Lesson 5
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Selecting and using lenses

Everything is so interrelated that is would be impossible to reach this point without mentioning a lot about lenses already, but rather than covering the technical detail, this section considers which lenses you might wish to use, and hints about using them.

Lens types

Because of the magnification effects of water, the angle of view of underwater prime lenses tends to be narrower than equivalent land lenses, and so the apparent focal length becomes shorter.

Some common angles of view, and equivalent focal lengths are:

Figure 12. Angles of view for land and underwater lenses

With the exception of the Nikonos 50mm macro for the RS, there are no true macro lenses for amphibious cameras, and close up shots are achieved by either using close-up supplementary lenses, or by using extension tubes. The main options are;

Figure 13. Reproduction ratios of macro lenses.

Reproduction ratios are expressed as "size of the image on film : size of object in real life".

Standard lens

It has been said that the standard underwater lens is best used for subjects which are at least the size of a wine bottle or which won't permit you to approach any closer than 3ft. Despite this, it can be good for schools of fish, and is a useful beginner's lens, giving quite acceptable results during the time he saves up for a wide-angle lens. At wide apertures it will have a fairly shallow depth of field and backgrounds may frequently be out of focus, which isn't necessarily a bad thing.

While an arms length (3 feet) is probably the best distance for this lens, remember also that you will get parallax distortion at distances nearer than 4 feet, and must correct for these in every shot.

This lens very much comes into its own when used with extension tubes for macro photography, details of which appear later. Failing that, it is very useful, if reversed, for viewing slides on a light box, and makes a charming paperweight.

Macro

Standard Underwater Photography Advice is "Always use a macro lens, unless the subject is too big, when you should use a wide angle."

This must hold true if the parameters we discussed earlier are correct. Above all, we want sharp clear pictures, free from any colour loss or backscatter, and the closeness of macro photography allows us to do this.

If using extension tubes, then your subjects should be chosen according to the reproduction ratio that you are using, which usually will restrict them to 1 to 4 inches in length. Get a complete image and if you find yourself having to crop, then you've probably got an inappropriate subject or are using the wrong extension tubes.

For beginners, extension tubes can be used by pre-setting all controls, aiming the strobe over the camera at the centre of frame, and positioning the subject in the middle of the frame - it's as easy as that.

Beware the shallow depth of field when using extension tubes, so make use of f/22 to avoid focus problems, which will also help turn any blue water into a black background, making your subject stand out. All the light falling on your subject will be from the strobe, so available light levels are of no importance, which is why macro is a good choice for night dives - but you'll need some sort of modelling light to see what you're doing.

Once you've mastered the basics, then you can start experimenting with other lighting angles, such as toplighting, sidelighting, and backlighting, but be careful never to aim the strobe directly at the lens, or permit the framer posts to cast shadows across your subject.

Housed cameras permit the use of various specialist macro lenses, with the Nikon 60mm and 105mm being popular underwater choices. These lenses will give reproduction ratios down to 1:1, but also have the ability to focus all the way to infinity, permitting shots of almost any sized object.

On a technical note, it is worth pointing out that the Nikon lenses actually have their own in-built extension tubes, which is why they can focus down to 1:1.

The 105mm lens has a much narrower angle of view, and therefore permits 1:1 reproduction ratios from a greater lens to subject distance than the 60mm, permitting macro shots of subjects which wouldn't let you get near enough with the 60mm. Neither of these lenses require framers, and are therefore less invasive of the subject than extension tube photography, and will permit shots that you wouldn't be able to obtain with framers. It is worth re-iterating that these lenses will have identical depths of field for any given reproduction ratio and aperture.

With these macro lenses, you may wish to consider leaving the aperture at f/16 or even f/11 in order to get maximum sharpness in the image. This will naturally reduce depth of field a bit, but this is the trade-off you must balance.

In all macro photography, you may find that subjects feel less threatened by a slow approach from underneath the subject, than an easier approach from the same level, or on top.

