No Light = No Picture. This is a basic rule which, unfortunately, many photographers have not learned. By “exposure” we are referring to the amount of light which strikes the film while taking a picture. In order to get an acceptable picture, the film must be properly exposed.
There are three factors involved in exposure:
- The sensitivity of the film.
- The amount of light which strikes the film.
- The length of time to which the film is exposed to light.
Proper exposure requires that the correct volume of light strikes the film for the right amount of time, as required according to the designated sensitivity of the film.
Film sensitivity is indicated by its “speed.” Film speed is determined by the International Standards Organization and is listed on the film package as an “ISO” rating – 100, 200, 400, 800, etc. Films with a lower number are called “slow” films while higher numbered films are considered to be “fast” films.
There is a definite mathematical relationship in film speed ratings: ISO 200 film is exactly twice as sensitive as ISO 100. Therefore, 100 speed film requires twice as much light for proper exposure as 200 speed; 200 requires twice as much light as 400; 400 requires twice as much light as 800, etc.
Slow films are generally finer-grained than fast films and are excellent for brightly-lighted subjects, such as sunlit scenes. (“grain” refers to clumps of the light-gathering sensitive material used in film emulsions). These fine-grained films are a good choice for large enlargements because the grain pattern does not show up in the finished photographs.
Fast films have larger clumps of grain and therefore are more sensitive to light than slow films. Fast films are a good choice for low-light situations or action photos where a fast shutter speed is required. However, when enlarged the grain pattern may appear on the finished picture.
The amount of light which strikes the film is determined by the diameter of the opening in the lens. This is called the “aperture.” A large aperture lets in a large quantity of light and a small aperture lets in small amounts.
The size of the aperture is indicated on the lens as the “f/stop.” The f/stop is the relationship between the diameter of the lens opening and the focal length of the lens. This relationship is usually engraved on the lens as a reciprocal. If the diameter of the lens is one-half the focal length of the lens, the f/stop would be indicated as “f/2.” There is a definite mathematical relationship between the f/stops indicated on a lens. This is based on a logarithmic scale. Some standard lens apertures are f/2, f/2.8. f/4, f/5.6, f/8, f/11, f/16, f/22, and so forth through the logarithmic scale. A higher number indicates a smaller aperture and a smaller number indicates a larger aperture – f/2 lets in twice as much light as f/2.8; f/2.8 lets in twice as much as f/4; f/4 twice as much as f.5.6; f/5.6 twice as much as f/8; f/8 twice as much as f/11, etc.
Another function of the lens aperture is control of the area of acceptable sharpness, commonly referred to as “depth-of-field.” Smaller apertures (but larger numbers on the lens – remember, it’s a reciprocal) increase depth-of-field, while larger apertures (but smaller numbers on the lens) decrease the area of acceptable sharpness. Not all cameras allow the photographer to adjust the size of the aperture, in which case you must accept a fixed depth of field, usually from about 4 feet to infinity. With a fixed aperture you also cannot control the amount of light entering the camera – it is pre-set.
The length of time to which the film is exposed to light is controlled by the shutter-speed. The shutter is the device which opens and closes to allow light to enter the camera or to prevent light from entering. Shutter speeds are indicated in fractions of a second and are indicated on the camera as reciprocals. A shutter speed marked as “500” on the shutter speed indicator means 1/500 of a second. There is a two-to-one mathematical relationship between shutter speeds. The most common speeds are 1/30, 1/60/ 1/125, 1/250, 1/500, 1/1000, and 1/2000. Each of these allows light to strike the film for one-half the time of the previous speed, respectively. Some lower-priced cameras have only one shutter speed which cannot be adjusted.
How correct exposure is determined.
The light meter in your camera is designed with the assumption that an “average” scene reflects 18% of the light falling on it. Your light meter reads the total amount of reflected light, then computes the exposure based on 18% reflectance. When your light meter is programmed with the correct ISO speed rating for the film you are using, the 18% reflectance rule will give the correct exposure for an “average” scene, such as a front-lighted scenic. This will give a good exposure most of the time, but definitely not all of the time.
Many of the subjects of your photos, especially people, may not be “average” scenes. We will discuss each of these “exceptions” in separate articles.