INEVITABLY MORE AND MORE DIVERS who have started using digital compact underwater cameras will be encouraged to progress to full-blown digital single-lens-reflex (DSLR) units. These allow you to look through the lens and capture your picture as soon as you press the button, rather than suffering the delay of the compact, and getting too many pictures of spaces once occupied by fast-moving fish. Most divers start with close-ups, but once you move on to wide-angle work, one lens seems to dominate the world of digital underwater photography - the Nikon 12-24mm digital zoom. DSLRs may look a bit like 35mm SLR cameras but, except for a few very posh ones, they have image-receptors with a smaller area than a conventional frame of 35mm film. The angle of view of a lens is a function derived from its focal length, usually expressed in millimetres, related to the width or diagonal of the frame. It will be less on a DSLR than on a 35mm film SLR. Some manufacturers have taken to expressing 35mm film-equivalence when it comes to focal length, thereby complicating matters further. In simple terms, a 20mm lens on a film SLR will be very wide-angle yet will equate to a 30mm lens on a DSLR, which is not very wide-angle at all. Still with me? If not, don't worry. All you need to know is that the Nikon 12-24mm zoom lens can be varied from very wide-angle (12mm gives 99?) to only slightly wide-angle (24mm gives 61?) on a DSLR such as a Nikon D70, D2X or D200. If a lens is designed to produce an image that will fill a full 35mm frame of film, what happens when you use that lens on a DSLR, with its smaller image-gathering area? The answer is that the rest of the image spills out inside the camera, and if the camera is not well baffled inside, you get lower-contrast results. This is why professionals always use lens hoods with telephoto lenses, to reduce peripheral light that might otherwise spill around inside the camera.
Hoick them out The snappily named Nikon AF-S DX Zoom-Nikkor ED 12-24mm f/4G IF comes with its own lens hood, though it's of no help when used behind the dome port of an underwater camera housing. It is, however, designed to produce an image circle that will only just fill the image receptor of the digital camera, so that's another problem eliminated by using a tool specially designed for the job. It also means that the lens is lighter in weight - and lighter on the credit card, too. It is an auto-focus lens, and its silent-wave motor is almost instantaneous in its effect. Just as well, because with a maximum lens opening of only f/4, it would prove quite difficult to focus manually in anything but the brightest conditions, despite the built-in rangefinder of the Nikon camera. I have always been a devotee of large dome ports for underwater housings used with cameras and wide-angle lenses, because they generally allow the lens to focus without the addition of optically imperfect supplementary close-up lenses. Look closely at a lot of underwater pictures and you will see that they go smudgy at the corners, because the photographer used a smaller port and a strong close-up lens to pull the centre of the shot into focus. Sharpness in the corners is the first thing to suffer when you do this. That said, with the 12-24 Zoom-Nikkor I was unable to focus closely on many things that I needed to photograph for these pages. This included diving computers that I might otherwise hold in my hand at arm's length, and my own head, breathing from a regulator on test. I usually do this by turning the camera on myself and holding it at arm's length. I resorted to a plus-2 dioptre supplementary lens, which did the job without being strong enough to mess up the sharpness in the corners. So how do the optics perform? I found that the lens was at its crispest when cranked back a little from its widest setting at 12mm to about 14mm. It also proves quite useful to be able to crank the lens so that you can get a bigger image of those animals that refuse to come close enough, but I usually use it at a wide-angle setting. I rarely use the widest lens opening, opting for apertures in the range of f6.7 to f/11. With small lens openings, the depth of field is enormous. The aperture mechanism uses a seven-bladed iris, so out-of-focus points are rendered in a pleasing way, and there are few signs of those hexagons that bright twinkles sometimes become with simpler lenses. There is no aperture ring on the lens. You set that via a control on the camera body. Observant photographers will note that I have been using this lens not on a Nikon DSLR but a Fuji S2. I have a couple of these Fuji digitals and the upper camera part is actually a Nikon. The 12-24 Zoom Nikkor works very well with them. My only criticism is that, at around£775, it makes your eyes water when you flood it - and, yes, I have. I am pleased to report that the replacement lens was as good as the first! DIVER is not a photography magazine, and without getting into esoteric arguments about other Nikon lenses, I would say that for wide-angle photography with a Nikon digital SLR, a 12-24 Zoom Nikkor is probably the only lens you need. It's certainly a good starting point.