Aperture and shutter speed control exposure, and in most cameras they are set automatically to give a photograph that matches as closely as possible, the way we see the image. (Show Photo) This means, for a typical outdoor day-lit scene, a picture that has a range of brightness from nearly white to almost black. The actual lighting conditions can vary enormously. On a foggy day, for example, there may be just a few close shades of gray, with no bright highlights and no deep shadows. In bright sunlight, on the other hand, some shadows can be 200 times darker than the most brightly lit parts. Certain kinds of artificial lighting at night can produce an effect that has too much contrast. Making an exposure involves capturing these many different ranges of light on film, and doing it so that the scenes remain recognizable.
Although it may be tempting to think of photographs in terms of "correct" exposure, there is in fact always a choice, and it is one that depends very much on the photographer. At dusk, for instance, there is a case for making a darker-than-usual exposure in order to convey the impression of evening. While the camera's automatic exposure system, if left to itself, will produce an image with average exposure, all but the simplest cameras allow the photographer to adjust or override it. There are, surprisingly, many picture situations in which different photographers would choose different exposures. Although there is one single exposure for any view that will give the most complete record of everything in the scene, this is no more than an objective ideal. Tastes differ, and individual preference is important.
Exposure Latitude
The human eye can rapidly accommodate different levels of brightness, flicking from highlight to shadow with little difficulty. Film, however, has a fixed range, below which there are no details recorded from the shadows, and above which the highlights are completely white. This exposure range is crucial to making a satisfactory exposure, and varies from one film emulsion to another. Most fast films, for example, record a relatively small range of tones (they have a lot of contrast). Some very slow emulsions, on the other hand, record very gradual changes in tone over a wide range.
Most films have an exposure range that is greater than the range of brightness in an average subject. In practice, this means that there are a number of different exposures that can be made without losing detail. This freedom of choice is called exposure latitude. Technically speaking it is the increase in exposure that can be made from a minimum needed to record shadow detail while still preserving detail in the highlights. If the film's exposure range is exactly the same as the brightness range of the subject, just one exposure setting will record everything well. If, however, the scene is harshly lit and the range between shadow and highlight is greater than the film's exposure range, no exposure will record all the details. In a case like this, the photographer should choose where to sacrifice detail - whether in the brightest or darkest areas. (Show Photo)
Metering
There are two steps in making an exposure, whether this is done automatically or manually. The first is to record the levels of light that actually exist in the scene in front of the camera. The second is to translate these measurements into the exposure setting that will best suit the film speed.
Most 35mm cameras incorporate light-sensitive cells that run their own metering systems, and readings are taken of the actual image about to be recorded on the film. This through-the-lens (TTL) metering automatically compensates for the different light transmitting properties of lenses and extensions. The sophistication of the metering system varies between camera models. One of the simplest is to measure the average brightness of the entire picture area.
The most advanced systems, under various trade names, measure a mosaic of the picture area and then weight the importance of each section according to a preprogrammed set of rules. Thus, a small bright area slightly off center, set against a dark background, would be accounted for and which a simpler, averaging meter system would over-expose. In between are systems that weight the exposure towards the center of the image (where most photographers tend to place the main subject of interest).
The simplest way of operating an exposure system is for the photographer to adjust the shutter or aperture (or both) to match the display in the viewfinder; this is called metered manual exposure. Moderately automated cameras require the photographer to set the aperture, and they then automatically adjust the shutter; this is called aperture priority exposure. Most sophisticated models allow a choice; aperture priority, shutter priority, in which the user sets the shutter and the camera adjusts the aperture, or any of various program modes, in which the camera sets both aperture and shutter according to a particular set of priorities. Some of these various modes are related to specific types of photography, such as sports or close-ups, which have special needs (such as the highest shutter speed possible or good depth of field).
With such automation, there are few occasions when there is any need to take separate readings manually. Nevertheless, there are certain types of lighting conditions that will fool most automatic cameras - snow scenes, for example, which ought to appear as a mainly white image, would be exposed by an automatic camera as gray. The principle on which all exposure meters work is to average the area they read; normally they then produce a setting that will give a mid-toned exposure. If the photographer wants the image lighter or darker than average, it is usually a simple matter to compensate, either by manually overriding the camera or, in some models, by operating a special over-under-exposure control. Beyond this, however, there are hand-held meters for taking more serious, independent measurements.
