Metering and Exposure
By Doug Box
/ Published by Amherst MediaGet a firm grasp of how shutter speeds, aperture, and ISO relate; how to meter a scene effectively; and how to trigger your flash wirelessly from a handheld meter with this excerpt from the Amherst Media book Doug Box's Flash Photography.
This excerpt from Doug Box's Flash Photography is provided courtesy of Amherst Media. To purchase the book and learn more about the publisher, visit the Amherst Media website.
When it comes to determining the correct exposure for a scene or subject, the trend today is to make a wild guess, then chimp (look at the back of the camera and make an “oooh, oooh” sound like a monkey when the image on the LCD looks somewhat like you thought it would). I am a big fan of using a meter. While I do use the LCD to confirm my exposure, I use my in-camera and handheld meters to determine what the exposure should be. I have learned my lesson about relying on the magic little screen. You see, sometimes the light conditions affect the appearance of the image on the screen, making you think you should recalculate your exposure. When you do, you discover that the image that looks fine on the LCD looks much different on your computer. There are other times when it is so bright outside that you can’t clearly see the screen.
Every image is made with light. By manipulating the amount of light striking your image sensor, you can change the way the image is recorded. There are three variables that work together to create a perfect exposure: shutter speed, f-stop, and ISO.
Shutter Speed. The term “shutter speed” is used to refer to the length of time that the shutter remains open and the film or sensor is exposed to light. Typical shutter speeds range from 30 seconds to 1/8000 second. A one-increment change in shutter speed will cut the amount of light creating the exposure in half (e.g., when going from 1/125 second to 1/250 second) or double it (e.g., going from 1/125 second to 1/125 second).
Aperture. The term “aperture” refers to an opening in a light-blocking plate inside the lens. The diameter of the opening is described in f-stops. A small f-stop number (e.g., f/4) is used to notate a large diameter opening. A larger f-stop, like f/22, is used to describe a smaller opening. If you change your aperture from f/4 to f/5.6, half as much light will enter the lens. Note that most modern cameras break the f-stops into 1/2 or 1/3 stop increments. This gives you more accuracy in your exposure.
Proper exposure is critical to achieving a great image. Invest in a high-quality meter and use it to make sure your images are the best they can be.
Depth of Field. When you use a small aperture, more of the area between the closest and farthest image element is in focus.
Several factors will affect the depth of field in your image: (1) the size of the aperture, (2) the focal length of the lens in use, and (3) image size and its relationship to the subject distance.
Given the same subject distance and image size, the bigger lens opening (aperture) used (like f/2.8, f/2, f/1.4, etc.) will have a more shallow depth of field.
On the other hand, if you want more depth of field, you can just choose a smaller lens opening (like f/8, f/11, f/16, or f/22) to extend the plane of sharpness, so everything will be in sharper focus.
It is important to understand that changing any one of the three settings will impact the effectiveness of the other two settings.
Remember the following relationships:
- The smaller the aperture, the more depth of field with the other two factors remaining the same. For example, if the lens focal length and the shooting distance stay the same, the depth of field is much deeper at f/16 than at f/2.8.
- The shorter the lens focal length, the more depth of field with the other two factors remaining the same. For example, comparing a 35mm lens with a 85mm lens at the same aperture and shooting distance, depth of field will be more with the 35mm lens.
- The greater the shooting distance, the more depth of field with the other two factors remaining the same. For example, if the subject is photographed from 4 feet and then from 8 feet, the zone of sharpness in the foreground and background is greater at 8 feet.
Another characteristic of depth of field is that it is typically deeper in the background than in the foreground.
ISO. An ISO is a film speed rating system. Low numbers indicate slow film (an emulsion with low sensitivity to light), and high film speeds indicate film that is quickly exposed. In digital cameras, the sensor’s sensitivity to light can be manipulated by changing the ISO settings. (In reality, the sensor has only one sensitivity and, when you increase the ISO, you are stretching the info that is there. The software in the camera interpolates the information and fills in the blanks.) With film, the ISO is a measure of the sensitivity of the film to light. As the film speed increases, for example, from 100 to 800, the film needs less light to make a proper exposure.
