Modern Photography/Exposure controls

Photography is the act of writing with light. A light-sensitive surface is exposed to light coming from the scene through the camera lens. As modern methods allow for this to be done in ever-smaller fractions of a second or over multiple days, we must control exposure—how much light enters the camera.

In order to get a perfectly-exposed image (one that has all of the desired levels of gray from pure dark to pure white) a photographer must balance and coordinate three settings—aperture, shutter speed, and sensitivity—against the scene to be shot.

Concepts

Level, tone, value

These terms are used interchangeably to describe how light or dark a region of a picture may be. For printed images, the lightest tone available is usually that of the bare paper on which the print is made, and less often a specific white pigment used in some printing methods; the darkest tone results from being fully saturated with black ink/pigment/dye/toner. For images viewed on a screen, the lightest tone occurs at full illumination, and the darkest tone with emitted light being completely obscured, or no light being emitted.

We aim to keep all visible details within the range of tones we can reproduce in print or screen—dynamic range. This range is *much* smaller than the range of tones we encounter in the real world—from full sunlight to complete darkness. Our eyes adjust to changing lighting conditions, instantly and involuntarily. Meanwhile, our cameras must measure more carefully, and determine what exposure settings to apply in response. However, cameras can still operate in conditions that humans would find blinding or too dark, and images can be processed in a way that exceeds the dynamic range normally available.

Contrast

Contrast is the overall difference between light and dark areas. Human vision is more sensitive to changes in brightness than changes in color. So it is that contrast in an image provides clarity through high-contrast edges, and impact through contrast between regions of an image. Images with low contrast can appear dull and uninteresting, or may even be difficult to comprehend. Extreme contrast—where most of the tones are pure white or black—is generally avoided for many images, but can prove very striking, and is also the primary aim of written text.

Key

Key is the predominance of tones in a scene, with "high key" referring to bright tones, and "low key" for dim tones. By necessity, a shot taken in broad daylight will often be high-key, while a shot taken in moonlight will be low-key. The subject may yet be properly exposed with a full range of tone, even though the remainder of the scene is crushed or muddied.

High key can lend a feeling of cleanliness (such as for product photos or architecture) or discomforting sterility (plain white walls of an institution). Low key can emphasize intimacy (a figure in a darkened bedroom) or terror (a forest at night, creatures hiding in shadows). Both can also evoke feelings of isolation or loneliness from a lack of visible surroundings.

This is a different concept from a "key light", the primary light used to illuminate a subject. "High key" here instead means a key light placed above the subject, and "low key" for light from below.

Stops

A stop is a difference of double a lower value, and one-half a higher value. Stops are used because illumination and vision are dependent on this geometric scale. The perceived difference in light between one and two lamps appears greater than the difference between eleven and twelve of the same, but would appear the same as the difference between one hundred and two hundred.

Stops are often used when speaking of aperture, as the term originates from the literal stops that secure the aperture ring along its adjustable range. However, stops can also describe the differences between shutter speed and sensitivity values all the same. Camera controls are typically available in thirds of a stop.

Exposure value

Exposure value is an alternate system of quantifying a scene using a logarithmic scale. Instead of large numbers and fractions, exposure values are given in whole numbers that grow or decrease by one with each stop. This can make calculations easier as one needs only add or subtract, instead of the multiplication or division needed for dealing with stops on a linear scale.

Overexposure and underexposure

Overexposure occurs when too much light is captured for the chosen exposure settings, and underexposure when too little. These are often undesirable when contrast is severely reduced.

Minor exposure problems can be corrected during processing, especially with standard negative film as it is exposed a second time to produce a print. Still, the lowered dynamic range from the original shot may result in a lower quality image.

A digital image processor may simply render over- and underexposed areas as pure white or black—clipping to the maximum or minimum available values. Detail cannot be recovered from clipped areas. Because the sensor is often a permanent part of the camera, it may be damaged by extreme overexposure. Care should be taken to avoid shooting direct sunlight or bright sources of light such as encountered with welding or lasers.

Shooter controls

Before the camera even comes into play, the shooter has an important role in deciding what will be shot in the first place, and adjusting how much light is in the scene.

Composition

Natural lighting comes from the sun, which casts very strong light in one direction. Natural lighting also comes from materials illuminated by the sun. This includes the sky, terrain, buildings, and large objects like trees. Light reflected off of these sources is generally diffuse and comes from multiple directions.

Weather and time of day play major roles in using natural lighting. It is not uncommon for shoots to be planned for a specific time of day or year, or to be cancelled due to undesirable conditions.

Artificial lighting is created by humans for visibility and public safety. This is much less intense than natural lighting. It may even appear in the shooting scene without severely disrupting exposure.

Indoor scenes can be challenging to expose properly because the light level is much lower than outdoors, and daylight from outside can easily overpower. Without adjustment, shots taken indoors may require such long shutter speeds or high sensitivity that the resulting image is undesirable.

Staging

The lighting within a scene may be manipulated by the shooter with various tools. Some shots may require use of a studio, a space where the photographer may more easily control shooting conditions.

Devices such as on-camera flash, studio lamps, and strobes provide additional light for the photographer. Lighting can also be redirected from elsewhere by using reflecting screens. Care should be taken to avoid including these devices in the shot.

Light can be removed from a scene by the use of flags and panels to block light, by closing doors and curtains while indoors, and other obstructions. Screens and scrims may be applied to lighting to reduce their effect on the scene, or applied outside windows to allow outdoor objects to be exposed along with the indoors.

