Every type of art relies on practitioners' control their technique. A painter who doesn't know how to control the flow of paint cannot reliably produce predictable results; a musician who doesn't know how an instrument will respond can't produce music. Happy accidents can be great for learning and can sometimes result in interesting effects, but if not harnessed as part of a broader technique or strategy, they rarely result in meaningful work.

The same is true of photography: to be able to create engaging images, a photographer has to have a measure of control over his or her medium. The aim of this article is to set out the most fundamental aspects of photographic technique, in the broadest strokes that would make it possible for a photographer to make conscious, controlled choices, and without relying on automation to make them instead.

Photographic materials

Photographers have used a variety of materials to make images, including plastic film, glass plates, collodion (essentially gun cotton dissolved in ether and alcohol), gelatin, albumen, starch, as well as a variety of compounds of silver, gold, platinum, palladium, chromium, mercury and gold. There is very little that someone hasn't tried using in photography. Of those that have produced workable results, the most popular are plastic base films with silver compounds dispersed in gelatin. However, even within this seemingly narrow category, the choices are very impressive.

Monochrome or color

One of the most fundamental choices a photographer has to make is whether the final image is to be monotone ("black and white") or color. Black and white photography was the standard in the first century after its invention, though even there "black" meant anything from true black to the polished silver of daguerreotypes, the sepia of salt prints, and pretty much every color one can imagine, thanks to chemical toning and hand coloring. Color photography, which really came into its own after the Second World War, uses a complex layering of silver gelatin emulsions, color filters and dyes to create a close approximation of the colors most people see in everyday life.

Positive or negative

Photographic materials produce either "positive" images that have a direct relationship to the intensity of light in the photographed scene, or "negatives", where that relationship is reversed. Most of the time, positives are used for viewing directly or mechanical reproduction, while negatives are used to make final, positive images by exposing other negative materials through them, therefore reversing the light-to-dark relationship once again.

Sensitivity

Photographic materials (for the sake of simplicity let's call it "film", since that is the most popular medium) come in various grades of sensitivity to light. Generally speaking, the "faster" the film, the more sensitive it is to light, but the larger grain it will exhibit. Slower films make smoother, finer-grained images, but sacrifice the ability to capture quick-moving scenes in dimmer light.

Size

Film comes in a number of standard sizes, of which 35mm daylight loading casettes are most common. Larger sizes include type 120 medium-format films and sheet films in sizes such as 4x5", 5x7" and 8x10". Even if this is the bulk of the market, only technical prowess (and unfortunately financial capacity) prevents a photographer from either adjusting these sizes or making his or her own materials.

Substrate

Finally, while films constitute the bulk of the photographic materials using for recording scenes, other media have been used, and continue to be used by some photographers. The most common alternative to film is photographic enlarging paper, but others include metal plates (tintypes, daguerreotypes), glass plates (dry plates, collodion wet plates). Photographic emulsions can also be applied to fabrics, and even everyday objects. That said, these types of experiments constitute a tiny fraction of the work being done with analog photography.

Camera controls

While photography doesn't have to involve the use of a camera (see Laszlo Moholy-Nagy's photograms for example), that is how most photographs are made. The ability to control what a camera does with light is essential for a photographer to make reliable decisions. Fortunately, outside the bells and whistles of automation, most cameras modulate the quantity and quality of light they admit using only a few basic controls:

Focus

Focusing means simply adjusting the distance between the film and lens to produce a sharp image of the object or objects being photographed. With most cameras, this is as far as one can go, though adjustable focal plane cameras, such as view and field cameras, can greatly expand the possibilities by altering the geometry of the camera itself.

Lens aperture, depth of field

"Aperture" refers to the size of the opening in the lens that allows light to enter the camera. The larger the opening, the more light enters, and the less time is required to obtain the desired exposure level. Aperture size also affects "depth of field", or the distance from the camera within which objects appear to be in focus. Shallower depth of field means that only some objects in a scene may be in sharp focus, while others will be rendered softly. Greater depth of field means that objects at various distances will be sharp.

Shutter speed

At the most fundamental level, a shutter is a device to open and close an opening on the camera, to let light fall on the film. Earliest "shutters", in days when typical exposures were counted in whole seconds and even minutes, were simply lens caps that were taken off and removed from the camera. Later these became dedicated timing mechanisms that were able to open and close at precise intervals. Typically, modern film cameras employ one of two types of shutters: focal plane or central. A focal plane shutter consists of two curtains that travel across the frame just in front of the film. Central shutters are most ofthen located within the lens, usually between groups of optical elements. Both have their advantages and disadvantages, but in general focal plane shutters can produce shorter exposures, while central ones are generally more flexible with flash photography.

Lens focal length

Simple lenses focus light rays at a fixed point in space. The distance at which this happens is called their focal length. For example, a typical 50mm lens will focus light from a source located "infinitely" far away 50 millimeters from its optical center, a 6 inch view camera lens will focus at 6 inches, and so on. A lens with a longer focus will produce a larger image than a short one. Assuming that both are used on the same size of film, the larger image will fill more of the frame, and therefore will appear "closer".

