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.
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.
Section III: Numeric Relationship between Exposure Variables.
The numerical relationships between the four exposure variables: Aperture, Illumination, ISO, and Shutter Speed. are expressed in a simple equation that can be used for reflected light metering systems in Nikon and other cameras. For incident metering systems, a slightly different version of the equation is used, that involves a different metering constant. Variations of this equation are used by cameras when they are configured in their various automatic modes.
The description below explores the following reflected light version of the equation.
N2/T = LS/K
N = numerical aperture: f/stop (e.g., 8.0)
T = time in seconds (e.g., 0.500)
L = Luminance/Illumination in Candela per Meter Squared (e.g., 64)
S = ISO (e.g., 25).
K = Reflected Meter Constant (e.g., 12.5). Commonly used by camera and exposure meter manufacturers.
Enter any three of the N, T, L, or S values, and the Excel spreadsheet below calculates the fourth value. The metering constant K, remains unchanged so long as a reflected metering system common in most cameras is used.
In this Shutter Priority example above, the photographer selects the desired Shutter Time of 1/2 second (0.5000 second) and ISO (25). When the shutter opens, the camera meters the illumination (assumed to be 64 cd/m2 for this example), then calculates the aperture required (F/8.0) to achieve an appropriate exposure at ISO of 25. The (checking) function was included in the Excel spreadsheet to assure that the equation balances: (8.0X8.0/0.5 = 64x25/12.5: 128 = 128) for this example.
In this Aperture Priority example, the photographer selects the Aperture (f/8.0) and ISO (25). When the shutter opens, the camera meters the illumination (assumed to be 64 cd/m2) for this example, then calculates the Exposure Time (0.500 seconds) to achieve an appropriate exposure at ISO of 25.
In this Auto ISO example, the photographer is working under an assumed Illumination of 64 cd/m2 and selects an Aperture of f/8.00, and an Exposure Time of 1/2 seconds (0.5000 seconds). When the shutter opens, the camera meters the Illumination and calculates the ISO required (25) to achieve an appropriate exposure at the given Aperture and Exposure Time.
In this example, the photographer wants to shoot at ISO of 25 to achieve the greatest image quality, an Aperture of f/8.0 to obtain the desired depth of field, and an exposure time of 1/2 second (0.5000 seconds) to achieve a slight blurring of a slowly moving object in a studio setting with adjustable studio strobes for illumination. The required illumination can be obtained through time consuming trial and error, adjusting the strobes until it is achieved. Or, to save time, the equation above can be used to derive a Luminance value of 64 cd/m2. With this value in hand, and using an exposure meter that can wirelessly trigger the strobes, the photographer can quickly set them to the required value and fine tune as needed.
The photographer wants to shoot at 1/500 second (0.002 second) to stop the action of the moving subject, while retaining the f/8.0 Aperture under the ambient illumination of 64 cd/m2. The equation indicates that an ISO of 6250 is required to achieve an appropriate exposure. In this case, the closest ISO value of 6400 would be selected, as most cameras do not permit non-standard ISO values.
For those wishing to create their own spreadsheets, the equations for each of the four variables in the Unknown column are shown below.
Incident Illumination Metering Systems
Studio photographers frequently use incident illumination exposure meters for complex setups involving studio strobes. Incident metering systems measure the illumination falling on the subject instead of the illumination reflected from it. One advantage of this approach is that the meter is not affected by subjects with a wide dynamic range of very dark and very bright components that can throw reflected illumination metering systems off.
Reflected illumination metering systems will select exposure variables that will render the subject in a neutral shade. Bright, sunlit snow scenes, for example, will cause reflected metering systems to under-expose while dark, shadowy scenes will be overexposed. Wedding photographers faced with a bride in a very bright white dress standing next to a groom in a very dark tuxedo will often resort to incident illumination metering.
A similar equation is used for incident illumination metering:
N2/T = ES/C
N = Aperture
T = Time
E = Exposure Value (EV)
S = ISO Speed
C = Metering Constant for incident illumination, instead of K, used for reflected illumination.
The Sekonic L758DR exposure meter, for example, permits incident metering and reflected illumination metering. Three separate metering constant values are used:
K = 12.5 When the meter is used to measure illumination reflected from a subject.
C = 340 when the Lumisphere dome is extended for measuring three dimensional setups, as in studio, outdoor nature and other similar situations.
C= 250 when the Lumisphere dome is retracted when measuring illumination for paintings, documents, and other flat subjects.
A number of different units are used in various illumination metering systems. Among them are Candelas per Square Meter, Lumens, Lux, and Exposure Values (EV).
The Candela is based on the illumination provided by a standard candle over a one square meter area. The Foot-Candle –now obsolete- was based on the illumination provided by a standard candle over a one square foot area.
Lumen (Luminous Flux) is the measure of the total quantity of visible light (luminous energy) emitted by a source per unit of time. A light source radiating one Candela spherically and uniformly in all directions has a total luminous flux of approximately 12.57 lumens.
Lux is a measure of the luminous power falling on a surface, and is measured in lumens per square meter.
Exposure Value (EV) represents a combination of a camera's shutter speed and aperture. All combinations that produce the same exposure will have the same EV value. EV 0 (zero) is defined as 1.0 second shutter time at an aperture of F/1.0. Each increment of 1.0 in EV equates to one half as much exposure (one stop), which can be achieved by cutting the shutter time in half or stopping the lens aperture down by one full stop. Tables of EV values relating shutter and aperture combinations from EV -6 to EV 21 are readily available on the web.
Exposure meters like the Sekonic L758DR, and others, permit the user to select the desired units when making incident illumination measurements.
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