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.
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