Basics of Exposure

Chapter 6 Overview

This chapter discusses the concept of exposure and how it applies to the use of digital cameras. After a presentation of the basics, including equivalent exposures, nonmetered and metered approaches are considered, including basic daylight exposures, use of reflective and incident meters, and metering. Specific approaches of substitution, tonal placement, dark tone, highlight metering, and average value metering are also presented.

Exposure Basics

Exposure allows the image to be captured. Because sensors, unlike film, cannot accept overexposure and are noisy in very dark areas of exposure, understanding how to set the exposure will provide cleaner and more useable images. Exposure is the capture of light on the sensor. A simple equation defines exposure: exposure (H) is equal to the illumination (E) × (T), or:

H = E × T

Exposure A measure of the light captured on the sensor. Exposure is calculated with the equation H = E × T, where H is the exposure, E is illumination (the amount of light), and T indicates time (shutter speed).

This implies that two variables affect exposure. The aperture of the lens controls the illumination, and the time is controlled by the shutter speed. This equation would suggest that only two variables are at work here, but it has been assumed that the ISO or speed of the sensor remains constant. Sensitivity, however, is variable for many sensors.

Thus, exposure depends upon the ISO, f-stop, and shutter speed. It is important to realize that all of these measures are based on a 2: 1 ratio. Each doubling of the ISO rating increases the sensor’s sensitivity by a factor of two. As discussed in Chapter 3, both f-stops and shutter speeds are based on this doubling/halving function. This allows coordination among the three factors. When any one of the factors remains constant, then the other two variables establish the exposure; thus, exposure is controlled by illumination and time if the ISO is held constant. Holding the ISO constant is the most commonly used approach for roll film. If the shutter speed is held constant, then exposure is controlled by the illumination and ISO. Finally, if the illumination as defined by aperture setting is held constant, then the shutter speed and ISO establish exposure. Push processing of sheet film allows use of this last approach, but with roll film varying the ISO is not feasible between frames. Although all of these models of exposure are valid for digital capture, varying the ISO limits the range of exposure.

These three controls provide the photographer with various ways to achieve equivalent exposures, which are a series of settings that each achieve the same level of exposure. Because f-stops, shutter speeds, and ISOs are based on the 2: 1 ratio, there are many potential settings. By holding one of the three variables constant, manipulating the other two variables can create equivalent exposures; for example:

• If the ISO is held constant, then f4 at 1/125, f5.6 at 1/60, f8 at 1/30, f11 at 1/15,… all provide the same exposure. 92 basics of exposure
• If the speed is held constant, then f11 at 400 ISO, f8 at 200 ISO, f5.6 at 100 ISO,… all provide the same exposure.
• If the f-stop is held constant, then 1/125 at 400 ISO, 1/60 at 200 ISO, 1/30 at 100 ISO,… all provide the same exposure.

With variable ISO, as mentioned earlier, as the ISO of the sensor is increased the quality of the image deteriorates. This is because the increase in speed reduces the overall signal strength, thus affecting the noise level, which results in greater noise in the image. The increased amount of noise reduces the effective dynamic range.

Basic Daylight Exposure

Although metering systems are built into digital cameras, in some situations understanding some principles of light will allow for better exposures. Primarily, daylight and ambient light situations can allow the use of a non-metered approach. With basic daylight exposure (BDE), the photographer can adjust exposure based on the condition of the light rather than on a meter reading. The BDE actually is a list of recommended exposures for varying light conditions. The concept is based on a consistent level of clear-sky sunlight, or the “sunny day” or “sunny 16” rule, which states that on a sunny day the exposure will be at f16 at a shutter speed of 1/ISO. This is the BDE standard. Using this as a basis, the following are exposures recommended in specific lighting situations:

Basic daylight exposure (BDE) Basic daylight or sunny-day exposure is based on the lighting conditions of a bright, sunny day with the subject in the sunlight. Sometimes this is called the “sunny 16 rule,” which states that on a sunny day the exposure will be f16 at a shutter speed of 1/ISO. BDE is an exposure calculated by adding or subtracting f-stops from f16 at a shutter speed 1/ISO to adjust for various lighting situations.

The following settings will require adjusting the shutter speed as well as the f-stop:

Light Meters and Basic Metering

With a large dynamic range of some digital capture and potential problems with large light values, exposure of sensors favors underexposure. With proper capture, the detailin shadows will be realized and the highlights will be controlled without blooming or other overexposure problems.

Determining exposure without metering is possible, but most photography is produced with light metering. Light meters work with sensors similar to those in digital cameras, but the sensors have fewer and/or differently manufactured sites. Metering uses the light received by the sensor to evaluate and then determine the recommended exposure as an f-stop and shutter speed combination based on an ISO setting of the sensor.

Meters evaluate how much light is reflected from the subject (reflective meters) or the amount of light falling on the subject (incident meters). Metering determines exposure by using a predetermined value as the average tonal representation in the subject. Middle or 18% gray represents this value. When the light is measured, it is assumed that the meter is reading this middle-tone mix of light values and that this is the value that the photographer wishes to reproduce. Therefore, if the scene contains this average middle tone, then the subject will be captured correctly.

