## Light and exposure values

Light is the stuff that is captured in our photographs, without it, we would hang up our cameras and go home. Yet what is it?

Our science teachers tried to instil in us that light is the part of the electromagnetic spectrum that our eyes are sensitive to, and that it is made up of waves, each different wavelength appearing as a different pure colour.

But in reality, pure colours occur very rarely in nature, and natural colours are a combination of many different wavelengths, which can be partially, but never totally, simulated by artificial means. A violet colour, if a pure colour, would diminish in brightness gradually with depth of water, but still retain the same hue, whereas an almost identical violet colour, made up of red and blue, would lose its red component quickly, and change to a blue colour in a relatively small depth.

I believe that it is helpful to the photographer to consider light also as particles as well as waves, which physicists like to identify down to their smallest amount, which they call a quantum. Thinking of light in terms of quanta allows us to understand the concept of an amount of light being present, an idea which I find difficult if I consider light to be only a wave.

### Exposure values

The amount of light present, specifically falling on the subject (in candelas per unit area), can be measured with a light meter, and quantified as an Exposure Value (EV), which photographers (or camera computers) can use to calculate exposure times and apertures for a given film stock. Examples of these numbers are:

• EV 7 Street lights
• EV 15 Bright or hazy sunlight

Many modern cameras can work over a wide range, perhaps EV 0 to EV 20, although not all functions will work correctly at all values (lower EV numbers can be too dark for many autofocus systems).

### Colour temperature

Of interest too to photographers is how that light is made up of the various wavelengths. Light with a bias toward the red end of the spectrum is often called "warm", while light at the blue end is "cold". The actual balance is referred to as Colour Temperature and is measured in degrees Kelvin, along the lines of the colour emitted from a specific object when it is heated. Generally these range from about 2,800°K for a 75W light bulb to 12,000°K for open shade on a bright sunny day.

While the human eye readily adapts to wide changes in colour temperature, films cannot, and the photographer must choose a film to match the colour temperature of the light if results are to be obtained without colour biases (easy to spot as whites don't come out white!). While the range of film types is very limited, the photographer can use special colour filters to balance the colour temperature of the light to the film being used, and some are even available that try to balance the colour absorbing effects of water, compensating for the blue of tropical waters or the green of UK waters.

Films are also balanced for particular colour temperatures, in order to reproduce whites correctly after exposure to particular types of light, primarily

• Daylight or electronic flash (5,500°K)
• Type 'A' Tungsten photographic studio lights (3,400°K)
• Type 'L' Tungsten photographic studio lights (3,200°K)

Certainly a good understanding of these colour temperatures, the various film stocks, and the colour correcting filters would permit the underwater photographer to experiment with various combinations to obtain creative effects.

Luckily, underwater we usually want to convey the scene in its colours as we see them, and by using daylight balanced film, and electronic strobes (which are matched to daylight also), we can achieve a lifelike result while restoring 'true' colours to close-up subjects.

### Film characteristics

All film consists of fine grains of light sensitive chemicals held firmly in an emulsion base on some sort of backing strip. A number of layers are necessary to contain the various chemicals sensitive to different primary colours in order to recreate a full colour image in the end.

Figure 4. Film construction.

There may be a number of different layers for each colour, containing chemical grains of different sizes, as large grains react quickly to light, and small grains react slowly to light. You can think of it in terms of each grain needing a certain 'quanta' of light to 'fire' it, with a large grain only needing one 'quanta' to fire it, but eight small grains, occupying the same space, needing eight 'quanta'.

The reason for this is that all films need a mixture of grain sizes in order to obtain contrast - if they were all the same size, then a low light level would cause none of them to fire, and a high light level would cause all of them to fire, causing a black or white result, but never any greys. (Some specialist lithographic copying films do actually work in precisely this way).

