1. Light and electromagnetic radiation
Every object becomes visible through the light that is emitted by it or reflected from it. This raises the question: “What is light?” The answer was first given in 1865 by the physicist Maxwell, who explained in his theory that visible light is an electromagnetic wave, part of the broader electromagnetic spectrum. This spectrum extends from cosmic rays and X-rays, through ultraviolet, visible and infrared light, up to radio waves. Different bodies emit energy in the form of electromagnetic waves (radiation), whose wavelength can span the whole electromagnetic spectrum. Some radiation is perceived as heat, other radiation as visible light. They differ only in wavelength. The part of the electromagnetic spectrum that extends from 400 to 700 nm is called the visible spectrum, or visible light. As can be seen, light, or the visible spectrum, is a very small part of the electromagnetic spectrum, and it is through this part that the beauty of the universe is transmitted to the human eye and can be recorded on the light-sensitive surface of photographic film.
In 1901, Planck’s quantum theory, supplemented by Einstein in 1905, explained certain properties of light on the basis of the particle theory. These particles are called photons, and the quantity of energy they contain is called a quantum of light. Today it is accepted that light behaves both as an electromagnetic wave and as a particle. The difference is that its particle nature shows at short wavelengths, while its wave nature shows at long ones. Electromagnetic waves travel in a straight line at a speed of 300,000 km/sec in a vacuum. This speed, however, also depends on the medium through which the wave travels. This means that the speed of a wave equals its speed in a vacuum divided by the refractive index of the medium; for example, the refractive index of crystal is 1.5, while that of water is 1.35. Applying this relation, the speed of light in crystal works out at 200,000 km/sec, and in water at 225,000 km/sec.
2. Exposing the light-sensitive surface
Film is exposed to light by means of the shutter and the aperture. The aperture regulates the amount of light that reaches the film, and the shutter regulates the length of time that this amount of light is allowed to act on the film. Light, which is obviously the basis of photography since it creates the image, has three main characteristics:
– quantity
– quality
– direction
a. The quantity of light must be sufficient to allow the image to be recorded on the film. The exposure time must therefore be determined as accurately as possible, because the final quality of the image depends to a large extent on it.
b. The quality of the lighting governs the smooth gradation of tones between the light and shadow areas of the image. A balance of tones thus produces an image that is harmonious, pleasing and, in the end, useful.
c. The direction of the light emphasises the shape of the object and, with the help of shadows, records its relief in the image. To make proper use of the direction of the lighting, one must know the basic effects produced by each lighting direction.
2.1 The basic lighting directions
2.1.1 Front lighting
When the light falls on the object from the direction of the shooting position, the object will look flat in the photograph, without any shadow to show its relief. By removing the shadows, front lighting removes the liveliness of the subject. For this reason, this type of lighting is recommended for photographing flat objects.
2.1.2 Opposing lighting (shooting against the light)
This lighting direction is the complete opposite of the previous one. The main light source is behind the subject and directs light towards the camera, thus creating a halo around the object. A small supplementary source is usually positioned to light the front of the image. This lighting is suited mainly to silhouette subjects, atmospheric landscapes, or the photography of transparent objects (e.g. crystal).
2.1.3 Side lighting
With side lighting, the light is directed at the object perpendicular to the shooting axis. An object photographed with side lighting shows strong differences between lit and unlit areas. The result of this strong contrast is a different effect in the image, which can often be undesirable.
2.1.4 Three-quarter lighting
The light source is slightly higher than the object and on the camera side, so that it lights more of the subject than side lighting does. This lighting emphasises relief, the shadows become soft, and their direction helps produce a dynamic result. Three-quarter lighting is suitable for landscapes, portraits and, above all, architectural works. It is perhaps the most suitable lighting for photogrammetric shots of objects, outdoors or indoors.
2.1.5 Overhead lighting
The main light source is directly above the object and produces heavily emphasised shadows with no detail. Photographing from this direction should generally be avoided, because the images are of little interest and the loss of detail is very great.
