Dr. Glenn Rand explains how to calculate exposure and how to test your digital camera to determine its true ISO equivalent.
This excerpt from Lighting for Photography: Techniques for Studio and Location Shoots is provided courtesy of Amherst Media. To purchase the book and learn more about the publisher, visit the Amherst Media website.
Basic Daylight Exposure. Photographers often use built-in or handheld meters to ensure proper exposure. However, there are times when knowing how to judge the light will arrive at good exposure.
In daylight and ambient light envelopes, you can use a non-metered approach to determining exposure. With the sun, for all intents and purposes, a constant distance from the earth, the light reaching the light envelope will be constant with normal clear sky sunlight. Therefore, we can set a standard for lighting using the sun as our main light.
Basic daylight exposure (BDE), also called the sunny day or sunny 16 rule, allows you to adjust exposures based on a set of recommendations related to the light rather than a meter measurement. The concept is built on the assumption that on a sunny day, an f/16 aperture setting and a shutter speed of 1/ISO can be used to produce a correct exposure. With a shutter speed of 1/ISO, the following are recommendations for BDE:
|Sunny on snow or sand
||BDE + 1 stop
||BDE - 1 stop
|Normal cloudy but bright
||BDE - 2 stops
|Overcast or open shadow
||BDE - 3 stops
|Lighted signs (to see the color)
||BDE - 5 stops
|Stage lighting (bright stage)
||BDE - 5 stops
The following scenarios will require adjusting the shutter speed and f-stop:
|Bright city streets (Times Square), fireworks, night football, store windows
||BDE - 6 stops
||+ 6 stops
|Stage lighting (spot lighting), night baseball
||BDE - 7 stops
||+ 7 stops
|Inside schools and churches (well lit)
||BDE - 9 stops
||+ 9 stops
|Flood-lit buildings or neon lights
||BDE - 11 stops
||+ 11 stops
||BDE - 13 stops
||+ 13 stops
This exposure was made at night and metering was not practical. Using BDE, an exposure was established that allowed for the capture of the color of the lights. Photograph by Glenn Rand.
Light Meters. Though determining exposure through BDE is possible, most exposures are determined by light meters. Light meters can be built into the camera or handheld.
Reflective meters evaluate the amount of light that is reflected from the subject. Incident meters, on the other hand, are used to determine the amount of light falling on the subject. Reflective meters are available in two types: averaging or spot meters. An averaging meter reads all the light reflecting toward the meter from the entire light envelope. A spot meter measures the light entering the meter from a specific area within the envelope. Either type may be built into the camera or handheld. A problem with reflected metering occurs when the subject is very light or very dark. This creates subject bias and will cause images of dark subjects to appear overexposed and images of light subjects to look underexposed. In such cases, an incident meter should be used. Because an incident meter measures the light in the envelope, it is not influenced by subject bias.
Substitution Metering. Subject bias can be avoided by using substitution metering. To use this technique, you use a reflected light meter to meter an object with a known reflectivity (e.g., an 18 percent gray card). The object you are metering must be part of the subject, placed at the subject location in the light envelope, or receiving the same intensity of light as the subject. If the only thing that the metering system measures is the substituted material, the results will match those of an incident meter.
Though a gray card, held perpendicular to the lens axis, is a popular choice for substitution metering, there are other commonly used targets. For instance, you can meter a clear north sky, which has a relative luminance equivalent to 18 percent reflection. You can also measure the light reflected from the palm of your hand, then reduce the exposure by one stop.
Flash Meters and Guide Numbers. When working with electronic flash, photographers typically use a flash meter, which also reads incident light. A still better option for ensuring proper exposure when using electronic flash is to use a guide number (also called a guide factor). A guide number is a value used to calculate the correct f-stop for the flash-to-subject distance and ISO rating/setting required to achieve correct exposure. To determine the correct f-stop, divide the guide number by the distance (the guide number is often provided with the flash unit). To determine best subject distance, divide the guide number by the f-stop.
