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7.1 – Basic Exposure

Exposure directly effects the light needed to make a picture. In fact the word "photography" means to write with light. It is through knowledge of exposure that is needed to make good photographs. Exposure is a term that refers to the amount of light that is allowed to make contact with a light sensitive surface. Stated differently it is the response of a photographic emulsion to light.

Because every photograph represents a different problem in lighting it is necessary to study the various lighting conditions as they pertain to correct exposure. Every time an exposure is made a certain intensity of light enters the camera for a selected period of time. Exposure combines the intensity of light and the time it is permitted to act on the film is the definition of exposure.

(Intensity of light x Time = Exposure)

One of the photographer's primary concerns in operating any camera is setting exposure controls. The two chief methods of calculating exposure are (1) by visual estimation and (2) use of an exposure meter. The following information will tell what exposure is, how it is controlled and a simple method of estimating exposure. Later more information will be given on the exposure or photoelectric meter.

Emulsions are made of light sensitive materials called halides (microscopic particles of silver bromide, iodide, fluoride or chloride) which when struck by light form a "latent" (invisible) image. The developer changes this latent image to a visible image, which is composed of black metallic silver. The amount of silver darkened in the image depends upon how much Light struck the emulsion and to what degree the emulsion is sensitive to light.

Films vary greatly in their sensitivity to light, but each is sensitive over a fairly short range. This means that when a picture is taken the photographer must precisely control the amount of light that will enter the camera. If too much exposure is given, then the negative image will be too dark; conversely, too little exposure will result in a negative image that is too light.

The image on film is made darker where it receives more light and the light areas on the film are the result of little or no light reaching it. Thus, the lighter parts of the scene (which reflects more light) will turn the corresponding areas of the film dark. The darker areas of the scene reflect less light and cause a light area on the film (negative).

On the negative, the light (or thin) areas which received little light are called the shadow areas. The darker parts (dense) areas are called highlights. Remember that on a negative the black and white are reversed. In the original scene if the subject is wearing a white shirt then on the negative the shirt will appear black.

The average negative should contain both highlights and shadows and a full range of tones in between. Its overall darkness is referred to as its density and the degree of difference between the densest areas and the least dense areas is known as the contrast of the negative. The latitude of the film is the ability of film to properly record the lightest and darkest areas of the subject.

The amount of exposure that film receives is determined by the (1) intensity (strength) of the light and by (2 ) the exposure duration (how long the light strikes the film). The more of each the film receives, the more it is exposed. The camera contains an aperture to regulate the intensity of the light passing through the lens, and there is also a shutter to control the duration (time) of the exposure. Keep in mind that there are only two controls on the camera that regulates the amount of light for exposure. They are the f/stops and shutter speeds.

The aperture is an opening through which the light must pass. The larger the opening is made, the more light that will pass through, just as more water will flow through a larger pipe than a smaller one. In dark lighted situations, the aperture is made larger (opened up) to pass more light. In bright light the aperture is made smaller (stopped down) to prevent overexposure.

The aperture sizes are indicated on the lens mount by a series of numbers called f/stops. The most common f/stop numbers are f: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, and 22. The larger the f number, the smaller the opening. Some older cameras may have different f-numbers. The difference from one f/stop to the next one (when opening up the aperture) is two times (2x change in the amount of light it will transmit. For example, f/8 will pass two times, (2x or twice) as much light as f/11. By the same fashion, when "stopping down" from an f number of 11 to 16 the new opening lets in ½ as much light. The f numbers can be very deceiving the number 16 is larger than 11 but only lets in ½ as much light. For example 4 is larger than 2 until these numbers are related to fractions, what would be larger ½ or ¼ of a dollar.

Thus, a change of one "stop" either cuts the light in half (when stopped down) or transmits twice as much (when opened up one stop). A change of two stops would make a 4x change of light (2x2) and 3 stops would be an 8x-difference (2x2x2), and so on. On most cameras, the aperture may be half way between two f-stops such as 3.4 is half way between 2.8 and 4.

The camera must also contain a timing device, called a shutter, that controls the length of time light will strike the film. It is made of a spring operated set of metal blades. When opened, they allow light to pass and when closed block entry of all light. During exposure they remain open for a reselected amount of time. The time is marked on the lens dial.

Most shutter speeds are listed by such markings as: T, B, 1, 1, 3, 8, 15, 30, 60, 125, 250, 500 and 1000. The T and B settings permit the shutter to be held open for time exposures, while the other numbers represent fractions of seconds. For example "60" means that the shutter will remain open for 1/60th of a second. Some older cameras may have different shutter speeds.

The same relationship between shutter speeds exists as between f/stops. That is, each change from one shutter speed to the next results in a change of 2x. 1/60 second is twice as long as 1/125 and 1/250 give 1/2 as much exposure as 1/125. A change of one setting to the next on the shutter produces a change of exposure of one stop, as does the change of one f/stop of the aperture. On most cameras never set the shutter speed half way between two settings (example 1/60 and 1/125). The shutter is not made for 1/2 stops.

With such a range of f/stops and shutter speeds to choose from, the photographer may wonder how to select the proper combination to obtain correct exposure. Usually a photoelectric device called an exposure meter is used to tell the correct exposure. At other times the photographer can rely on their own experience under similar situations, or another possibility is to make a visual estimation of the proper exposure. To make such a visual estimation the following variables must be considered: (1) film sensitivity, (2) light intensity, (3) light direction, and (4) subject brightness.

