellent visibility.

Summing up, the 12mm is a very useful lens, but there are several things that need to be made clear.

1 You will need two flash units to cover the area of the lens.

2 TTL flash metering is useless with this lens. You will have to calibrate exposures and the optimum distance between flash units for each working distance. This is best done by trial and error in the swimming pool, unless your physics is very good.

3 The action on the aperture knob could be a bit more definite. You can feel each aperture notch click in, but it's not as 'clunky' as I'd like it to be when I'm trying to bracket without taking the camera from my eye. 4 This lens is not cheap but it seems close in price to its nearest competitor, the 15mm UW Nikkor for Nikonos, which has only a 95-degree angle of coverage.

The 12mm Fisheye is available, price £892, from a variety of camera and dive outlets. Stockists' information from Sea & Sea, tel: 01803 663012, fax: 01803 663003.