Photography Underwater:
Lenses and Ports:
The articles below relate to:
Cameras with interchangeable lenses.
Housings with interchangeable ports.
Cameras and Housings which can use supplementary lenses.
See also: Ports.

Q: What is the best lens to use for underwater photography?

A: There is no single best lens for underwater photography, but the general rule is that pictures should be taken from as short a range as possible (within reason) unless special considerations dictate otherwise. This means that large objects and general scenery should be photographed using a wide-angle lens if possible, and small objects should be photographed using a close-focusing macro lens. SLR users should note, that for photographing sedentary objects, macro lenses of 50-60mm focal length (35mm equivalent) are best because they result in the least amount of water between the lens and the subject; but for photographing fish and other animals which are likely to be disturbed by the proximity of the camera, macro lenses of around 100mm focal length are to be preferred.

Note that, although it is best to be close to the subject, this does not mean that there is no practical limit on the desirable angle of coverage for a wide-angle lens. The best choice for general underwater photography is to use a lens system with coverage in the range of about 75 to 100°. Beyond that lies a special realm of photography in which optical distortion is difficult to correct and effective flash illumination is all but impossible; which means that if you buy a super-wide lens system you should do so for a particular purpose, such as photographing whales or shipwrecks by natural light.

Q: Can I use my SLR with a standard zoom lens underwater?

A: If your reason for wanting to use a zoom is that you already have one and you don't want to buy any more lenses, then the answer is that you can, if you must, but optical compromises are involved. Zoom lenses (especially 28-70mm standard zooms) are designed to eliminate the inconvenience of changing lenses across a variety of common situations. Going underwater is not one of those situations. The main compromise, with most zooms, is that the minimum focusing distance is large (even on macro settings), which precludes close working. Zoom lenses also tend to have a small maximum aperture, which makes the viewfinder image dark in low-light (i.e., underwater) conditions. If you use a zoom lens with a dome port, poor minimum focusing distance usually makes it necessary to fit a close-up (+ Diopter) lens to the front of the main lens, to cancel the de-magnifying effect of the port. Adding a + Diopter lens degrades optical performance to some extent, although the effect is minimal if you use an Achromatic close-up lens (as made by Nikon and Canon). If you using a digital SLR, note that any chromatic aberration you introduce can be removed in software (see the software lens correction article).

Q: How do I work out which port to use for a particular lens?

A: The normal practice, in the case of ports used to adapt SLR lenses for underwater use, is to use a flat port for standard, macro, and other lenses with long focal length, and to use a hemispherical dome port for wide-angle lenses. Consequently, given the marginal utility of standard, portrait, and telephoto lenses, the flat port is often referred to as a `macro' port. Assuming the 35mm format, you can use flat ports with lenses down to 35mm with little degradation in optical performance, but the angle of coverage of the lens will be reduced. The reduction in coverage corresponds to an increase in magnification, so the flat port increases the power of macro lenses; but it also introduces a small amount of pincushion distortion and chromatic aberration. The reason why flat ports are not favoured for use with wide-angle lenses, is that vignetting may be a problem, the reduction in angle of coverage turns a wide lens into a not-so-wide lens, and aberration and distortion get worse as the focal length of the main lens gets shorter. A 28mm or wider lens behind a flat port will give pictures which have noticeable colour fringing in the corners; but if you are using a digital camera all is not lost, because you can restore the image quality to be the same as it would be in air (and possibly improve it) by applying software optical correction .

Note that ports, in general, do not have anti-reflection coatings. Consequently, under some lighting conditions, the camera may pick up a reflection of any writing or markings on the front of the lens. The solution is to go over any white or light-coloured markings with a black alcohol-based (permanent) felt-tip pen. Inking out the lens markings also improves performance when using ordinary filters and diopters above water. If you want to restore the lens to its original condition, you can remove the ink with cotton buds and alcohol (either industrial methylated spirit or isopropanol).