2.1.6 Lighting from below
The main light source is directly beneath the object. This lighting looks unnatural, because we are used to seeing light always coming from above the subject. It is the worst lighting for the metric use of the shots.
3. Light sources
There are two basic kinds of lighting: natural and artificial. Natural lighting is, of course, the sun. Most photographs are taken during the day. Nevertheless, photographing with the sun poses quite a few problems, mainly for objects that cannot be moved (buildings, landscapes etc.), since the sun’s path changes from season to season and the day is shorter in winter. The intensity also fluctuates constantly when there are even small clouds. So when the photography has to be done at a particular time of day, the time lost in preparation is precious.
3.1 Colour temperature
The variation in the colour of the light radiated by different light sources depends, as is well known, on their temperature. Recall that when a body is heated, it radiates mainly red; heated further, it radiates white; and finally, at very high temperature, it radiates blue. On this principle, temperature values are assigned to the various bodies that act as light sources. For colour temperature, as this phenomenon is called, the absolute temperature scale is used, with degrees Kelvin (°K) as the unit of measurement. Recall that 0 °K = -273 °C. Colour temperature is a very important factor in choosing the right film and filters for photography. The human eye adapts so easily that it perceives almost any kind of lighting, natural or artificial, even that of a candle, as white. Light-sensitive surfaces, by contrast, do not have this ability. So, for example, a film made for daylight (5500 °K) will give a reddish cast to a photograph taken by the light of a candle (1800 °K). As can be seen, colour temperature is inversely related to what has come to be regarded as a warm or cool colour. Colour temperatures must be measured accurately before photographing, especially in difficult lighting conditions. For this purpose, special instruments called pyrometers or, more commonly, Kelvin meters are used. The choice of the right film and filter for correct rendering is the subject of a later chapter (see Ch. 9 and 12).
3.2 Types of artificial lighting
Artificial light sources, however, can overcome these difficulties. They come from luminaires that can provide diffuse or directional lighting. Luminaires for diffuse lighting may have more than one lamp, placed inside reflectors so that the light is better concentrated and directed. They may also be covered at the front by a semi-transparent surface such as Plexiglas, frosted glass etc., to reduce contrast and render detail better. The simplest method is to use a reflector that directs the light from the main source where it is wanted. The ceiling, the nearest wall, a projection screen, a photographic umbrella or even a sheet of white paper are some of the reflectors that will produce diffuse lighting. Directional lighting is provided by luminaires known as spots. In front of the lamp there is a powerful condenser lens and behind it a concave mirror. The distance between the lamp and the lens can be varied, so that the diameter of the emerging beam changes accordingly. The condenser lens of spots is a Fresnel lens. Today there is a wide choice of artificial light sources, the most important of which are:
3.2.1 Fluorescent lamps for general diffuse lighting. In this case a special filter must be used to balance the colour temperature of the light.
3.2.2 Photoflood lamps, mainly in two power ratings, 250 and 500 Watt. The colour temperature of the light they emit is 3200 K – 3400 K. The lifespan of the lamps is limited.
3.2.3 Quartz-iodine lamps, from 150 to 2000 Watt, 3200 K – 3400 K. The lamps of these luminaires must on no account come into contact with bare hands.
3.2.4 Flash. Often the light available in the spaces to be photographed is not satisfactory in quantity or quality to expose the film. In such cases, flash is the ideal solution. Flash must be used with great care, since the light it produces is of very high intensity, causing strong differences between light and dark areas. Another point
to be taken into account when using flash is the synchronisation of the burst of light with the opening of the shutter. In general, flash should be avoided for metric shots.
4. Light meters
To achieve a good, correct photograph, you must first of all obtain a good negative. How good the negative is depends on the quantity and quality of the light that falls on it. If, when the photograph is taken, the light striking the film is much less than it should be, the negative will be underexposed (thin) and the detail in the dark parts of the image will be lost. If, on the other hand, the light striking the film is much more than it should be, the negative will be too dense (overexposed), and the detail in the light parts will have disappeared. For aesthetic reasons, and not only those, there must be a balance between the light and shadow parts of the image, just as there are intermediate tones in the scale. Certain areas of the subject may even be overemphasised or downplayed, or may appear as silhouettes, or a lot of detail in the background may be lost. To achieve all this, exposure has to be strictly controlled. Correct control of the lighting is achieved only by using a light meter.