Reflective meters The photocell in a reflective meter reads the light reflected from the subject.

Incident meters The dome on an incident meter covers a photocell that evaluates the amount of light transmitted through the dome. This allows a determination of the amount of light falling on the subject.

Reflective meters are the most frequently used light meter. The three types of reflective meters are average, programmed, and spot, and the most common of these is the averaging procedure. Average metering takes all the light from the scene, finds the average for all light energies entering the meter, and sets that as the middle value, or the middle tonal mix.

Because light meters use sensors similar to those in digital cameras, using the camera as a reflective meter is a software function. In-camera meters read the light directly from the scene to be captured and set the exposure based on the light coming through the lens. Through-the-lens (TTL) meters are the most common meters used in photography today. Software in many cameras automatically sets the exposure using TTL metering. While the “auto exposure” setting gives an overall reading, it is not the only exposure choice on some cameras.

Through the lens (TTL) meters Through-the-lens meters are reflective meters that read the light in the optic path after the light has passed through the lens.

The sophistication of sensors and metering associated with them in today’s cameras allows metering to be programmed according to specified patterns rather than just as an average or simple spot reading. This allows weighted exposure, or emphasizing predetermined areas of the frame; for example, programming can give a center weight or take a reading from the lower portion of the frame. The most specific pattern is a spot in the center of the frame that acts like a spot meter. Cameras that allow choices of average, pattern, or spot metering have a circuit that changes the number and activity of sensor sites throughout the frame. In many advanced digital systems, the sensor can be used to take light readings from multiple points. These points can be varied depending on the image and can create a specific light weighting for each image.

In general, meters are based on the concept that the metered area, pattern, or spot is middle toned; however, if the subject varies from this middle-tone mix, then the meter will have subject bias and the exposure will not portray the subject accurately. Subject bias can be seen when the subject is made up of a large amount of light tones, such as snow. Average metering will result in the subject being underexposed. Even when the subject is comprised primarily of light tones, many meters assume that the scene is a mix of values that result in middle gray, and the calculated exposure will try to render the image as middle gray. The reverse is true for darker values, in which case the metered exposure produces overexposure.

To avoid subject bias, incident metering can be employed. Incident metering reads the amount of light illuminating the subject. An incident meter uses a translucent dome over the sensor. The dome diffuses the light falling on it and averaging the light illuminating the subject. Unlike a reflective meter, an incident meter is not pointed at the subject but instead is pointed at the camera from the subject’s position. Because of the shape of the dome and because it is pointed toward the camera, this meter averages not only the light directed at the subject but also any ambient light reflecting onto the subject surface that will be in the image. Flash meters are generally incident meters.

Substitution Metering

An incident meter is a specialized piece of equipment, and many photographers have only an in-camera meter or a reflective meter; however, the notion of measuring the light falling on the subject still can be used by these photographers to avoid subject bias. Substitution metering uses a material of known reflectivity; for example, gray cards (18% reflective cards) provide the same reflectivity as the settings of meters. Substituting a gray card for the metering value of the scene allows the illumination on the scene to be measured as it reflects from the known light value card. This establishes the exposure as an incident meter. If the only thing that the metering system measures is the card, then the exposure will be based on the light falling on the area that the card occupies. The angle of the card should be perpendicular to the lens axis to capture the effect of the light as well as the amount of light.

It is also possible to use other materials for substitution metering. With a known relative reflectivity of any material it can be used as a substitute for average metering. The palm of the hand normally reflects about 36% of the light illuminating it. Measuring the light from the palm of hand and reducing the exposure by one stop works well. The clear north sky has a relative luminance equivalent to 18% reflection, or an incident meter. Pointing the reflective meter or the camera at the clear north sky and using that light reading as a substitute for an average meter reading gives the exposure for the light created by the daylight.

Substitution metering with digital capture is a very valuable tool. A system that allows multiple exposure points allows the photographer to use either a gray scale on a color checker or a series of flat-painted (black, middle gray, and white) balls, which can be put into the subject for multiple substitution exposure points. These multiple points allow for control of highlights and shadows, as well as the middle gray of the image.

Tonal Placement

Tonal placement metering involves choosing a specific tone within the scene and adjusting the exposure to ensure that this tonal area will reproduce as desired. To accomplish this, a reflective meter reads the light coming from the chosen area, and the photographer increases or decreases the exposure the number of stops that the chosen tonal area will be from middle gray. A light middle tone will require opening up one stop, and a detailed highlight will require opening up two stops. Dark tones require stopping down. The camera is stopped down one step for a dark middle tone, two for shadow detail. The tonal placement steps are

• Determine the desired tonal value.
• Meter the critical area of the scene; that area will be metered as middle gray.
• Open up to lighten or stop down to darken the area as desired.

Tonal placement gives control of only one tonal area in the scene. As the chosen tonal area becomes lighter or darker, the rest of the tones in the image will move proportionally in the same direction.

Dark-Tone Metering

Perhaps the most useful non-technical approach for exposure of film is darktone metering. While effective for film, it is not well suited for digital capture because of the sensor’s inability to accept overexposure. This method, while inappropriate for digital capture, is discussed here because there may be times when film needs to be employed for capture with scanning of the final output used to create a digital file.