Light balancing was used to produce this image. Since shutter speed is less of an issue for electronic flash, the light outside was balanced to the flash exposure by using an equivalent exposure to set the camera’s shutter speed to the f-stop recommended by a flash meter. Photograph by Glenn Rand.
This photograph was made for Women’s Circle Crochet magazine. The lighting of this subject was controlled to produce only three stops of difference between the reflected spot-meter readings from the white yarn and the black detail. The lighting was adjusted while taking meter readings until the light was tuned to create the dynamic range required for the magazine’s printing. Photograph by Glenn Rand.
Tonal Placement. Tonal placement metering is an important tool to accomplish the desired look in our images. In fact, it is the basis of the Zone System. The concept is built on the direct relationship between stops and zones and holds true for all midtones but loses its effectiveness at the dark and light extremes of the light range. The concept holds that a one-stop change in the light or exposure will change the zone value of the metered area one zone. To use this technique, then, the photographer uses a reflected light meter to measure the light coming from a selected area of the image. Since the meter is built to calculate exposure of middle gray, opening up one stop will lighten the selected tone to be brighter by one zone and stopping down one stop will darken by one zone. When darkening or lightening any specific area within the image, the tones in the rest of the image will shift in the same direction.
Dark-Tone Metering. Perhaps the most useful and nontechnical approach to film exposure based on the Zone System is dark-tone metering. In this approach, we would determine the area of critical shadow detail in the picture, use a reflective meter to take a reading, then stop down two stops from the recommended setting. In a controlled-lighting scenario, we can take an overall incident meter reading then adjust the fill light in the shadow area to provide a two-stop difference from the metered setting.
Highlight Detail Metering. Highlight detail metering is the flip side of dark-tone metering and is most effective for determining exposure for digital capture and transparency film. In this method, you meter the area in which you want to show highlight detail and adjust the exposure by opening up by two stops.
To avoid blooming or blown-out highlights, when working with a tethered system check the histogram for your image and ensure that the maximum highlight value is no higher than 242.
Exposure was chosen based on the important highlight tones of the pearl diver. A spot meter was used to take a reading from the diver at a previous surfacing and used for photographing this assent. Photograph by Glenn Rand.
Average Value Metering. Within the light envelope there are generally both highlight and shadow details that the photographer will want accurately presented in the image.
To use average-value metering, use the following steps:
- Determine the area that requires highlight detail and take a reflected light meter reading.
- Determine the area that requires shadow detail and take a reading.
- Choose the f-stop that falls in the middle of the two readings you obtained and use that as the exposure setting. For example, if the highlight area metered f/16 and the shadow metered f/4, you would use f/8 as the exposure setting.
If the distance between the highlight and shadow detail areas of the image is beyond seven stops for film or nine stops for digital, the system will not work well. Note that when the distance is an even number of stops (e.g., f/16 to f/5.8) the average value will be a half stop. In this case, you should select the value below the midpoint for film and the aperture setting above the midpoint for digital work.
Special Concerns for Digital Exposures
Testing. Image sensors can only accept a specific amount of light at each pixel. Beyond this point the energy will bloom to adjacent pixels or block up as white. To determine our image sensor’s ability to accept overexposure, we must conduct a simple test to determine its exposure index (EI).
To run the test, you will need the following:
- A meter designed to meter the light in the scene (e.g., an incident light meter; or you can take a reflective meter reading of an 18 percent gray card).
- A test target that consists of a white, 18 percent gray, and black area that will be large enough on the image to measure in imaging software. These can be overlapped separate cards.
- A model. The model should be wearing a textured white shirt or sweater. If need be, a piece of white material with heavy texture, ribs, or cabling can be draped over the subject’s shoulder.
- The camera must be in manual mode so that you can adjust the shutter speed and aperture and set it to capture raw files.
Test setup. Photograph by Tim Meyer.
To ensure the test is properly conducted, complete these steps:
- Find or create an evenly illuminated scene with an even, solid, dark background. If there is a bright background or backlighting the test will be inaccurate.