Hand-Held Meters
Though by no means essential, hand-held meters are very flexible instruments, and can be used in situations that are difficult for the camera's TTL meter. All have a light-sensitive cell, a read-out that is usually digital but may be analog (such as a moving needle against a scale), and a calculator. Whereas camera TTL meters measure the light reflected from a subject or a scene, most hand-held meters offer the option of measuring the actual light itself. The two methods are known respectively as reflected-light and incident-light readings. (Show Photo)
Spot-meters are a special variety of hand-held meter, with a lens and viewer. They take reflected-light readings but through an optical viewing system with an angle of acceptance of as little as degree. With such precision, they can measure the brightness levels from the smallest areas of a scene. One technique is to measure the differently lit parts of the view and then calculate the average (a digital spot-meter can usually make this calculation itself). Another is to measure one small representative area. Although a spot-meter takes time to use, it is the most accurate of all the meters and comes into its own when the main subject has a completely different level of brightness from its surroundings.
Recording the Image
The task of the camera and lens is to form an image; the job of film and, increasingly, electronic media is to record the image. Although at first glance, film and electronic media may seem diametrically opposed, there is in fact considerable correspondence between them. Essentially, both record images as a mass of tiny, individual units of tone and color. Viewed at a normal distance, this finely patterned mosaic looks continuous. Mechanically, they work quite differently; electronic media have the special, invaluable property of digitizing the picture. Once digitized, an image, like a set of numbers or a block of text, can be processed and manipulated in almost any way. The process of creating an image on film occurs in two main stages. The first stage is the exposure, when light-sensitive crystals, or grains, in the film emulsion undergo electrochemical change in response to being struck by photons (particles of light). At this point a latent (hidden) image is formed. The second stage is the film development, when the sensitivity of those crystals that were struck by light is increased by the chemicals in the developer, about 10 million times, so that the image finally becomes visible.
Silver Halide Emulsion
The first photographic emulsions were black and white. Restricted to enthusiasts and some professionals, its chemistry remains the basis of all films. Without getting into details, an invisible electrochemical change is triggered in the silver halide crystals when they are exposed to light. The pattern of these changes across the entire surface of the emulsion forms a latent image. Color film works in essentially the same way, except that there are three layers of emulsion sandwiched together, each sensitive to a different part of the spectrum. (Show Photo)
The active ingredient in film is silver halide (a light sensitive chemical compound). It takes the form of a mass of crystals, commonly known as grains, distributed as evenly as possible in the gelatin layer that makes up the body of the film emulsion. The size and distribution of these grains varies according to the type of film, but in an average emulsion each measures about 1/1000th mm across. The silver and halide components are held together in a lattice arrangement by electrical bonds; when the photons strike a grain they disrupt its electrical status quo by adding energy to it.
Most of the improvements and alterations that film manufacturers make to their products have to do with the size and shape of these grains and the way in which they are suspended in the emulsion.
Although all film reacts in the way described, the end product can be a color negative, a black-and-white negative, a color transparency, or even a black-and white transparency. The negative images are, in turn intended to be converted into enlarged prints. In each case, the original exposed grains must be treated chemically. In standard black-and-white film, the grains are converted to black metallic silver. In color film, dyes replace the silver halide grains. Transparency film requires an extra processing sequence in which the tones, and colors, of the developed negative image are reversed.
Apart from color quality, which is dealt with a little later, the two characteristics that most concern photographers are sensitivity to light (speed) and resolution (ability to show fine detail). To contrast and improve these, film manufacturers have had to experiment with the granularity of the emulsion.
Grain And Speed
To make a film more sensitive to light (increase its speed), a greater area of each affected grain needs to be exposed. (Show Photo) Traditionally, this was done by using larger grains, but a more modern development is to use grains that are flatter rather than lumpy; if they lie flat side towards the light, the result is greater sensitivity without more silver halide bulk. Film with larger, or flatter, grains reacts more readily to light and is therefore "faster". However, the price that has to be paid for this increase in film speed is a coarser image, more so with large than with flat-grain emulsions. The increased granularity gives rise to a grainier appearance, which in the fastest films or with great enlargement actually breaks up the image into a pronounced speckled texture.