Each film is a therefore a balance, or realistically a trade-off, between large and small grains which affect

• Speed of reaction to light
• Amount of contrast
• Overall Grain size in result when enlarged
• Overall Sharpness

Although films are primarily classified by their speed of reaction, or sensitivity, measured by their ISO/ASA number, this is nevertheless a very good guide to their other characteristics.

Figure 5. Film characteristics.

What actual film to use is generally a personal preference, as each individual film stock returns slightly different results. I find that underwater photographers generally use a medium speed film (ISO 100) for wide-angle and other general work, but may use speeds down to ISO 25 for close-up and macro shots which are entirely lit by strobe. Some very popular choices of slide film at the time of writing are:

• Kodak Elite II or Ektachrome E100S
• Fuji Sensia or Provia
• Fuji Velvia 50
• Kodak Kodachrome 64 (especially popular with the press)
• Kodak Kodachrome 25

The first three are developed using the E6 process, which is a commonly available service in high street stores and professional labs, but Kodachrome must be processed by Kodak at one of their laboratories.

Professional films, like Ektachrome E100S, Provia and Velvia, are almost no different in manufacture, but appear to be more carefully quality controlled and stored to help professional photographers obtain identical colour balances from rolls of the same batch. There appears therefore to be no justification for amateurs to choose such films solely on the basis that they will give better results than the non-professional version.

Films come in all sizes, from the sub-miniature spy cameras to large view cameras giving 8"x10" negatives. Almost all underwater photographic equipment uses the common 35mm standard film (expressed as the width of the film strip), which gives 24mm x 36mm images. These films come in three main lengths, either 12, 24 or 36 exposures per cassette, although special attachments are available for professional press cameras which will take up to 250 exposures if you "roll your own". The 36 exposure cassette is most commonly used underwater, as most underwater photographers want as much film on hand as possible on each dive, and offers a very good compromise, as few people would carry enough air or strobe-power to expose a 250 roll.

Films come also in two different types, either colour print films giving a negative image which can be printed, or colour transparency (or reversal) films which are suitable for projection. The ability to give slide shows, and the preference by professional publishers for transparencies means that the majority of underwater photography is taken using this medium.

However, just in case you thought underwater photography wasn't difficult enough, transparency film is much more sensitive to overexposure or underexposure than print film. This property is called exposure latitude, and is typically ±5 stops for print film, but only ±1½ stops for transparencies. On the plus side though, transparencies give better contrast and colour saturation, for the same reason, and are well worth the additional effort to get right - even though its harder to get a good print made from them

### The camera shutter

The camera shutter is a finely engineered mechanism that is one of the two ways that give you control of how much light falls on the film. When released, the shutter opens, permitting light through onto the film, and stays open for a fixed period of time, then closes. Times are in seconds, successive ones usually opening for half of the time of the previous, with common ones, likely to be used underwater, being;

• 1/15,  1/30,  1/60,  1/125,  1/250,

although 'non-standard' speeds are also used, for example 1/90 for strobe synchronisation on the Nikonos V, and 1/100 on the Motormarine II.

Shutters may operate physically via blades (called 'leaf shutters') or via sliding curtains (called 'focal plane shutters'). Focal plane shutters operate with two 'curtains', made out of fabric or metal, the first of which moves to reveal the film, and the second which closes at the end of the period. Focal plane shutters can achieve higher effective speeds when the second curtain rapidly follows the first, resulting in a narrow slit of light passing quickly over the film. For this reason, focal plane shutters have a physical limitation when used with strobes, and the maximum speed that can be successfully used with strobes is that when the first curtain is fully open before the second has started to close.

### The lens aperture

The other mechanism that gives you control over how much light falls on the film is the lens aperture. The aperture as an adjustable 'hole' in the lens design.

The size of the 'hole' to allow a set amount of light through will vary from lens to lens. Apertures are therefore measured by the amount of light they let through, known as an f-number or f-stop, ranging from 1 to 32 or greater.