With all extension tube photography, there is a light loss involved, which is why you might see the 60mm or 105mm Nikon lens, which has a minimum aperture of f/32, showing an effective aperture of either f/48 or f/64 in the camera viewfinder. If you wish to obtain maximum sharpness from your lens, do not be fooled by these numbers which relate to the amount of light coming in, not the actual diameter of the aperture. So if you want to use f/11 or f/16, then set it with the lens focused on infinity, and don't touch it afterwards whatever the viewfinder says.

Wide angle

The use of wide angles lenses, as mentioned in the previous section, is for when your subject is too big for macro. Of course this is an oversimplification, and wide angle lenses give the underwater photographer enormous creative scope, and an almost infinite depth of field

The perspective of wide angles means that near subjects can easily fill the frame, but need only retreat a few feet to become very small images indeed, and for this reason, you must always use the reflex or accessory viewfinder to properly see how your subjects are arranged in the frame.

Having said which, wide angle lenses are used primarily to reduce backscatter, not change perspective, but the ability to do so is one of the more powerful creative effects that can be achieved by these lenses. On land, the use of wide angle lenses is usually immediately obvious, with the distortion of recognised perspectives, yet underwater there are few features or lines to give the viewer an indication of what the true perspective should be, and wide angle shots come out looking entirely natural.

The techniques of forced perspective can allow you to make any foreground object look considerably bigger than it is, by putting a figure of known size, e.g. a diver, into the background. This way, openings to little caves can appear as caverns, and even medium sized fish can appear larger than the divers.

The wide angle of view will allow you to take photographs, preferably in the vertical format, that include both parts of the surface, and parts of the depths. This gives your photographs a splendid graduated blue background, from a mid-blue at the top to a dark indigo at the bottom.

Close focus wide angle

Unlike normal perspective lenses, such as the Nikonos 35mm, wide angle can be used for close-ups as they are capable of focusing down to 1.3 feet or even less, and have a front depth of field to bring that even closer. Lenses should be set to f/16 or f/22 and minimum focus (even when using supplementary wide angle lenses despite the instructions to set the prime lens to infinity!). With reflex cameras, obviously focus should be properly set on the subject itself.

This use of wide angle lenses is entirely consistent with our aims of getting close and filling the frame with our subject, although the distortion associated with wider angles becomes more apparent as your subject starts to fill up the frame. While distortions with wide angle lenses are not normally apparent, having a part of the subject very close, and another part further away, will accentuate this effect, and it's very easy to make divers' proportions look very strange indeed.

Sadly, parallax errors, mentioned earlier, will be remarkable at very close distances, and it is all too easy to forget about them in the heat of the moment.

Figure 14. What I thought to be a perfectly framed shot of a Napoleon Wrasse turned out to be another victory for the parallax pixies.

Because of the great depth of field, you have the opportunity to place a secondary subject, usually a diver, into the frame above and to one side of your primary subject, which will enhance your picture by creating a secondary point of interest in your picture

Lenses for housed systems

The following prime lenses can be used with Nikon equipment in Subal housings, and presumably all other housings made for the F801s or F90.

Figure 15. Lenses commonly used underwater with Nikon cameras.

The is only a sample list, and a wide range of lenses from various manufacturers is available for these cameras, but unlike the above, not all lenses will have ports specifically designed for them by the housing manufacturer, so your choice may be somewhat more limited.

The Sigma 14/15 lenses are noted as being very good, with no perceptible quality difference from the equivalent Nikon lens. They give exceptional close-focusing ability, and tend to be considerably cheaper as well. The 90mm macro is a good alternative to the Nikon 105mm, and costs under half as much, but will only focus down to 1:2 which may or may not be a limitation to your photography.

Do not dismiss the use of zoom lenses underwater, particularly those that offer a sensibly wide angle of view, and realistically 24mm can be wide enough for most circumstances. Again the availability of dome ports for a specific lens will be a major criterion here, and it must be borne in mind that some wide lenses and zooms have their own dome shaped front lens elements which preclude the attachment of the necessary diopter lenses for housed work, although some permit attachment at the back of the lens.

With all lens for underwater use, you are strongly advised to consult your housing specialist before undertaking any purchases

This page was last updated on 11 August 1998
Please address any comments to Mark Mumford