It is important to understand that changing any one of the three settings will impact the effectiveness of the other two settings.
Once you have determined the settings you need to achieve a perfect exposure, you may find that you need to adjust one of the controls. As I mentioned in the previous section, changing one of the three settings will affect the other exposure settings too. For instance, if you were photographing a soccer player running down the field, you might decide to shoot at f/2.8 at 1/500 second. You might then realize that the lens you brought to the game had a maximum exposure of f/4. An f/4 aperture allows half as much light into the camera as an f/2.8 aperture. Therefore, you would want to change the shutter speed to 1/250, which lets in twice as much light as the original shutter speed, 1/500. Because you wanted to freeze the motion of the running subject, though, you might need to use the 1/500 shutter speed. Therefore, you would need to choose a faster ISO. Every time you halve or double a shutter speed or ISO, you change the exposure by 1 stop. So, in the above example, you could set your camera to ISO 200 and get proper exposure at f/4 at 1/250. If you set your camera to ISO 400, you could shoot at f/4 at 1/500 second. See how this works?
Sunny 16: The Sunny 16 rule can be used to estimate the correct exposure on a bright sunny day—without a meter. To use it, set your camera’s aperture to f/16 and set the shutter speed to the reciprocal of the ISO rating or film speed. If you were shooting at ISO 125, for instance, your exposure would be f/16 at 1/120 and ISO 125.
Exposure Modes and Settings
Program. The camera sets the shutter speed and aperture according to the subject’s brightness and a preset program built into the camera.
Shutter Priority. In this mode, you set the shutter speed and the camera sets the aperture according to the subject’s brightness and the metering mode.
Aperture Priority. In this exposure mode, you set the aperture and the camera sets the shutter speed according to the subject’s brightness and the metering mode.
Manual Exposure. You meter the subject (using the in-camera meter or handheld meter), and you set both the shutter speed and aperture. (This is my favorite option.)
Exposure Compensation. This feature allows you to set your exposure to up to ±3 stops different than the camera’s autoexposure thinks the exposure should be. Exposure compensation is ideal for correcting in-camera metering errors caused by the subject’s reflectivity. No matter what metering mode is used, an in-camera light meter will always erroneously underexpose a subject such as a bride against a white or lightcolored background. Photographs like this will require +1 or more exposure compensation, whereas a low-key image may require negative exposure compensation.
Bracketing. The camera will automatically make three or five images when you click the shutter, with half of the exposures above and half below the normal exposure. You can set the stop difference between each exposure. For example, if you set the camera to 1 stop, bracketing will produce an image with correct exposure (0), –2 stops, –1 stop, +1 stop, and +2 stops.
Accurate exposure is fundamental to creating a high-quality photograph. There are two basic kinds of meters: reflected light and incident light.
Incident Light Meter. An incident meter is used to measure the light falling on the subject. To measure incident light, you place the incident meter at the subject and point the dome or disk toward the light source. This is the easiest way to get an accurate exposure. To measure incident light, you can typically use a handheld meter.
Reflected Light Meter. Light bouncing off of a surface is reflected light. To measure reflected light, point the reflected meter (your camera lens) at the subject and read the light reflected off the subject.
In-camera meters are reflected light meters, and there are also handheld reflected light meters available. The recommended exposure can be swayed by the subject’s tone or brightness level.
Spot meters allow you to measure the brightness of small areas in a scene from the camera position without walking over to take a close-up reading. Like reflected light meters, the recommended exposure can be swayed by the subject’s brightness level. (Note: Sekonic offers a “combination meter,” which functions as either a spot meter or an incident meter.)
Reflectance: Every surface reflects light differently. A white surface reflects 90 to 100% of the light that strikes it. A black surface reflects only 1 to 5 percent of the light that strikes it. An 18 percent gray surface reflects 50 percent of the light that strikes it.
