Back in the sixties Kodak came out with Kodachrome, an emulsion without colours. The colours are added during development through a laborious process, known to Kodak alone. This transparency film is used by organisations such as National Geographic, who also take exceptional care in storing their photographs in cool, dry and dark archives. In this manner the film can be retained for over 300 years. Other slide films, in ordinary filing systems, last only a human's life time or less. But now we can save images electronically on CDs. The aluminium CDs we buy can perish through aluminium 'rot' but the gold 'write once' CDs could last forever. Once in electronic form, images can be cropped, cleaned up, colour and contrast corrected and sharpened. For under water photography such manipulations can be of enormous benefit.

Slide, negative or digital? At the moment there are three basic options to consider: slide, negative or digital. Slides and transparencies Transparencies are the professional's choice for delivering pictures to printers and publishers. Their advantages are obvious: what you see is what you get. The slide is both the original and the finished product. It can be shown in a viewer or a projector. Each slide is packaged on its own, can be manipulated and stored on its own. Slides have good colour rendition and sharpness. They look like the printed image and the printing industry knows how to handle them and turn them into colour plates. But slides also have their down side. Because they are the unique and finished product, they can easily be damaged or lost. Printers often remove slides from their frames and greasy fingerprints, and miniature scratches are often left behind. Although good copies can be made of slides, printers often demand the originals. Slides fade when used regularly in projection, so only copies should be shown. Slides have high contrast, desirable to blacken the dark parts for projection. They show the world not exactly the way it is, with brighter colours, deeper shadows but the public has become accustomed to this, often demanding the postcard look rather than reality, particularly for advertising.

Because of its contrastiness, exposing slide film correctly is rather critical. Even half an f-stop (25% light intensity) can spoil the result. To cope, photographers take many repeats of the same shot with varying exposure settings ('bracketing'). When an artificial light source is used such as a strobe, the number of repeats becomes very high, resulting in high wastage, costs and time.
For under water use, slide film is often disappointing. Under water one cannot reload films. One has only very few chances to shoot more than a few photos of the same subject in the same situation. Bracketing then becomes a hard liability.

Negative film
The colour saturation and sharpness of negative film has improved quite considerably in recent years (1990s). It is the favourite choice for amateurs but also for wedding photographers who often can not bracket their shots, or re-do them. Prints made from negative film can be handled without the risk of damaging the original. But negatives need to be left in strips of four to eight frames. So when a printer needs the original, he receives a strip of several frames. Printers dislike negative film because the result may change and there is no visual original to compare the printed image with, unless the corrected print is supplied as well. For a photographic library or stock clearing house, negatives are unacceptable because of this. But for the private photographer, shooting negative film gives the best value.
Negative film is very tolerant to exposure, producing good results within a three f-stop tolerance. Negative film has low and linear contrast and portrays the world the way it is, much more so than slide film does. Prints can be enhanced by reprinting them lighter or darker. Negative film has very fine grain which allows one to use faster films. For nature photography under water, shooting negative film is very rewarding.Now that photographic images can be saved, filed and traded in electronic form, negative film has lost its earlier disadvantages and is rapidly becoming more popular for the professional photographer who wants quick, reliable and cost-effective results.
 

A baby pink maomao is found sleeping in a safe place inside the giant Rikoriko cave at the Poor Knights Islands, New Zealand. Because the camera's aperture failed to narrow from f4 to f22, this photo was over exposed by 5 f-stops! Hardly discernible on the very black negative, it still produced this amazing image, with true colours. Film: Fuji NPH400 Lens: approximately 100mm

Digital cameras
Whereas progress in film emulsion technology has been slow, progress in electronic imaging has been rapid. Cameras now claim as many as 6 million pixels per image (a scanned 35mm Kodak Photo-CD image has 3000x2000 = 6 million pixels, compressed to some 6MB, about 80 lines per mm, the resolution of most slide films)
Digital images show the result immediately, allowing for on-the-spot improvements to lighting, exposure and even contrast. Digital cameras are more sensitive, being able to take pictures in low light conditions. The CCD (Charge-Coupled Device) is small, allowing for short focal lenses with vastly superior depth of field characteristics. Very recent digital cameras now provide resolutions comparable to the 35mm frame.

