Four variables are involved in capturing an image photographically: Shutter Speed, Lens Aperture, ISO, and Illumination.
Some instructors believe that students should not be exposed to all four of these variables in the initial phases of their photographic journey. Some will present shutter speed and aperture alone, leaving ISO and Illumination for later. Others will present shutter speed, aperture and ISO, as the three variables controlled by the camera, leaving illumination for later. Some feel that, since illumination is not controlled by the camera, it should not be included in exposure discussions. From a somewhat broader perspective, illumination can be considered the sine qua non of photography. Without illumination, there can be no photography.
There is no single teaching method that is right or wrong, or better than others. The approach that works for an individual instructor, leading to a firm grasp of exposure by the student, is the one that should be employed.
The following article provides a description of the four variables and their relationship to each other.
Section I provides a definition and brief discussion of the variables.
Section II discusses the four variables and presents a non-numeric chart that describes their interrelationships.
Section III explores the arithmetic used to calculate the variables for reflected illumination metering systems: Given any three of the variables and their numeric values, the fourth can be determined. This section also includes a brief discussion of the equation required for use with incident exposure meters.
Section I: Basic Definitions
Shutter Speed is the time that the shutter is open, exposing the film or digital image sensor to light passing through the lens. Shutter speed may range from very short intervals (1/8000 of a second: 0.000125 second) controlled by the camera, to several minutes or more using manually controlled time exposures. Slower shutter speeds are frequently used in static situations, where the subject is stationary and illumination is adequate: portraiture, landscapes etc. Faster shutter speeds are used for freezing subjects in time: sporting events, racing, etc.
Lens Aperture is a variable opening in the camera that lens determines the intensity of the illumination that passes through the lens on its way to the film or digital sensor while the shutter is open. Large apertures (small f/number, f/1.4 for example) will permit a greater intensity of illumination to pass through the lens (think brighter image) and a shallow depth of field, while smaller apertures (f/11.0 for example) will allow proportionally lower intensity of illumination to pass through the lens (think dimmer image) and a greater depth of field.
ISO (International Standards Organization) indicates the relative speed at which the film or digital sensor can absorb enough of the photons of light passing through the lens while the shutter is open to render an acceptable exposure on the film or digital sensor. Lower ISO numbers (100 for example) are considered slower, as they require more photons to capture the image, resulting in higher image quality. Higher ISO numbers (6,400 for example) can capture an image with considerably less photons, resulting in lower image quality.
Illumination consists of photons of light in the visible portion of the electromagnetic spectrum that illuminates the subject. It may be ambient (sunlight, indoor lighting, etc.), or it may be supplemental (studio lights, on-camera speedlights, etc.), or a combination of ambient and supplemental (using fill flash to illuminate the shadow side of a subject standing in the shade outdoors, for example). Illumination reflected off the subject, passing through the lens and shutter and striking the film or digital sensor, creates the image.
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