Using Your In-Camera Meter
There are four different metering methods available on most cameras.
Evaluative or Matrix Metering. In this type of metering, the scene is split up into a grid of zones which are evaluated individually. The overall exposure is based on an algorithm specific to the particular camera, the details of which are closely guarded by the manufacturer. Often they are based on comparing the measurements to the exposure of typical scenes.
Spot Meter. A spot meter is center-weighted metering method that measures about 3.5 percent of the viewfinder. Spot metering allows you to meter the subject in the center of the frame (or on some cameras, at the selected autofocus point). Only a small area of the whole frame is metered, and the exposure of the rest of the frame is ignored. This type of metering is useful for brightly backlit, macro, and moon shots.
Partial Metering (or Center Weighted). Partial metering is similar to spot metering, but in this method, the meter evaluates about 8 percent of the scene visible through the viewfinder.
Center-Weighted Average Metering. This method averages the exposure of the entire frame but gives extra weight to the center. It is ideal for portraits. This is the most common metering method used in point-and-shoot cameras, which don’t offer metering mode selection.
Using the Sekonic L-358 Flash Meter
This meter offers accurate flash and ambient light reading in both incident and reflected modes.
Metering in Manual Mode. To use the meter with your camera in manual mode, set your meter to 1/3 stop mode and turn on your meter. Next, set the meter’s ISO to match your camera. Remember, the lower the ISO, the finer the image quality. Finally, choose available light mode or flash mode.
Available Light. With your camera in manual mode, you’ll be able to set your aperture and shutter speed independently. I pick an aperture and the meter will tell me which shutter speed to use. This is where the handheld meter can be helpful.
Flash. There are three flash metering modes available on the Sekonic L-358:
- flash—In this mode, you press the button on the side of the meter in order to prepare it to read the flash. Next, trigger the flash using the open flash (or test) button.
- flash cord—In this mode, you plug the PC cord from your flash into the PC connector on the front of the meter. When you press the button on the side of the meter, the flash will fire and read the flash intensity at the same time.
- flash with radio trigger—Sekonic has an accessory available for the L-358—a small PocketWizard transmitter that will trigger your flash when you push the button on the side of the meter, if you have a PocketWizard receiver attached to your flash.
Flash with cord.
Flash with radio trigger.
Metering in Aperture Priority Mode. To use this meter in the aperture priority mode, press the mode button and turn the jog wheel until you see the sun symbol encased in a box (this will tell you that your meter is set to available light mode). As you turn the wheel slowly (while you press the Mode button), you will see that the sun setting offers two subsettings: one where the “T” (shutter speed) is in a box, and the other where the “F” (aperture) is in a box. Turn the wheel to make the box land on the “F.” This means the meter is in Aperture Priority mode.
Metering in the Shutter Priority Mode. Sometimes you want to pick your shutter speed and let the meter tell you what aperture you should use (this is helpful in sports photography). Here’s how to do it:
While pressing the mode button, turn the wheel until there is a box around the sun symbol and a box around the “T”. This will tell you that the meter is in available light and shutter priority mode. Now simply turn the dial (without pressing any other buttons) until you see the shutter speed you want to shoot at. Aim the meter’s white dome at the light source or camera position and press the activation button once. The display will show you the recommended aperture setting.
The Exposure Calculator for Digital Photography
The Exposure Calculator for Digital Photography was designed to help you understand how the three components of exposure work together. To use the calculator, you first select the ISO that you will need to use for the shot. Next, you take a meter reading of the light on the scene and enter the resultant f-stop and shutter speed into the calculator.
The exposure calculator is a great tool that will help you find the correct aperture, shutter speed, and ISO combination that will suit any lighting scenario and subject.
The back of the card features an exposure target.