Properties of light such as diffusion and color can be altered with various materials.

Camera controls

Three primary exposure controls are located on the camera. Aperture is controlled by the iris diaphragm inside the camera lens. Shutter speed is controlled by the shutter curtain inside the camera body. Sensitivity is a property of the speed of film chosen, or the digital image sensor used.

Aperture

The aperture is a hole in the camera lens through which light is transmitted from outside the camera. Changing the size of the aperture allows more or less light through as needed. The aperture size also has an effect on depth of field—how much of the scene before and behind the focal plane is in focus. There may also be minor side effects depending on the quality of the lens.

Shutter speed

Shutter speed is the length of time for which the shutter is released, permitting light from the lens to reach the sensor. It is measured in seconds, or fractions thereof. Shutter speed has a direct effect on motion blur captured in the image, whether through the motion of subjects or of the camera itself.

Though normally controlled by a timer, long shutter speeds may be achieved through a "bulb" setting. With long shutter speeds, it may be necessary to use neutral density filters to prevent overexposure during the day. The camera should also be stabilized with a tripod or setting it on a sturdy surface. Very long shots, such as with astrophotography, may even require a tracking mount that counteracts the rotation of the Earth.

Sensitivity

Sensitivity or speed—often given as an "ISO number"—is the rate at which the film or sensor is able to capture or react to light. By changing the sensitivity, the same amount of light can produce different tones in the captured image. Each successive grade (double the number) requires half as much light to be captured for the same resulting tone.

Film

Film manufacturers achieve varying degrees of sensitivity by altering the grain size of the light-sensitive crystals embedded in the film. Slower films will have smaller grain than faster films. The grain size affects how the film captures fine detail, and how smoothly it can reproduce gradual changes of color.

Film speeds are generally obtainable from ISO 100 (slow) to ISO 3200 (really fast). In order for the camera to properly calculate exposure, the matching film speed should be set on the camera body's sensitivity control.

Digital

For digital sensors, sensitivity has an indirect relation with image noise—unwanted variations in color. Higher sensitivity (corresponding with a higher ISO number) allows for quicker exposure but risks increased noise; lower sensitivity requires more time but reduces noise captured. Digital cameras may provide a wider range of sensitivity, from ISO 50 to ISO 12800.

Changing the camera body's sensitivity control changes how the image processor translates the sensor readings, simulating a change in sensitivity. Depending on the quality of the image processor, there may be subtle changes in contrast and color rendition as sensitivity is adjusted.

Metering

The proper exposure settings for the scene to be shot can be determined by 'metering—using a light meter that instantly reports the level of light. The readings may simply inform the shooter while using manual exposure, or they may be used by the camera's autoexposure system to automatically set exposure controls.

Manual exposure

With manual exposure, the exposure controls are set by hand. These cannot be changed by the camera, even if the shot is sure to result in over- or underexposure.

Through-the-lens

The camera body often contains a built-in reflected light meter that measures the amount of light entering the camera through the lens (TTL). The suggested exposure settings are usually indicated in the viewfinder.

To better ensure a proper reading, a calibration card may be positioned in view. An 18% gray card provides a neutral, middle gray tone.

External metering

An incident light meter may be used to measure the actual amount of light present in a scene. The user holds the meter where the primary subject is positioned. These often have a white hemisphere that allows light to be collected from all directions. The meter reports what exposure settings should be used, and allow for locking of certain controls. Some can measure flash lighting by reporting the peak value detected.

There are devices that allow a smartphone to be used as a light meter, with an accompanying app to report settings.

Rules of thumb

Approximate settings can be figured by easy-to-memorize rules:

Sunny 16—On a sunny day, set aperture to f/16, and shutter speed to the reciprocal of sensitivity. If sensitivity is ISO 100, then shutter speed is 1/100. The aperture can be adjusted for different conditions, such as overcast clouds or shooting in full shade.

Looney 11—When shooting the surface of the moon at night, set aperture to f/11, and shutter speed to the reciprocal of sensitivity.

Automatic exposure

The camera body can use automatic exposure controls using the TTL meter and a programmed shooting mode.

Because scenes vary, the camera offers some basic metering modes:

Spot metering: Measures a small field, typically centered with the lens. Spot metering is recommended for use with a calibration card.

Center-weighted metering: Measures a larger field to account for areas with high contrast. The reading will split the difference between the brightest and darkest areas.

Matrix/evaluative metering: The entire scene is analyzed in sections, with priority given to focused areas. Some camera models may compare readings against a database in order to pick settings.

Average metering: All available light in a scene is simply averaged, yielding a crude but quick result.

Exposure compensation

Light meters assume that the subject will be exposed to a middle tone. This may not be the case if you are shooting in excessively bright scenes (snow, sand), or if your subject is strongly colored (dark skin or paint). The meter reading can nonetheless be used by setting exposure compensation that adds or subtracts stops.

On a film camera, the sensitivity control may be used for exposure compensation, by setting it a certain number of stops away from the actual film speed. Setting to a lower speed will increase exposure; setting to a higher speed will reduce exposure. This allows for using the shutter speed and aperture controls as intended.

On a digital camera, there is a dedicated exposure compensation control (+/–). The camera will adjust exposure controls depending on what shooting mode is chosen. Camera customization may allow more than one control to be adjusted to ensure a shot can be taken. In other cases, the camera may refuse to shoot and alert the user that its limits have been reached.