A light meter is an instrument that measures light and nothing else. It consists of the photocell, the microammeter and the system that converts the needle readings into exposure combinations. The photocell is the basis of the light meter and converts the light falling on it into electric current.
Light meters are classified, according to the type of photocell they use, into selenium, cadmium sulphide and silicon meters. Meters that use a selenium cell can convert the light they receive into electrical energy. They do not adapt easily to strong fluctuations in lighting. Silicon meters combine all the main characteristics of selenium and cadmium meters, with high cost as their only drawback.
The microammeter is the instrument that measures the intensity of electric current. This measurement is expressed by the movement of the needle on a suitable scale or, in modern light meters, by a digital display. The greater the luminance of the subject, the stronger the electric current created by the photocell, whatever its type.
Finally, the system that converts the needle readings into exposure combinations helps the user choose the elements of a correct exposure. This conversion is achieved by combining two discs, which list the various apertures and shutter speeds.
Manufacturers determine experimentally, for each point indicated by the needle, the corresponding aperture and speed.
Light meters have two measuring scales, one for medium lighting and one for low. Readings are indicated either by the microammeter needle or digitally. The first step in operating the light meter is to set the film speed (see Ch. 9). On handheld meters the needle must always be zeroed. On cadmium meters you must check whether the battery that powers them has run down. The meter scales that relate each intensity number to shutter speeds and apertures are, broadly, designed for a negative that, with a normal printing method, gives a photograph with a medium grey tone whose value corresponds to 18% reflectance. The exact sample for this reflectance is the so-called grey card, made by Kodak.
4.1 Types of light meter
Handheld meters are divided, according to the method by which they measure light, into reflected-light meters and incident-light meters. Reflected-light metering is done from the camera position, by pointing the meter’s photocell at the subject being photographed. The metering angle in this case is approximately 30°. For greater exposure accuracy there is another category of meters, the so-called spot meters. Their angle of view is usually variable from 1° to 15°.
When measuring reflected light, the following may be considered:
a) metering of the general lighting
b) metering based on the shadow areas of the subject, when they predominate
c) metering based on the highlights of the subject, when they predominate
d) average luminance metering, when the contrast between the highlights and the shadow areas is very high.
Incident-light metering is done very close to the subject, by measuring the light that falls on it from the light source, with the photocell pointing towards the camera. Usually there is a plastic, semi-transparent, hemispherical dome in front of the photocell, with a 180° angle of view. This cover reduces the intensity of the radiation so that it equals that of the reflected light, and it has the property of diffusing the incident light. Light meter manufacturers have designed a range of additional accessories, for different uses, for certain meters. For example, metering on the focusing screen for aerial photography, for the microscope, metering for flash etc.
The meters built into cameras are reflected-light meters and fall into two categories:
a) Meters mounted externally on viewfinder cameras, with the meter’s angle of view set to match that of the lens, and
b) Meters located inside the camera that take a reading through the lens (TTL – through the lens), so that the type of lens being used is also taken into account.
In this second case there are two metering systems:
i) Average, in which the reading is taken from the whole frame of the photograph
ii) Spot or narrow-beam, in which the reading is taken only from a very small circular area at the centre of the focusing screen.
With through-the-lens meters, when filters are used, there is obviously no need to take into account the factors of any filters that may be in use.
Source: NATIONAL TECHNICAL UNIVERSITY OF ATHENS
SCHOOL OF RURAL AND SURVEYING ENGINEERING
DEPARTMENT OF SURVEYING
PHOTOGRAMMETRY LABORATORY
ELEMENTS OF
PHOTOGRAPHIC TECHNIQUE
NOTES
A. Georgopoulos, Professor, NTUA
R. Chliverou, Specialist Photographer, TEI