Apertures the underwater photographer will commonly use are f/5.6, f/8, f/11, f/16, and f/22. Not all lenses will have all openings, but all of the above will be found on most common lenses used underwater.

The number is arrived at by taking the focal length of the lens and dividing it by the diameter of the aperture (as seen from the front). Consequently, the brightness of the image on the film is inversely proportional to square of the f-number. This means to me that:

• f/2 is a bigger hole than f/2.8
• f/2 lets in twice the amount of light as f/2.8

Moving to a smaller aperture (f/2 to f/2.8) is known as 'stopping down', and moving to a larger aperture (f/2.8 to f/2) is known as 'opening up'.

The primary reason for using this rather strange mechanism of classifying lens apertures is, because they are related to focal length, a given lens aperture, for example f/5.6, will let through the same amount of light regardless of whether the lens is a wide angle or telephoto, and this makes calculating exposure much easier.

Wide apertures aren't commonly needed underwater, so don't go and buy lenses with a maximum aperture wider than you are likely to need, as it increases bulk and particularly expense. For example, in Jan 1995, compared to a Nikon 35mm f/2.8 lens, the f/2 version of the lens cost 50% more, and the f/1.4 version cost almost three times as much!

### Focal lengths

Different lenses are categorised by their focal lengths (usually in millimetres), relating to the image size that would have been produced from a pinhole at a certain distance from the film plane. This is a simplistic definition which obviously would not allow for angles greater than 180 degrees, but works well for standard lenses.

Lenses come in a variety of focal lengths, and lenses with shorter focal lengths have wider angles of view, also called angles of acceptance or angles of coverage

Figure 6. Coverage angles of different lenses.

The immediate consequence of this is that shorter focal lengths will cover a wider area when shooting from the same distance, than a longer focal length lens.

Figure 7. Apparent coverage of different lenses at a constant distance

Or alternately, a wide angle lens means you can get closer to the subject to obtain the same image size (although you may find a few good reasons not to!)

Figure 8. Distance to subject for different lenses for a constant image size.

Now this is really important to underwater photographers, as we need to get closer in order to minimise backscatter - so wide angle lenses are widely used, and an indispensable part of the underwater photographer's equipment

### Focus and Depth of field

Most lenses are adjustable for their point of focus from 1 foot to infinity. However focus isn't an all-or-nothing matter, and acceptable focus remains for a distance both behind and in front of the point of focus. Of course 'acceptable' is rather a personal thing, but a generally agreed standard does exist. (See Appendix B for further details). This area of acceptable focus is known as the depth of field.

Figure 9. Depth of field in relation to plane of focus.

It is convenient to think of two separate areas, a rear depth of field and a front depth of field. Assuming that lens is focused on the subject, the rear depth of field is the distance from the subject to the farthest point that is acceptably sharp and the front depth of field is the distance from the closest point that is acceptably sharp to the subject,. It is worth noting the front depth of field is usually only half as deep as the rear depth of field when the lens is focused near infinity, but are almost equal at close-up distances.

More commonly though the term depth of field is used for the combination of these two, i.e. the distance from the closest point that is sharp to the farthest point that is sharp. Depth of field however is not constant, and can vary quite considerable as:

• It gets smaller the closer the subject becomes
• It gets greater with smaller apertures
• It will be greater for wide angle lenses than telephoto lenses at the same aperture

Problems are caused as extreme close-ups have a very shallow depth of field, and need perfect focusing. But on the plus side, by selecting a wide aperture, cluttered backgrounds can be thrown out of focus, allowing the subject to stand out.

It is an important point to note however that, when using close-up lenses, depth of field remains remarkably constant, and is a product of the magnification, rather than the focal length of the lens. 60mm and 105mm Nikon macro lenses both focus down to 1:1 reproduction ratios, and at this ratio, or any other, they should have identical depth of fields.

### Hyperfocal distance

Imagine that with a given lens and aperture, there will be some particular distance which, if focused on, will have a rear depth of field which just includes infinity. This special distance is known as the hyperfocal distance.