Graininess runs counter to the ideal of good resolution. For this second important quality, film manufacturers set smaller grains into a thinner emulsion to reduce the internal reflections that degrade the image in a thick layer of gelatin. This type of emulsion has a fine-grained appearance, but reacts less quickly to light and is therefore "slow". The improved sensitivity of flat grains is also used in combination with cubic grains for fine-grained emulsions, which can produce exceptionally sharp images. An even distribution of the grains and a fine internal structure contribute to this.
Ultimately, the photographer must choose between these two ideals, sensitivity to light or detail. If high speed is essential, as in low-light or action photography, then fine grain must normally take second place. On the other hand, if it is more important to preserve the finest detail, then the exposures will have to be longer.
A quality that is related to graininess is sharpness. It is important to realize that sharpness is a subjective impression, but is a good indication of how well edges are recorded. The objective measurement of this is called acutance. Together, graininess and sharpness make up the even more subjective impression of definition. Of the two, graininess is normally considered less objectionable than loss of sharpness. Indeed, graininess can slightly compensate for loss of sharpness because the edges of the grain-clumps themselves are clear.
Color Film
A basic silver halide emulsion reacts simply to light - across approximately the whole spectrum. The developed image is also monochromatic. Most photography, however, is in color, and the steps needed to create a color image from black-and-white emulsion are fairly complex. What makes color reproduction possible is that the full white-light spectrum can be broken down into three equally spaced wavelengths: blue, green and red. These colors are each located in a third of the spectrum. Combined equally they make white light, but combined in other proportions they can reproduce almost any other color. This is the three-color principle of reproduction.
There are two ways of using these three colors for reproduction: additive and subtractive. In the less commonly used additive method, blue, green and red are combined. The subtractive process, which is the basis for most color films, uses dyes or pigments to subtract selectively from white light. To do this, it makes use of the three colors that are complementary to red, blue and green - these are cyan (blue-green) yellow and magenta (purple-pink). Each one absorbs its opposite primary color; thus, cyan absorbs, or blocks, red light; yellow absorbs blue, and magenta absorbs green.
Using this three-color principle, a color film can be constructed with the same basic emulsion as black-and-white film (i.e., using a silver compound that is sensitive only to the amount of light reaching it). Color film has three layers of emulsion, each one of which has been made sensitive to one of the three primary colors - blue, green, red - by means of built-in dye filters. The technical term is integral tri-pack film.
Color negative film: During exposure, three separate latent images are formed. When developed, they become three negatives, corresponding respectively to the blue, green and red light in the subject. (This is very similar to the separation negatives necessary to print the color images in this book.) There is just one development, and the process is completed by bleaching out the unwanted silver to leave three color dye images. When projected in an enlarger, the three transparent layers combine to give an image that is reversed not only in tone, but also in color. Color negative film has a distinct orange appearance; this acts as a mask, designed to compensate for color distortions that would otherwise occur in printing. (Show Photo)
Color reversal film: Reversal, or transparency, film has essentially the same tri-pack construction as negative film. Its development, however, involves an extra step in order to create a positive image from the negative. In this second stage, after the negative image has been developed, the film is briefly "exposed" (originally to light, now chemically) so that the silver compounds that had remained unaltered now form reverse, positive images. This is the reversal that gives this process its name. A second color development is then given to these newly activated compounds. Following this stage, the three layers of emulsion each contain a silver negative image (which is not needed) and a positive color image. The silver is bleached away, leaving three colored layers that, when seen against the light, give the effect of the original subject colors.
A different process - K-14 - is used by Kodak for its Kodachrome reversal films, which are the only remaining brands of what are called non-substantive emulsions. These films, as used by the photographer, contain no color couplers; they are added later at the processing stage. This adds to the complexity of the processing, and few laboratories are equipped to handle Kodachrome. The advantage, however, is that the emulsion is thinner when exposed and the dye can be spread more evenly, giving reduced graininess and very good resolution.