For the amateur under water photographer who wants immediate results that can be reproduced professionally up to sizes of 100mm across, the digital camera is the solution. In the past few years digital video has stormed the world, providing superior pictures that can be obtained with ease. The digital colours of 3-CCD cameras are 'additive' as opposed to the 'subtractive' colours of slides and negatives, making a whole new range of 'fluorescing' intensive colours available. Unfortunately these can not be rendered in print. Most still digital cameras, however, use a single CCD colour technique which renders colours subtractive. The most standard method of saving the image, is in the standardised sRGB (scanner-Red-Green-Blue) computer format, which prints beautifully, the way one sees it on the computer screen.

The digital image is made up of (square) pixels, each containing three colours, red, blue and green, much the way a television image is composed. Each colour can assume only discrete values, ranging from 0 to 255. At mid-exposure of 150 units, a picture looks rather dark whereas a one stop over exposure (2 times)  at this point would overexpose the image to 300 units, which is 'clipped' not to exceed the value of 255. So working with a digital pictures can be even more critical than working with slides.
Fortunately, the high quality end of the digital cameras produce images with 10 or even 12 bits colour depth('raw' format as opposed to JPEG), thereby extending the range considerably (from 255 steps to 4096). But the JPEG compressed file format has only 8 bit precision.

The art of exposing correctly is to dim the intensity of the real world, to that required by the film, and we do so by means of shutter speed and aperture. The diagram depicts a typical slide film with a range of 8-9 f-stops in density. Just to put this scale in perspective, prints in books have a density range of only 4 f-stops (1:16); prints in newspapers only 3 f-stops (1:8) and slides projected in a dark room 6 f-stops (1:64). So the scale at bottom right runs over 6 f-stops from white to black. But because of the contrastiness of the slide film, this corresponds to only 4 f-stops in the real world (which covers 10 f-stops from direct sunlight to deep shade), reason why exposure of slide film is highly critical. This is much less so for negative film. But the situation is even worse because the colours in the highlights are truly bleached, missing colour information, AND those in the shadows are mixed with black, which also reduces the amount of colour information. To sum it all up, colour slides provide only 3 f-stops of true colour, which explains why they are so difficult to expose right.

In the diagram, the 1:1 contrast line has been dashed. A film with this density curve would be ideal as a duplicating film to make copies of slides. A steeper incline means less contrast (soft film) whereas a flatter line means more contrast (hard film). As you see, most slide films are soft in their highlights but contrasty in their shadows. This explains why correcting an under exposed slide by duping (duplicating), results in very high contrast.

In the diagram below, a number of typical slide films of the 1980s are compared. As you can see, there are distinct differences between them, reason enough for careful selection. Notice how Ektachrome 100 follows a smooth soft density curve. It has been the preferred film for under water photography for many years.
As far as negative film is concerned, a similar comparison can be drawn. But the result depends also on the qualities of the print material used.

Most under water photographers are obsessed with sharpness of detail, resorting to low speed fine grain film. But in doing so they cannot capture natural light and they waste much film on movement blur, poor depth of field and so on. Adapt your film type to how much light is available (much in the tropics, little in temperate seas) and accept graininess as part of the trade-off. For macro photography where enough artificial light is available, go for the finest film if you like.

My choice has been overwhelmingly for negative film because its softness captures the under water atmosphere best. It is tolerant to exposure and gives me a large number of good results. I am mainly using Fuji REALA (100ASA) which has good colour separation and density but also very fine grain. For higher speed, I used the Fuji SUPERIA range of films, of which the 200 and 400 ASA films stand out by their sharpness, colour and fine grain. Note that the SUPERIA 100 is much grainier than the REALA film. Recently I have been push-developing Reala 100ASA to 400 ASA with fine grain results but increased contrast. After digital scanning this option gives me the best results: both finest grain AND finest colour reproduction. Note that the most recent batches of Fuji Superia films now also contain a fourth colour layer, like Reala 100. This new technology reproduces fine colour nuances much better than any other film on the market.
 
 

Lens types Camera lenses are available in a continuum of focal lengths from 16mm to over 100mm (for 35mm cameras). But the practical range for under water is 13mm to 100mm. The super wide angle 13mm lens has good depth of field but is not able to take fish portraits. For sharp macro photography often a 100mm lens is used. The table compares the properties of wide, medium and tele -angled lenses. In general, the 13-20mm lens is for people, large animals and seascapes. The medium angle lens is often a 28-80mm zoom lens, most suitable for fish photography and close-ups of people. The tele lens is often 60-100mm and is good for macro photography and detailed fish portraits or small animals.

The wide angle lens distorts the real world as if there is more space. Objects seem farther away. It creates a lot of emptiness and should be used with care. By contrast, the tele lens makes objects seem stacked closer together. Both lenses introduce new creative possibilities and surprises.