In general, lenses with a focus similar to the diagonal dimension of the frame for which they are used are considered "normal" - they produce images that correspond to renderings of reality in classic art, with relatively little apparent perspective distortion. Shorter focus lens are considered "wide angle", as the smaller images they make fit more of the scene into the frame than a "normal" lens. On the other end of the scale are long focus lenses, which make objects appear closer than normal. These are often referred to as telephoto lenses, though technically telephotos are lenses that have a shorter focal length on one side, to allow a lens to be physically shorter than its optical focal length. Not all long lenses are telephotos, even if most people refer to them that way.

Since the 1980s, zoom lenses have been gaining in popularity, and currently they make up the bulk of all camera lenses on the market. These allow photographers to change the focal length of the lens by altering the relationship between the components of the lens, whether by rotating or pulling a control ring, or by pressing a physical or screen button. The quality of these lenses is now generally seen as on par with simple "prime" lenses, though earlier ones were usually optically inferior.

The process

Measuring the light

Knowing how much light is in a scene is crucial to achieving predictable results in photography. In the earliest days of the medium, photographers relied on experience to determine the exposure needed to produce optimal results. Since then, technology for measuring light has evolved, and most cameras produced since the 1970s have had built-in light meters, often coupled with automatic exposure systems.

Setting the exposure

Photographic materials react to light predictably within a certain range of exposure to light. When "correctly" exposed, they record the greatest possible amount of detail in both light and shadow areas, and generally have the greatest range of tones between the two. However, for various reasons, including availability of light and artistic expression, photographers sometimes choose to allow more or less light than the "ideal" amount to fall on their photosensitive materials. Both will generally result in a compressed tonal range, meaning images that do not include the full range of values from black to white.

The ability to consciously adapt exposure to the intended effect is one of the most fundamental skills of a photographer. While light metering systems and camera automation continue to advance, a light meter has no way of knowing what a photographer wants the image to convey. A photograph intended to capture the maximum amount of technical detail may need a different level of exposure than one meant to convey the mood of a place, even in the same location and with the same amount of light present. It is all a question of intent, and until cameras can read photographers' minds, it is definitely useful to understand exposure thoroughly.

Traditional silver-gelatin emulsion film reacts to light by turning darker the more it is exposed. This relationship between exposure and the resulting tone depends on a lot of factors, including the sensitivity of the material and the development process, but in the broadest therms within a "normal" range is predictable and linear. This means that adding exposure will increase the density of the silver deposit on the film by a predictable amount. However, this is true within a certain range of light levels, called the film's exposure range or latitude. At very low light levels, the film will not react at all, while above it all the available silver compounds in the film will be converted to metallic silver and further exposure will not result in any further change. It is important for a practicing photographer to know these limits.

Conventionally, exposure is expressed in "stops". Each stop is equivalent to doubling the amount of light of the stop below it and halving that of the one above. Most cameras control the amount of light entering using an aperture, which controls the amount of light entering, and a shutter, which controls the amount of time for which the light is allowed to enter the camera. Each of these devices is calibrated in stops, to make it easy to adjust exposures quickly and efficiently.

Aperture stops are labeled with "f" numbers, where the "f" stands for the lens's focal length. This means that an f/2 aperture is half the size of the lens focal length, and f/4 means one fourth of "f". This makes it easy to compare apertures of lenses of different focal lengths. In modern cameras, the sequence of stops, with each halving the amount of light entering compared to the previous one, is f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, and so on. Notice that the numbers themselves double every other one - that is because the amount of light is determined by the aperture's area, not its diameter. An aperture twice the diameter of another will have an area four times as big, and will therefore let in four times as much light.

Shutters, on the other hand, are usually labeled as the denominators of the fraction of a second for which they are open. So a shutter speed of 1/250 will be shown simply as "250" on an adjustment knob or display, while "30" typically means 1/30 of a second. In cameras where the shutter can be set for times above one second, the whole second times are typically a different color than the fractional ones to avoid confusion. The typical progression of shutter speeds on modern cameras is 1, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000 and so on, each being half the duration of the previous one.

With the total amount of light striking the film determined by both the aperture size and the shutter speed, exposures are typically given as a combination of the two numbers. For instance, on a sunny day in moderate lattitudes, an ISO 125 film might be exposed for 1/125s at an aperture of f/16. Since each stop on either device means a doubling or a halving of the exposure, the above combination is equivalent to an exposure of 1/250s at f/11, or 1/1000s at f/5.6. This allows a photographer to vary the final look of the image, while maintaining the same exposure. Faster shutter speeds will "freeze" motion better, while smaller apertures (i.e. ones with larger "f" numbers) will allow greater depth of field. It is also possible to change the exposure from the "correct" level indicated by the light meter. Underexposure, which in the example above might be 1/500s at f/16, will emphasize detail in the light areas of the image while compressing it in the shadows. Overexposure will do the opposite - dark shadows will have fuller detail than normal, while light areas might lose information.

Regardless of the particular intent of the image, it is worth remembering that while modern light meters and camera exposure systems are great at setting "correct" exposures, equipment manufacturers calibrate these for average conditions. Taking a picture in a coal mine or a sun-blasted snowy slope diverge from "typical" conditions significantly, and in order to achieve predictable results a knowledgeable photographer will adjust the exposure in each instance. In the examples above, since coal is black, it will fool the light meter into "thinking" that there is less light in the scene than there really is, resulting in overexosure. Muddy snow scenes are often the result of a photographer letting a camera do its own thing, where the large amount of light reflected from the snow tells the system that the scene is far more brightly lit than it is in reality. Again, there is no substitute for conscious choice.