- Place the card(s) on a stand near the model’s face. Accuracy will only happen when the gray, black, and white cards are parallel to the sensor’s plane (the back of the camera). Also ensure that you can see part of the white fabric with detail.
- Check to see that the light is non-directional (i.e., diffuse) on the model and card(s). Do not conduct the test under specular light.
- The ISO for the light meter must be the same as the selected ISO for the camera. (Because of noise considerations it is recommended that you use ISO 100 or the lowest available ISO for baseline testing.)
- Take an incident meter reading and write down the f-stop and shutter speed.
- Take a reflective meter reading of the white test patch. Subtract the incident reading (step 5) from the reflected white reading. The resulting value will be used when determining the optimal exposure.
- Make an exposure with the aperture opened up two stops from the reading obtained in step 5.
- Close down the lens 1/3 stop and take your second exposure. If your lens has 1/2 stops, do the test in 1/2 stops. Record the information obtained for each successive frame.
- Close down another 1/3 stop (or 1/2 stop if appropriate) and take another exposure. Repeat until you have a complete four-stop range.
To determine your camera’s true ISO, read the white card with the color sampler tool in your image-editing software. Neutralize the file, then take a series of readings across the white card. The neutralizing should not include adjusting the white point. The correct exposure will be the frame where your white card value is approximately 242 with no exposure adjustment.
If you organize the shooting and corresponding white point data in order of exposure, from most exposure to least exposure, you can see which exposure’s white point is closest to 242. That is the exposure closest to the ISO set in the meter. The optimal ISO setting (EI) is determined by counting the number of stops (1/3 or 1/2) from the camera’s set ISO. If the EI is below the meter setting, you must reduce your exposure. Conversely, if the EI is greater than the meter setting, you must increase the exposure. Obviously, this is only viable if the camera is used in manual mode using the light meter used for testing. It is also important to record the light meter readings for the white card and the textured white area of the test scene.
The two examples show a normal exposure (top) and a two-stop overexposure test (bottom). Our task is to find the highest level of exposure in which detail is maintained in the white textured area of the image. Photos by Tim Meyer.
This photograph by Tim Meyer shows the benefit of EI testing a digital camera. The image was captured in JPEG, and without EI testing, shadow details in the background and highlight details in the gown and wall would not have been captured.
Exposure Profiles. An advanced but complicated way to ensure accurate exposure is to build exposure profiles. Because the process is quite complex and is beyond the scope of this book, this section is designed to merely introduce you to the basic concept.
An exposure profile is an intermediary function that will allow us to work within a changing light environment with predictable results. The profiles include information about the type of light, its intensity range, the camera sensor’s limits, the sensor’s functional EI, the type of file that will be used, and the limits of the intended output device. As can be imagined, taking into account all these variables and the way they interact can be very difficult.
By establishing your camera’s EI, you can ensure that the highlights in your image will not exceed the camera’s capture range. The RAW channel histogram shows that few light levels reach the maximum highlight value. Photograph by Glenn Rand.
Key in developing an exposure profile is the tool in the middle, the light meter. The meter is used to measure light variations, and the values that are calculated are used in the creation of inter-relational and coordinated lookup tables. These look-up tables connect measurements from the light meter, camera limits and processes, and outputs. Each look-up table compares two or more of the variables and provides exposure information based on these or related interactions.
Manually creating exposure profiles is difficult because one must take into account so many interrelated variables. Fortunately, there’s a tool that can be put to the task to alleviate this burden. The Sekonic DigitalMaster L-758DR light meter uses proprietary software and testing tools to calculate exposure profiles, which can then be transferred to the meter via USB connection. Once stored in the memory of the light meter, the profile is accessible as an operating function of the meter. When operated in the profile mode the meter graphically shows the photographer where overall (incident) illumination falls within the dynamic range of the sensor and also indicates if the highlight and shadow points fall outside the dynamic range of the sensor and will be “clipped.” This allows the photographer to adjust the lighting or exposure in order to ensure that the photograph can be captured and output as previsualized.
Photograph by Glenn Rand.