Additive color film: Only a handful of additive color processes have been used significantly in photography. The most recent is Polachrome instant slide film, a black-and-white emulsion laid over a screen of fine lines in the three primary colors. These give the color effect, but the drawbacks are a dense image (more murky than conventional slides) and the limited enlargement possible because of the screen lines.
Color Quality
Our judgment of color is a complex process, both physiologically and psychologically. For the film manufacturer the problems are not simply those of trying to reproduce color accurately, but also of meeting the preferences of photographers - and these are not all the same. That different makes of color film produce different results is not necessarily because one is more advanced than another; film manufacturers have different policies towards color balance. For example, some makes of color film deliver noticeably more intense blue skies than do others - more intense than in real-life, too - because a part of the photography market prefers this richness in holiday and travel pictures. Apart from this, the dyes used in film are all, to some extent, imperfect. Achieving accuracy in one hue may make it extremely difficult to reproduce another hue precisely.Trade-offs between colors in the manufacturing process account for differences between film brands, and consequently the preference that some photographers have for certain brands.
As color film depends on the basic silver halide emulsion already described, manufacturers face the same dilemma in trying to produce fast films and good resolution at the same time. The two qualities are essentially incompatible, even though the metallic silver responsible for graininess in black-and-white film is bleached out of color film. Instead, the color dyes that replace the silver during processing form "dye-clouds", which give a similar impression to graininess. Fortunately, these dye-clouds measure only a few thousandths of a millimeter across and the unaided eye cannot distinguish between them, or the three different colors of dye-cloud would make color film look even grainier than black and white. As it is, the magenta clouds cause most of the grainy appearance, because they absorb the green light to which the eye is most sensitive.
Light Balance
One major distinction between different color films is the type of light for which they are intended. There are important color differences between daylight and artificial light. Humans have no difficulty adjusting their eyes from one light source to another, but film cannot adapt in the same way as the human eye. In practice, if a film that is intended to be used in daylight is used indoors with tungsten lighting, the result will be an orange-tinted photograph.
Photographic lighting is provided by flash, which has the same color as daylight, or tungsten, which is much more orange. For this reason, a few films are made balanced for tungsten lighting. With color negative film, the difference in color cast from different light sources is not quite so important as it is with transparency film, as it can to some extent be corrected during printing. Ordinary non-photographic tungsten lamps, as in most domestic interiors, are even more orange than photographic tungsten, and if they need to be neutralized, a bluish filter should be used over the camera lens even with tungsten-balanced film. Other artificial light sources, such as fluorescent light, do not have specially balanced films made for them, and call for filters.
Filters
The principal use for filters with color film is to balance the color of the light source. (Show Photo) One range of filters, from bluish to amber, balances the color temperature of the light. For example, a strong bluish filter from this range can be used with regular daylight film in tungsten light to neutralize the color cast. Another range of filters is available in different strengths of the primary colors, blue, green and red, and yellow, magenta and cyan. These can be used to correct deficiencies in either the film (which can change color with age or because of poor storage conditions) or the light source.
Selective filtration is not easy, as most filters simply change the overall color. A polarizing filter, however, is one of the few ways of making a noticeable change in colors. Light from the sun travels in waves that vibrate in all directions, but when light bounces off certain kinds of material - notably glass and water - the waves are reoriented to vibrate in just one plane. This reflected light creates reflections, but the structure of a polarizing filter, which acts like a grid with extremely narrow parallel slats, passes only those light waves oriented at the same angle, and can be used to eliminate reflections. Blue sky also reflects light, and for the same reason that a polarizing filter works on reflections from glass or water, it can darken the appearance of blue sky at right angles to the sun. It also enhances color saturation by suppressing white-light reflections from particles in the air.
An ultraviolet (UV) filter also improves haze penetration, reduces the overall blue cast associated with atmospheric haze and increases color saturation. The effect is particularly marked when using a long-focus lens on a distant scene. Another occasionally useful filter is a neutral-density (ND) filter, which simply reduces overall exposure without altering color. Occasionally, for instance, there may be too much light for a particular shot, as when a slow shutter speed is needed on a bright day to create deliberately blurred movement. Neutral-density filters are available in a range of strengths.