So what is the work-horse of lenses? It depends what you are after. In general the super wide angle lens (13-16mm) is difficult to use although it gives surprising results. You have to combine it with a super-wide strobe light and long strobe arms. The lens looks at the sky and at your fins all at the same time, introducing very high contrast. To counteract this contrast amounts to a fine art. But if you want to photograph reflections in the surface, the snell's circle, cathedral light, models and sea mammals you can't do without it.The medium focal lenses of 28-35mm tend to be good work-horse lenses, but for some reason (price?) they do not have a wide focusing range. I find the 50mm macro lens also very productive because it allows one to keep some distance from fish and without changing lens, focus down to 15cm.

A note on macro photography The macro tele lens with or without close-up attachment lenses, places the camera at a sufficient distance from the object to allow the strobe light in. A macro lens is able to focus over a range of 5 diopters (from infinity to 20cm), whereas a standard lens would range over 2 diopters (from infinity to 50cm). With a closeup attachment lens of 2 diopters, the focal range would reach from 50cm to 14cm for the macro lens and from 50 to 25cm for the standard lens. As far as depth of field goes, it would be better to use a medium angle lens (35-28mm) with extension tubes, such as offered for the Nikonos cameras. A 28mm lens has 16 times the depth of field of a 100mm lens but would need to be placed very close to the subject.  Note that the Nikonos lenses are corrected for the frontglass effect, and deliver very sharp images under water (but less so above).

Flat port and dome port A port which is as flat as the front glass of a dive mask causes the same kind of distortion that makes all objects seem bigger and closer than they are. In fact, 30% bigger. It is interesting to note that our brains see this in three directions (1.3 x 1.3 x 1.3 = 2.2) and estimates a fish to be twice its weight. For the photographer the apparent image distance is quite confusing to estimate because, again our brains get used to the situation. But auto-focus cameras will always focus correctly. One immediate effect of the flat port is that it reduces the opening angle of the lens, turning a 38mm lens into a 50mm lens and so on, which in turn reduces depth of field by some 60% (double the effect).

The dome port places a perfectly spherical glass in front of the lens at the precise distance of the virtual centre of the lens (which may not be the same as the lense's focal length). The aperture is supposed to sit in this centre so that it will never project itself onto the film. The idea of the dome port is that light rays pass through perpendicularly, thus avoiding the distortion associated with flat ports. In practice this is not entirely true because the lens may move forward and backward while focusing and many dome ports are not very precise. The best dome port is the one that is part of the lens as is the case with the Nikonos wide angle lenses (Nikonos 5 and Nikonos RS). These are noticeably sharper.

A side effect of the dome port is that it introduces a negative water lens in front of the camera, which makes objects appear further away and which requires an equally strong positive correcting lens on the inside of the dome. The strength of this lens depends on the curvature of the dome and is often around 3 diopters. Note that without it, most lenses would not be able to focus at all. The correcting lens can be placed in front or behind, between lens and film. But please note that a 13mm fisheye is not just a 13mm lens in front of the film, because this would leave no space for the mirror. Instead, the lens is placed at the normal 50mm distance and additional optics convert it to behave like a 50mm one on the film side, and 13mm on the water side. Instead of a 3 diopter lens on the film side, you need one of much less strength (3/4 diopter).

As a point of interest it should be mentioned that the technique of taking half under and half above water shots, the part of the dome above water needs no correction whereas the part under water does. For such photos a special half lens is placed in front of the camera together with half a neutral density filter to correct for the brighter top half, by 1-2 f-stops. This half lens could also be placed behind the lens, between lens and film. Note that the Nikonos 13mm wide angle lens cannot be corrected this way unless a negative correcting lens is used between lens and film. With this in place, the lens is fabulously sharp above water (See Nikonos chapter).

Rebikoff-Ivanoff correction lens Demitri Rebikoff was one of the pioneers in underwater photographic equipment in the era 1940-1980. He designed an underwater correction lens that is also a wide angle converter, based on an inverted telescope. Because it does not change the camera's focus, it is said to be a-focal. As shown in the diagram, this correction lens consists of a negative lens as port and a flat positive lens placed 30mm further towards the camera. The lens can be put together from the parts supplied by a technician for eye glasses.  It has a number of attractive advantages, not the least that it can be used both above and under water. It is also used as a 'wet-mate' underwater attachment. The Rebikoff 'port' is often used for underwater television cameras, but it is disappointing for still cameras, and cannot match the sharpness of the Nikonos lenses.

For a detailed course on mastering the digital darkroom, visit the corresponding chapter.