Figure 10. Hyperfocal distance and depth of field.

This is extremely relevant when taking distant shots. If we were to focus on infinity with our lens, then we would have a front depth of field in which things were still in focus. However, by focusing on this hyperfocal distance, we can take advantage of both front and rear depth of fields, and still have infinity in focus.

With wide angle lenses, at medium apertures, e.g. f/11, using this technique, we can get everything from about one metre to infinity to be in focus.

Exposure Starting to put it all together, we have previously discussed · Light and EV's · Film speeds · Apertures · Shutter speeds which are all the elements we need to ensure the right amount of light falls on the film, to achieve the 'correct' exposure.

However, we must remember that EV's are related to the amount of light falling on the subject, not the amount that is reflected. Camera exposure meters (and films) are commonly balanced for approximately normal Caucasian skin tones, meaning that other darker subjects (e.g. dark foliage) or brighter subjects (e.g. white sand or snow) must be compensated for, by up to 2 stops, a large amount considering the low exposure latitude of slide film.

Nevertheless, assuming a subject of average reflectance (which works well underwater), for a given amount of light, you can alter the aperture or shutter speed to achieve correct exposure with a given film stock, and even select a number of correct shutter/aperture combinations within the variation offered by your camera and lens.

### Lens technical bits

The design and build of lenses is a particularly complicated science. The designer is trying to build an effective lens, while trying to minimise a number of aberrations and distortions that occur when you try and bend light to your will.

Aberrations are image defects that result from limitations in the way lenses can be designed. Better lenses have smaller aberrations, but aberrations can never be completely eliminated, just reduced.

The well recognised aberrations and their usual definitions are:

• Spherical aberration. Light passing through the edge of the lens is focused at a different distance (closer in simple lenses) than light striking the lens near the centre.
• Chromatic aberration. The position of sharp focus varies with the wavelength of the light.
• Curvature of field. Points on a flat subject get focused sharply on a curved surface, rather than a plane (the film). With subject at infinite distance the net effect is that when the centre is in focus the edges are out of focus, and if the edges are in focus the centre is out of focus.
• Coma. Magnification varies with the distance from the centre of the lens. Off axis points are rendered with tails, reminiscent of comets, hence the name.
• Astigmatism. Off-axis points are blurred in their radial or tangential direction, and focusing can reduce one at the expense of the other, but cannot bring both into focus at the same time.
• Barrel distortion. The image of a square object has sides that curve in or out. (This should not be confused with the natural perspective effects that become particularly noticeable with wide angle lenses.) This happens because the magnification is not a constant, but rather varies with the angle from the axis.
• Pincushion distortion. As above, but the image of a square object has sides that curve in. Underwater, these two distortions are of little relevance, as there are few square objects or even straight lines!

The effect of all aberrations except distortion is reduced by stopping down,

The last distortion, diffraction, is caused because when a beam of parallel light passes through a circular aperture it spreads out a little. It can be reduced by opening up.

Bearing this in mind, there is therefore an important trade-off between aberrations and distortion, which effectively means that a lens is going to perform at its best at some point between its maximum and minimum apertures.

### Image sharpness

Naturally we want our images to be as sharp as possible, and as well as aberrations and distortion, there are a number of other factors to consider which affect image sharpness.

Lens build and quality is important, and the best lenses have special multiple coatings on the surface of each element in order to reduce flare and other internal reflections that can detract from the quality of the final result.

In addition, lens condition is very important, and you should keep your lenses, (or any filters or close-up lenses you use), free from scratches, grease, dust, and condensation.

