Understanding D-SLR Lenses for Underwater Photography

By Steve Warren

When it comes to choosing your lenses for underwater photography, D -SLR owners need to consider a number of factors. Failing to understand these can easily result in a mistake, which is expensive in terms of hard cash and costly in terms of lost shots. These factors include the subject you want to photograph, the range of visibility in which you will be working, whether you are using artificial light and the model and format of your camera.

Choosing the right lens for the job begins with identifying what your subject will be. The general purpose zooms often provided as part of D-SLR packages handle few subjects well under the water. Underwater photographers work with extremes of subject under extreme conditions. The lenses required tend to be specialised ones. The four most commonly used lenses are fisheye, wide-angle zoom, standard macro and telephoto macro. The most important point to consider when selecting wide-angle lenses is angle of view. When selecting macro lenses angle of view and reproduction ratios are both important.

So let's begin by looking at the subjects we might want to photograph. Most underwater photographers pretty much want to shoot everything they see diving. And, for better or for worse, that's a huge range of very diverse subjects. It could be an enormous warship in Truk Lagoon or a tiny nudibranch in the Philippines. We'll start by looking at shooting larger subjects first. To shoot the wreck we would need to use a fisheye lens. This is the only lens that sees a wide enough picture area to take in a large, identifiable, section of the ship, like the bow, bridge or propeller. If we enter the wreck to photograph inside the engine room, the fisheye lens is also our best choice. This is also because it sees a large area, making it ideal in confined spaces where you cannot back away to get more in the picture. The angle of view of a fisheye lens on a D- SLR is usually 180 degrees.

The two reasons a normal wide-angle lens won't work well for this type of subject are due to the size of the area we need to photograph and restricted visibility. A wide-angle lens only sees about half the area a fisheye lens does. That's roughly 90 degrees degrees. This means that to photograph the same area of the wreck, we need to be twice as far back as we would using our 180 degree fisheye. This leads to an obvious problem - if we're inside a wreck, we probably can't back off because we'll hit a bulkhead. Our second problem is restricted visibility.

Most land photographers would not contemplate taking photographs in fog. Ecstatic as divers get when we can see 30 metres underwater, it's like looking through fog. Colour loss, reduced contrast, lower light levels and matter in the water column physically blocking the view of the subject are all hard for a camera to cope with. Our eyes and brain are surprisingly good at adapting to the problems of seeing through water. Even the most advanced D- SLR is much less adaptive. You simply cannot capture the image on your camera the way you see it yourself underwater over moderate to long distances. The solutions to getting good underwater photographs all depend upon getting as close as possible to your subject. This explains our dependence upon wide-angle lenses and especially fisheye lenses. The distance over which we can shoot by available light and still get a clear picture is only about one third of that we can see. So if you have 30 metres viz, you can really only shoot subjects successfully 10 metre away. If you are using an underwater strobe and want good colours the maximum distance you can shoot is only about 1.5 to 2 metres. This is because water is such an effective filter and removes colour both on the way to your subject and on the way back. So for a strobe to deliver good colours from a subject 2 metres away, it actually has to work over a distance of 4 metres as your subjects colours are reflected back to the camera. To shoot even a moderate sized subject, like a diver, full length, you have to use a wide-angle lens if you are to remain within the magic 2 metres and get realistic colours.

Having established some of the reasons for using fisheye and wide - angle lenses, let's now look at where some of the confusion lies and why underwater photographers sometimes end up buying the wrong "wide - angle" lens for the job. These guidelines should help you avoid making the same mistake.

Lenses for D- SLR cameras are usually described by focal length. Common focal lengths include packaged lenses like the mid range 18 - 70mm zoom, telephoto zoom lenses like 75 - 300mm and macro lenses like a 100mm. In the days of 35mm SLR cameras, this was usually enough information to choose a suitable lens because almost all 35mm SLR cameras were full frame models. It's different with D- SLR's because there isn't a single uniform sensor size. This lack of standardisation means that while one lens may fit and work on a number of camera models, it's effect on your photograph will be very different between camera types - meaning that, if you are not careful, the lens won't do the job you bought it to do. To choose your lens you need to know its angle of view on the sensor /format of the D- SLR you will be using.

At the time of writing, the most popular D- SLR's for underwater photography are either APS - C or full frame format. APS - C is a compact sensor size measuring roughly 23 x 16mm. Full frame measures 24 x 36mm and is the same size as a 35mm full frame camera - so that's why these cameras are often referred to as "full frame". Popular APS- C format D- SLR cameras for underwater photography are the Canon 50D and 7D, Nikon D90, D300 and D300s and for full frame format, the Canon 5D mk11 and Nikon D3 and D700. Four Thirds and Micro Four Thirds D - SLR cameras have not yet attracted a following among serious underwater photographers, though this is expected to change over time. Micro Four Thirds camera systems offer many features and benefits that will allow them to compete directly with more conventional D- SLR camera equipment. Canon's APS - H format is restricted to a very few specialised camera bodies and is rarely chosen by underwater photographers. DX is a common way to refer to the APS- C format and FX to full frame.

