Section II: Discussion
The four exposure variables may exist in seemingly endless combinations, as a function of the conditions confronting the photographer. Some of the conditions may be under the photographer's control, shutter speed, aperture and ISO, for example. Illumination may be completely beyond the photographer/s control, shooting outdoors with cloud cover changing rapidly for example. This forces the photographer to select different combinations of shutter speed, aperture, and ISO to adapt to the changing illumination. In some situations, photographers may elect to add supplemental illumination to overcome exposure difficulties presented by dim or changing illumination. Studio photographers, on the other hand, have control over all four variables and can configure them precisely to achieve the desired exposures.
Most cameras today can be operated in a number of automatic modes, whereby the photographer selects one or more of the camera controlled variables, and the camera calculates the value for the remaining variables. Examples include:
Shutter Priority (S): In this mode, shutter speed is more important than aperture. The photographer selects an ISO value appropriate for the situation, then selects the desired shutter speed, and places the camera in the shutter preferred mode, where the camera is forced to use the shutter speed selected by the photographer. When the shutter is opened, the camera measures the intensity of the illumination coming through the lens and calculates an aperture that will provide the desired exposure. Shutter preferred mode is often used to maintain a shutter speed necessary to capture a rapidly moving subject.
Aperture Priority (A): In this mode, aperture is more important than shutter speed. The photographer selects an appropriate ISO value, then selects the desired aperture and places the camera in the aperture preferred mode. When the shutter is opened, the camera measures the intensity of the illumination coming through the lens and calculates a shutter speed that will provide the desired exposure. Aperture preferred mode is often used when the desired depth of field (DOF) provided by the selected aperture is to be maintained. It can also be used to keep the subject in sharp focus, while blurring the background for artistic effects.
Manual Mode (M): For unusual situations, photographers may elect to shoot in total manual mode, where they will select three, and possibly all four, variables. When working with ambient illumination, they will select Shutter Speed, Aperture, and ISO. In studio or other environments where supplemental illumination is available, they will select the power settings to achieve the desired effect.
Programmed Auto (P): There are several variants of this mode. The Nikon D800e, for example, includes a Programmed Auto Mode (P), where the photographer is willing to let the camera make the Shutter Speed and Aperture decisions under the given illumination and ISO values.
Cameras capable of this mode will usually provide a chart in the instruction manual indicating the range of values that will be used in determining exposure. When the shutter is opened, the camera's metering system determines the illumination level, and then selects a shutter speed and aperture that will produce an acceptably exposed image.
Automatic ISO: In this mode, the photographer selects a desired shutter speed and aperture, and is willing to let the camera select the ISO required for the desired exposure. When the shutter is opened, the camera's metering system evaluates the reflected illumination passing through the lens and selects an appropriate ISO value that renders the desired exposure. The mode would be used when the photographer is working in dim illumination that may be changing rapidly, while shooting moving subjects (faster shutter speed) with a desired depth of field (selected aperture).
Photographers just beginning their journey may have difficulty understanding the relationship between the four exposure variables: shutter speed, aperture, ISO, and illumination, and the various combinations and tradeoffs that can be made in producing an acceptable exposure. To simplify, Chart One, below, provides a simple, non-numeric method for understanding the four variables and their relationships.
To use this chart, select one of the four variables in the "Selected Variable" column, and one of its two values in the "Value" column. Then select one of the three remaining variables under the heading "Relationship to Other Variables" you wish to evaluate. Finally, assume that the two remaining variables will remain constant.
The chart and examples below compare Aperture with Illumination, ISO, and Shutter Speed.
Aperture and Illumination Relationship
For a Small lens Aperture (large f/number), at a given ISO and Shutter Speed, a brighter source of Illumination is required. For a Large lens Aperture (small f/number) at a given ISO and Shutter Speed, a dimmer source of Illumination is required.
Aperture and ISO Relationship
For a Small lens Aperture (large f/number), at a given Illumination and Shutter Speed, a faster ISO is required. For a Large Aperture, at a given Illumination and Shutter Speed, a Slow ISO is required.
Aperture and Shutter Speed Relationship
For a Small lens Aperture (larger f/number), at a given Illumination and ISO, a slower Shutter Speed is required. For a Large Aperture, at a given Illumination and ISO, a faster Shutter Speed is required.
Similar relationships can be explored by selecting the other exposure variables in the left column: Illumination, ISO, and Shutter Speed.
Those interested only in this non-numeric exploration of the relationship between the four exposure variables can stop reading here. For those interested in pursuing the numeric details, continue on to Section III, below.
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