Both lenses and cameras are carefully and precisely manufactured, and loss of sharpness can easily be caused by damage, such as dropping equipment, causing misalignment of the elements in the lens, or changes to the alignment of the camera and its lens-to-film plane alignment

Shooting conditions also play a big part, and camera shake is a common fault to be found even amongst experienced land photographers, caused by camera movement while the shutter is open. The greater the 'magnification' of the lens the more likely this is to occur, and there is a generally accepted formula that the minimum shutter speed that can be effectively hand-held (as opposed to tripod mounted) is 1/focal length. This means 1/125 for a 125mm lens, 1/60 for a 60mm lens etc. The effect may also be present to a greater degree at large magnifications with macro lenses.

Camera shake is unimportant when using strobes as, however long the shutter might be open, the light duration from the strobe is very fast, usually in the order of 1/1000. Luckily, cameras are more or less neutrally buoyant in water, and the water itself probably acts as a damping (no pun intended) mechanism, so it is entirely possible that you would be able to successfully hand-hold a camera for longer times than the guidelines suggest.

Underwater, particles in water can have a great impact on the overall quality of the result, as can two other conditions, thermoclines, where two layers of different temperature water meet, and haloclines, where two layers of differently salty water meet, as in river run-offs, can cause shimmers and distortions in the water, both of which are best avoided.

Finally, you cannot capture an image that is sharper than your choice of film can record, and understanding the limitations of the film you are using will help avoid disappointments when you come to order that 3 foot by 2 foot enlargement of your favourite shot.

### Monochrome

Very little underwater photography these days is done in black and white, certainly I have not used any of this type of film. I have however seen spectacular results published recently, although my own attempts have been less than satisfactory.

My feeling is that taking good black and white photographs is as hard underwater as it can be on land, and while I have seen some very good shots, I have also seen many monochrome reproductions of what were probably colour originals, and which looked very sad. Perhaps part of the problem is that monochrome films have quite a large exposure latitude, and like colour print films, do not give very contrasty results underwater.

If you are really set on slides, only one monochrome reversal film is currently on the market, which is the new Agfa Scala and is rated at ISO 200. There are also options to develop monochrome negative films such as FP4+, Plus X, and T-Max 100 using a reversal process for those who are prepared to do their own developing.

Monochrome is probably best suited for shots where colours are muted, or one colour (probably grey) dominates, and contrast is low. Deep wreck shots fall nicely into this category, but at the same time have a wealth of detail in them that can be captured on a high resolution film. Contrast can be adjusted in the printing stage to give a stunning image.

### Dome ports

Dome ports are needed for housed wide-angle lenses, as the external air-to-glass-to-water interface on the outside of the port causes bending of the light which either doesn't occur, or is compensated for by the design, in purpose-built underwater lenses.

When a wide angle lens is mounted behind a flat port, in addition to a reduction in the effective lens angle of coverage and focusing range of one third (remember the magnification effect of water), there is also a loss of edge definition due to chromatic aberration, giving colour fringes, overall image softness, and a loss of geometric quality, caused by pincushion distortion (although we mentioned that this was not necessarily a problem).

With the use of a hemispherical dome rather than a flat plate of glass at the front of the port, some of these problems are dissipated. However, the dome itself, when used in contact with the water, will become a negative power lens, and a compensating positive supplementary (or close-up) lens is needed, the power of which depends on the curvature of the dome, its position in relation to the land lens, and the close focusing ability of the land lens in the first place.

The dome correction port is unfortunately not perfect, and still suffers from two problems:

1. Curvature of field aberrations which are most pronounced with wide lenses and small diameter domes, but which can be compensated for partially by the greater depth of field in wide-angle lenses
2. Internal reflections of bright subjects, which can be minimised by masking the unused portions of the dome surface and blacking out any shiny or reflecting surfaces inside the dome.

While it is obviously important to match dome ports to lens design, or vice versa, most ports are supplied by housing manufacturers, and may be designed to suit a number of different lenses. It may be worthwhile, when purchasing a dome port, to consider your future needs (perhaps you have only a 28mm lens at present but intend to upgrade to a 24mm or 20mm in the future), and purchase a dome port that will suit the lenses that you will be using in the future as well as now.