Black-And-White Film
Color film's realism, and the natural pleasure that colors give, account for it dominating modern photography. However black-and-white film has special graphic qualities that keep it popular among a small number of photographers. The essential simplicity of an image that varies only in tone makes it favored in photography that aims for a powerful image, and also for some subjects that benefit from being distilled and simplified, such as certain types of landscape and photo-journalism. (Show Photo)
As with color films, there is a choice available from very slow and extremely fine-grained at one end of the scale, to ultra-fast but grainy at the other. Because there is only a single tonal range - and a single emulsion layer - rather than the tri-pack structure, black-and-white film is easier to manipulate. In particular, alteration of the film's development is an uncomplicated procedure; this can be used not only to compensate for too little or too much exposure, but also to change the contrast.
One different monochrome process borrows some of color film's technology. In so called "chromogenic" films, the original silver is removed after the first development and replaced with a dye. The result is considerable flexibility of exposure and a virtual absence of the traditional grainy texture. Also, because such films can be processed in standard color negative chemicals, it is easy to find a commercial processing lab to handle them (traditional black-and-white processing is relatively rare on a commercial basis).
Filters For Black-And-White Film.
Although the single layer of silver halide produces a monochrome image, it still reproduces colors - but as different shades of gray. Technically speaking, its sensitivity to different wavelengths varies, so that it reacts more strongly to blue than other hues. Most general-purpose films, known as panchromatic, incorporate dyes to adjust their sensitivity to all the visible colors. Nevertheless, they still remain rather over-sensitive to blue and under-sensitive to red.
In our experience of colors, some hues are brighter than others: an average yellow, for example, seems brighter than an average blue. If the tones in which different colors are reproduced in a black-and-white photograph approximate to this experience, the image seems realistic. However the blue sensitivity of black-and-white film can result in clear skies looking too pale; a yellow filter , which absorbs blue, restores the expected appearance.
Special-Purpose Films
A few films are made for specialized fields of photography. One of the most striking in its pictorial effect is infra-red film, available in color and in black and white. The three layers of color infra-red film are sensitive to infra-red, green and red respectively, rather than blue, green and red as in a normal film. This is technically false-color film, and intended for surveying healthy versus dead vegetation. A yellow-orange filter gives the most normal-looking results, but healthy plants reproduce a striking red. Other filters give less predictable results, depending on the subject matter. As regular exposure meters do not measure the infra-red component, it is important to follow the packed instructions and I/O bracket exposures.
Black-and-white infra-red film is very sensitive to infra-red wavelengths far into the infra-red region of the spectrum - as far as 1200 nanometers, which is much further than we can see. Hot objects emit infra-red light in the far infra-red region, and so are "bright" to this film. Also, the infra-red reflectivity of materials is not the same as their white-light reflectivity. Water droplets, such as in clouds, reflect infra-red quite strongly, and clouds appear a brilliant white in black-and-white infrared photographs. Healthy vegetation behaves in a similar way, and leaves and grass have a snowy appearance. These films need an opaque filter to give their most intense effect, to prevent the infra-red "view" being weakened by the ordinary white-light appearance.
Electronic Imaging
In electronic imaging systems, film is replaced by an imager - typically a CCD array- and a means of storing the electronic information, usually in the form of a floppy disk or hard disk. Built-in color filters make color images possible. The resolution depends on the density of the array and is measured in pixels. For instance, if the pixel size is 16x16 micrometers, a CCD covering the full 35mm frame could resolve to more than 1 million pixels: 1024 rows by 1280 rows in the case of the Kodak Professional Digital Camera System.
Another system for handling images that have already been shot on conventional film is Kodak's Photo CD. In this system, processed slides or negatives are taken to a photo finishing outlet and recorded digitally onto a compact disc. Images can also be recorded in negative, slide or print form directly onto the computer's floppy or hard disk. Both of these methods were used in the production of this CD-ROM.
Electronic images can be viewed on a television monitor, or hard copies can be made using a printer. If the image is loaded into a computer, the choice of output devices is increased: as well as prints, slides can be produced, or even direct separations for printing in a magazine or book. This last option is particularly useful for desktop publishing.