The reason the format of your camera is important is because the angle of view of your lenses will be different with different format cameras. For example, Nikon's 14 - 24mm zoom lens is a true wide-angle lens when used on FX cameras like the D700 and D3. With these cameras the angle of view runs from 114 degrees, but only 90 degree on the DX format cameras, like the D90 and D300s, which it also fits.

Some lenses are designed exclusively for specific formats. It's still worth checking the actual angle of wide-angle lenses (and, as we'll look at later, reproduction ratios of macro lenses) rather than just looking at focal length though. An example of where just looking at focal length can be misleading is to compare the Tokina 10 -17mm DX only format lens to the Canon 10 -22mm DX only format lens. The Canon, on its widest setting of 10mm, has a picture angle of 107 degrees. The Tokina at 10mm has an angle of 180. It's a massive difference. The Canon is a wide - angle lens, but the Tokina is a full frame fisheye, even though both lenses have an identical 10mm minimum focal length. Another confusing example is Nikon's 14mm lens. On the FX format is has an angle of view of 114 degrees. We tend to assume that the shorter the focal length, the wider the angle - but a Nikon 16mm lens sees 180 degrees on the same FX format.

Another issue to consider with wide-angle lenses is what optics you will use on your housing with it. Most wide-angle lenses work well with simple dome ports. But the design of Nikon's 14 - 24, for example, really requires a more specialised optical solution to ensure an acceptable image. Not all housing manufacturers have got their heads around this yet.

Macro lenses can also be confusing. Macro lenses are used primarily for photographing tiny subjects like invertebrates on critter dives. They also work well with small to medium sized fish. Many macro lenses fit both DX and FX format camera bodies. As with the wide-angle example above, the angle of view will be different with different format camera bodies. Nikon's 105mm macro lens has an angle of view of 23 degrees with FX bodies, but only 15 with DX bodies. At the minimum focusing distance the reproduction ratio is lifesize or one to one with either format. But the DX format camera will see less of the scene than the FX format camera. The DX format is often referred to as a "cropped sensor" because it is smaller than the FX sensor and this is why it sees a smaller picture area. It is sometimes thought that when you use a lens designed for a full frame sensor on a cropped sensor cameras you'll get higher magnification. This is not true. The effect is more akin to putting on skiing goggles. Your peripheral vision will be "blinkered" or "cropped" by the skirt of the goggles, but the mountain you are looking at won't look any bigger. Another way to think of it is to imagine a 10 x 8 print you've made of your favourite photograph and how it would look in a 6 x 4 frame. After you'd trimmed the photograph and lost a chunk of the edges to make it fit the smaller picture frame, you'd have less of your original photograph but what remained wouldn't look any larger.

This effect can lead to a problem with macro lenses, especially in low visibility. With a smaller format D- SLR you'll have to back away to get the same picture area as you would with the same lens on a full frame camera. This simply may not be possible in limited viz.

Where macro lenses get most confusing is there maximum reproduction ratio. Most macro lenses, like the popular Nikon and Canon 60mm models, reproduce at pretty much one to one, or lifesize. This means that if you photograph a 5 pence coin, it is reproduced exactly the same size on the sensor as it is in reality. If we were shooting on film, the 5 pence coin would be lifesize on the negative. However some macro lenses, like Canon's 50mm macro lens only reproduce at half life size - the 5 pence coin would only be half its real size on the sensor or 35mm negative.

Remember, as touched on earlier, there's a misconception about how lenses behave when used on smaller format cameras. A lens like Canon's 100mm yields lifesize at its minimum focus with both full frame cameras, like the 5D mk11 and compact sensor cameras like the 7D. However to get the same field of view with the 7D as the 5D mk11 sees, you'd have to back away. This is not because the smaller sensor on the 7D magnifies the image as is sometimes wrongly stated, but because it is a cropped sensor - it simply sees a smaller part of the image at a given distance. Backing away will make the image seem much smaller.

For underwater photographers who want to shoot supermacro - higher than lifesize magnification - most macro lenses need to be adapted. This will increase the lenses magnification or reproduction ratio. Options include adding a close up lens to the front of the lens or housing port or a teleconverter behind the lens. Using teleconverters reduces the light reaching the camera which can cause the autofocus to fail and become slow and hard to use. So you may need to consider a housing that lets you focus your macro lens manually.

The camera body you choose can also limit which lenses you can use. For example some budget Nikon camera models lack motors to drive the auto focus. For most land photographers, this isn't a problem. Nikon provide lenses with built in motors designed to work with these camera bodies and meet most photographer's needs. But an important exception is the Nikon 10.5mm fisheye lens - a workhorse lens for many underwater photographers, though not a lens many land photographers would own. To use the 10.5mm you need a Nikon camera with built in motors to drive the lens focus.

A lot of thought needs to go into selecting your lenses for your underwater D- SLR lenses. You'll also need to research how these lenses will interface with your housing body and availability of lens gears and ports. If the choices look simple and straightforward, you probably haven't looked into it thoroughly enough.

Steve Warren

INON UK Underwater Photography Instructor - Examiner