Let’s consider a hypothetical shoot to illustrate how the calculator works. I entered ISO 100 because I wanted the best-possible image file my camera could record. Because I planned to shoot with my camera on a tripod, using a fast exposure would not be a problem. My meter reading was 1/30 at f/4. I could use that reading if I wanted a shallow depth of field. If I wanted the maximum amount of depth of field, I could choose f/22 at 1 second. Each combination will produce a correct exposure.
Let’s say that I want a lot of depth of field, so I would choose an f/11 aperture. But when I saw that, at ISO 100 and f/11, I would need a 1/4 second shutter speed, and didn’t have a tripod, I would be forced to find an equivalent exposure.
To get the shot I wanted, I would have to increase the ISO. Increasing the ISO from ISO 100 to ISO 200 would allow me to go from f/11 at 1/4 to f/11 at 1/8. If I feel that it is still not fast enough I can increase my ISO to 400 and that will increase my shutter speed to 1/15. ISO 800 would allow me to go to 1/125 in the above situation. Here you can see how the three elements work together. If you would like to own an exposure calculator, go to www.exposurecalculator.com. It comes with an 80-minute DVD that plays on your computer and explains how this whole thing works.
When you turn the exposure calculator over, you’ll find an exposure target. This will help you achieve the right exposure and correct white balance.
Light Is Cumulative
One thing to keep in mind is that light, no matter the source, is cumulative. For example, if you are working in a dark room and flash the subject twice, the subject will be brighter than if you fired the flash only once. If you use an on-camera flash with an off-camera flash, the light from the two flashes will be brighter than light from one source—and if there is ambient light in the room, whether from a lightbulb or window light, that light will also add to the overall exposure. This is great because it means you won’t need as much flash to get the correct exposure.
For the most part, you can meter these lights separately and predict the outcome. Also, you can use the shutter speed to control the effect of the ambient light.
The exposure calculator is available with an 80-minute video presentation that explains metering, equivalent exposure, and the relationship between f-stop, shutter speed, and ISO. It is available at www.dougbox.com/shop/.
Metering Your Flash with the PocketWizard Chip and PocketWizard Plus II
To meter your flash using the Sekonic L-358,with the PocketWizard chip installed and a PocketWizard Plus II on the flash, hold down the mode button, turn selector knob clockwise until you are between the flash C mode and flash antenna mode. A number will blink. This is the channel indicator. Release the mode button. Next, turn the selector knob until the desired channel appears in the window. (The PocketWizard Plus II allows you to choose from channel 1, 2, 3, or 4. All four channels work the same. The only reason to choose one over the other is if another photographer is working using the channel you selected.)
Next, hold the mode button down again and turn the selector knob clockwise one click to the flash antenna mode. Set the same channel on your PocketWizard.
When you push the trigger, the flash will fire.
When metering the flash and ambient light together (for the ambient fill method), first meter the ambient light in the ambient light mode (e.g., f/4 at 1/60). Next, switch the meter to the flash antenna mode. Set the shutter speed to match the meter reading (e.g., 1/60). Push the button to fire the flash. The resulting reading will be a combined reading of flash and ambient light.
In the image above (top), the final reading was 1/60 at f/5, 2/3 stop more than the ambient reading of 1/60 at f/4. Perfect. Notice two things: at the bottom of the window there is a graph with f-stops. There are three small lines above the numbers. The first one represents the flash part of the exposure. It is above 2.8. The second line indicates the ambient reading. It is above f/4 and the final reading is above f/5. This is the combined reading.
Also notice that 30% appears in the upper-right area. This tells you that 30% of the exposure is from flash and 70% of the final reading is for ambient light. This is about the percentage of flash that I like in my ambient fill images. There is not too much, not too little—it’s just right.
The meter’s screen tells you a lot more than how to set your shutter speed and aperture for the proper exposure.
The biggest advantage of using the Sekonic L-358 meter in combination with the PocketWizard radio trigger is the ability for the meter to fire the flash for metering the light because you can add a PocketWizard chip in the meter.
Category:Exposure and Metering
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