Dark-tone metering is a specific application of tonal placement; it commonly uses textured shadow (shadow detail) as the metering point for proper exposure. Many photographers using film have learned, “Expose for the shadows, and develop for the highlights.” With film, shadow detail is critical because if it is not captured in the exposure it will not be present in the final developed film image. For digital sensor exposure, dark-tone metering should only be used in calculating exposures where the highlights are unimportant or are low in relation to the shadow, because this method can lead to overexposure.

With dark-tone metering, the photographer determines what part of the scene will represent shadow detail and then takes a reflective light meter reading in that tonal area. Because the meter is reading for middle gray exposure, it reads two stops more exposure than is required. Stopping down two stops from the meter reading gives the correct exposure. Some photographers make a mistake by choosing the darkest tone in the scene as their dark tone. When they do this they overexpose by two or three stops. Because sensors do not handle overexposure well, this error should be avoided.

Dark-tone metering is part of a multiple-point metering system. With systems that allow multiple-point reading or tonal adjustment for the exposure of selected points, this method can be used to eliminate noise. Within the commonly used digital format, there are 256 gray tones. The purest black would be 0, but using a value of 0 for the black exposure may lead to a loss of shadow detail or noise. For these reasons, it is suggested that photographers use a digital value of 12 for the black setting.

Highlight-Detail Metering

While not as common as dark-tone metering with film, highlight-detail metering is both effective and desirable for digital capture. Similar to dark-tone metering, this technique is a specific application of tone placement when the photographer wishes to select highlight-detailed areas as a critical image area. In this method, the photographer meters an area in the scene that should contain the highlight detail and adjusts the exposure by opening up two stops.

Like transparency film, digital does not handle overexposure well. Because of this inability to accept overexposure, controlling highlights becomes essential in situations with bold or critical highlights. Because “blowing out” highlights is to be avoided, with systems that allow adjustments to known values the highlight should be set at 242 in an 8-bit-per-channel system to control for highlight detail. If there is overexposure, clipping will occur within either the capture process or post-capture software.

Clipping When the amount of light exposing the sensor exceeds its ability to record all the energy, the system eliminates these higher energy levels or software eliminates higher digital number values from the image file.

Average-Value Metering

Average-value metering uses tonal placement to produce good exposures. Dark-tone and highlight-tone metering are key to capturing the proper exposure. Because these tones are the extremes of detail in the picture, controlling these areas will allow capture that includes both highlight and shadow detail. The advantage to this adaptation of a tonal approach is that it will reduce errors occurring because of pictures that are tonally biased to either a bright tone or a dark tone within a majority of the scene. Working within a range of light allows capturing detail in both shadow and highlight. The photographer selects and meters these two tonal areas, which establishes a range of tones defining the middle portion of the scene that includes the important detail of the image. By dividing the two tones, the midpoint becomes the exposure. If the midpoint is not clearly determined, then exposure should tend toward underexposure by moving the exposure point toward the highlight. With adjustable-value systems, a black point value of 12 and a white point value of 242 are recommended.

Summary

• The basic concepts of exposure hold whether using film or digital capture. Exposure is controlled by the amount of light, the time of exposure, and the speed of the sensor.
• Equivalent exposure uses the interrelationship between the quantity of light as expressed in f-stop, shutter speed, and the speed rating of the sensor to create a set of exposures. Holding one of the three variables constant and then varying the other two creates equivalent exposures.
• Instead of a metering system, basic daylight exposure can be used to determine exposure based on observed lighting.
• Incident light meters read the light falling on the subject, and reflective meters read light reflecting from the subject. Reflective meters can determine the light reflecting off of the entire subject or just a portion.
• A reflective meter establishes the exposure by averaging all the light reaching the meter. By substituting an object of known reflectance, taking a meter reading at a critical point in the scene, whether shadow or highlight detail, or determining an average value by selecting both highlights and shadows, the average meter provides an accurate method for placing exposure for digital capture.

Glossary of Terms

Basic daylight exposure (BDE) Basic daylight or sunny-day exposure is based on the lighting conditions of a bright, sunny day with the subject in the sunlight. Sometimes this is called the “sunny 16 rule,” which states that on a sunny day the exposure will be f16 at a shutter speed of 1/ISO. BDE is an exposure calculated by adding or subtracting f-stops from f16 at a shutter speed of 1/ISO to adjust for various lighting situations.

Clipping When the amount of light exposing the sensor exceeds its ability to record all the energy, the system eliminates these higher energy levels or software eliminates higher digital number values from the image file.

Exposure A measure of the light captured on the sensor. Exposure is calculated with the equation H = E × T, where H is the exposure, E is illumination (the amount of light), and T indicates time (shutter speed).

Incident meters The dome on an incident meter covers a photocell that evaluates the amount of light transmitted through the dome. This allows a determination of the amount of light falling on the subject.

Reflective meters The photocell in a reflective meter reads the light reflected from the subject.

Through-the-lens (TTL) meters Through-the-lens meters are reflective meters that read the light in the optic path after the light has passed through the lens.