Films differ in several respects. The major difference with film is film speed or sensitivity to light. Films are roughly classified as being slow, medium, and fast. The fast film requires less light to produce the same density, as do the slower types. Films are assigned a numerical rating known as their exposure index (ISO) rating. A slow speed film might be rated as ISO 10, a medium speed around ISO 100, and fast films might be ISO 1200. Those numbers are directly proportional, meaning that a film rating ISO 100 is twice as light sensitive (fast) as one of ISO 50 and four times faster than one rated ISO 25. Incidentally "ISO" was called ASA which referred to the American Standards Association, an agency that tested films and assigns them their speed ratings. The new standard is the ISO, International Standards Association.

Another factor that affects exposure selection is that of intensity. This is a term that refers to the strength of the light. Photographers have to make pictures under widely varying conditions of light intensity. For now we will consider only those of daylight conditions which are:

1. Bright sun lighting where the sun is not obscured by clouds and which casts distinct shadows.
2. Hazy sun lighting is where the sun is partially covered by thin clouds, which leaves weak shadows. Such lighting will necessitate one-half stop of exposure increase.
3. Cloudy bright lighting is where the sun is barely visible in the clouds, very weak shadows, requires one full stop of exposure increase.
4. Cloudy dull lighting is where the sun is so obscured that its location cannot be made, no shadows are present. This condition requires from 2 to 3 stops exposure increase. Shooting in the open shade of building is a comparable situation.

Another factor, which we must consider, is the light direction. For side lighted (sun at 90 degrees to the surface being photographed) the exposure should be increased by one stop. For backlighted (sun behind the subject) subjects, the exposure should be increased 2 stops, i.e., if detail is wanted in the surface facing the lens. If the subject is looking directly into the sun, it will cause them to squint.

When light strikes an object, the object absorbs part of the light and part of it is reflected. The image on the film is formed by the light that is reflected and the amount any object reflects varies with its brightness. In order to record proper detail on the film, a bright subject would require that the exposure must be "stopped down" (give less exposure), and for a dark subject, "opened up" (give more exposure). Subjects vary in brightness types as follows:

1. Average subject - An average subject is one, which reflects 18 per cent of the light striking it. That is, an average gray. Probably most subjects will fall into this classification--people in medium color clothing, most buildings, landscapes with trees, etc.
2. Dark subject - A dark subject reflects only about 9 per cent of the light. Thus, it is only half as bright as an average subject is so it requires a 2x increase in exposure (1 f/stop). For a dark subject open up one stop. Examples are dark buildings, dark colored scenes, and people in dark clothing.
3. Bright subject - A bright subject reflects twice as much light as an average subject and requires one stop less exposure. Such a subject might be fair skinned people, light clothing, light colored buildings, etc.

The following all let through the same amount of light. Which exposures to choose is determined by what the photographer is concerned with a large or small depth of field or slow (blur) or stop motion.

Speed f DOF SHUTTER

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1/60 22 great slow chance of blur

1/250 11 medium fairly fast to stop action

1/1000 5.6 small very fast to stop action

The photographer could choose any of these and be right. The chance for camera movement is greater at the slower shutter speeds. The large f setting would offer a small depth of field. The photographer must decide on their priority.

The hardest and most critical skill a beginning photographer must learn is how to estimate exposure. Some examples of the use of exposure estimation are: changing films, using a different film speed, light transitions from bright to shadow to inside, digital cameras, and automatic and manual cameras. All of these instances require the photographer to be smarter than the equipment, estimate lighting, know their equipment, and know their film.

The general rule is to set the shutter at the films ISO and set the f at 16. This is for outside shooting in normal daylight. An example would be using ISO 100 film, set the shutter at 1/125 (this is the closest to 1/100), the f setting at f16 and expose. If the camera gives different readings then the photographer must consider the possibility that the battery is going bad, the lightmeter is malfunctioning, or the camera is not working properly.

Most all photographers do not make a lot of changes in equipment. Once that they are used to the feel of a piece of equipment and purchase the necessary accessories then they don’t want to change and can’t afford to. Once the photographer is used to a particular film, they know what to expect in terms of grain structure, sharpness, and its latitude and reliability. The new photographer must learn to estimate exposure in a variety of lighting situations with positive results to gain confidence in themselves, the film and the equipment. Automatic cameras require more skill to use and override than the cameras of the past.

The main purpose of photography is to strive for a "normal" print one that has a full range of tones that matches the subject. The following project will help you understand how exposure and development effect the photograph. Below are listed ten facts that you will be expected to know and will be tested on.

1. Under exposure creates a lack of shadow detail.
2. Over exposure creates dense shadow areas.
3. Underdevelopment shows detail in a thin negative.
4. Overdevelopment builds contrast; that is, the highlights get darker and the shadows remain the same.
5. Exposure builds density.
6. Over-exposure and normal development loses detail in the highlights.
7. Over exposure and underdevelopment flattens contrast.
8. Underexposure and over development will increase contrast.
9. Underexposure and underdevelopment produces a thin, unusable negative.
10. Overexposure and over development produces a dense, usable negative.

The highlight and shadow areas noted above refer to a negative image, the terms would be reversed when speaking of a positive image.

DAYLIGHT EXPOSURE CHART

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