Tools

Before entering this specialised field of photography you should become familiar with some of the most useful types of equipment that are currently available for this purpose. Of the many types available, the three that are most practical for underwater photography are:

1. Extension tubes

2. Teleconverters

3. Supplementary positive lenses (wet and dry). Extension Tubes

An extension tube is a simple hollow cylinder that is inserted between the camera body and the primary lens in order to move the lens further away from the camera's film/sensor plane. By doing so, it decreases the lens' minimum focus distance and increases the maximum achievable image magnification. How much the magnification increases depends on the amount of extension added to the system. For each lens focal length worth of extension added, the magnification ratio increases by one. For example, the addition of 100 mm of extension to a 100 mm 1:1 macro lens will result in the ability to shoot 2:1. Since it does not contain optical elements, an extension tube will not degrade image quality and is a relatively inexpensive option for achieving reproduction ratios beyond 1:1.

An extension tube does, however, have some disadvantages. One potential drawback of separating the lens from the camera body is the loss of auto focus capability — only some models/brands of extension tubes include electronic linkages between the body and the lens. Without that linkage you'll be stuck at a single focus distance, unless your housing port has a manual focus control.

Another drawback is light loss. For each additional lens focal length worth of extension, the light intensity at the film/sensor drops by a factor of two (one f-stop) due to the spreading of the same light over a larger area (the 'bellows effect'). If f11 on a 100 mm lens gives perfect exposure, the insertion of a 100 mm extension tube will require the aperture to be opened by one f-stop to f8.

Since the extension tube attaches directly to your camera and becomes inaccessible once it's sealed inside the housing, you'll have to commit to using it for the duration of your dive. This could be very annoying, since the decrease in the minimum focus distance sacrifices the ability to focus at infinity. When a barracuda swims by you can look but not shoot. A final point — the addition of an extension tube to your camera may also require the addition of port extension rings.

For the above reasons, this super macro tool is far from ideal for underwater photography.

Teleconverters

A teleconverter (TC) is also inserted between the camera body and the primary lens. Unlike an extension tube, however, it contains several optical elements which act together to diverge the light rays coming from the primary lens and 'optically crop' the image being projected toward your film/sensor, thereby causing an increase in magnification. The amount of additional image magnification achieved depends on the teleconverter's multiplication factor (typically 1.4x, 1.7x, 2.0x or 3.0x).This factor, when applied to the primary lens'focal length, also gives the 'effective focal length' of the system (a 100 mm lens combined with a 1.4x or2.0x TC becomes a'140 mm'or'200 mm'lens, respectively). I use the word 'effective' because the optical properties of the primary lens remain unaffected, even though the image is magnified. With teleconverters, high levels of magnification can be achieved from relatively large distances, making them excellent for particularly shy super macro subjects.

However, a teleconverter also has several disadvantages, which collectively explain the dust that has accumulated on mine over the years. Like an extension tube, its use underwater requires the same level of pre-dive commitment, since you can't remove it once you're submerged. A teleconverter also diminishes the light intensity at the film/sensor, with the drop in intensity being a function of the teleconverter's multiplication factor. For example, a 1.4x TC produces a 1-stop light loss, a 2.0x TC a 2-stop loss, and a 3.0x TC causes a staggering 3-stop light loss (one-eighth the intensity!). Combine these disadvantages with the degradation of image quality caused by the additional optical elements and the necessity to insert port extension ring(s) for the additional lens length, and teleconverters become somewhat less attractive as a super macro photography solution.

FIG 8.54 2 teleconverter.

Supplementary Positive Lenses

A supplementary positive lens, which is placed between your primary lens and the subject, functions in the same way as everyday reading glasses: it creates a virtual image far enough behind the object so as to be within the focusing range of the primary lens. This virtual image becomes the object for the primary lens, or more precisely, the virtual object since it only appears to be at that position. In effect, this 'trick' allows the real object to be brought closer to the primary lens while simultaneously providing it with a 'virtual object' that is erect, larger than the real object, and far enough away that it can be focused. The net result is a decrease in minimum focus distance and an increase in magnification.

Commonly called 'close-up filters','close-up lenses', or 'dioptres' (a dioptre is actually a unit of measurement for quantifying the strength of a lens), these optical devices are usually constructed from either one or two lens elements, forming a positive dioptre lens system. 'Multi-element achromatic' lens designs are capable of producing much higher quality images by reducing/ removing optical distortions and colour fringing, so look for those words when researching available close-up lenses.

With a supplementary lens in place, the farthest focus point (normally infinity) is reduced to a distance of one focal length (of this lens) in front of it, and the closest focus point will be correspondingly decreased by some amount. Although seemingly restrictive, this is of small concern when doing super macro photography, since minimisation of the lens-to-subject distance is often desirable. This narrowed focal range expands with weaker close-up lenses and contracts with strongerones. But regardless of strength, and unlike the previously mentioned tools, this super macro tool has the advantage that it does not incur any amount of light loss.

There are two main types of supplementary lenses to be aware of: 'dry' and 'wet'. 'Dry'close-up lenses, which are manufactured by Nikon, Canon, Cokin, B+W, Hoya and several others, come in a variety of strengths/diameters that can be screwed directly onto the primary lens. Like extension tubes and teleconverters, once they're on, they're on for the dive. 'Wet' close-up lenses, on the other hand, are quite the opposite. These mount on the outside of your housing (in the water), either by means of a port adapter or by screwing onto the port's threaded front edge. If a wet lens' refracting surfaces are in contact with water, approximately two-thirds of its power will be lost underwater, so lenses which have the refracting surfaces sealed in contact with air are much preferred, since 100% of their magnification is maintained.

A good example of an air-sealed achromatic wet lens is ReefNet's +10 dioptre SubSee. This wet lens, combined with my old trusty Nikon 105 mm macro lens, makes up my super macro kit. The SubSee attaches to the front of my Ikelite modular flat port with an adapter ring, and has a hinged arm which holds the lens, allowing it to swing in and out of place as needed. When flipped into place over my 105 mm lens, I'm able to get approximately 2.2:1 image magnification (filling my Nikon D300's frame with an 11 mm x 7 mm subject), and yet I still have the freedom to shoot normal macro shots at any point. Additionally, the lens can be removed from its holder to be used as a normal hand-held magnifier if desired, so I can get a good look at what I will be shooting ahead of time.

For beginners in this field, I would strongly recommend achromatic wet lenses (like the SubSee) as a starting point. Once you're comfortable with it, making a switch over to the more limiting teleconverters and extension tubes will be a walk in the park.

If you decide to push the magnification envelope even further, you can always combine this type of wet lens with one of the other types of tools. the results can be shocking!

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