Proper Exposure vs. How the Camera Exposes
Exposure is a complex beast. Mastering it is of the utmost importance. Exposure and composition are the two most critical components towards making a great image. Exposure is based on three components:
- The ISO or sensitivity to light
- Aperture or the size of the opening that will let in light, and
- Shutter Speed which defines the duration the light will have to pass through the aperture
Whether you shoot in Manual, Aperture Priority or Shutter Priority; there is no difference in how the meter evaluates the scene.
Measuring the light, or brightness of the scene you are trying to capture, is a critical component in determining the perfect exposure. To establish this you need a meter that can read the levels of brightness.
Exposure is measured using light meters. There are two types: one measures the light falling on the subject or scene, called an incident light meter; the other measures the light reflected off the scene or your subject, and is called a reflected light meter. All meters that are built into digital cameras are reflected light meters, and we will be addressing that type in this article. The more you understand these meters and how they work, the better you will be at understanding and interpreting what they are telling you. Keep in mind that incident light meters are far more accurate than reflected light meters.
How does your camera determine exposure?
Reflected light meters attempt to read the amount of light in the scene you are trying to capture. Unfortunately, those readings are guesses. You may have had the misfortune of photographing a very dark or black subject and have it come out looking over exposed, or a snow scene where the snow looks grey or under exposed. The reason for this behavior is that the camera’s light meter believes that most scenes should average to a mid-grey, also referred to as 18% grey. This middle grey is a mid-point between the darkest shadows and the brightest highlight. Since the camera meter has no idea about white or black, you need to help it using some form of exposure compensation based on the tonality of your subject or scene.
Metering Modes
To help with exposure, and determining how much to compensate, cameras have various metering modes. Typically, the three basic modes are Matrix (also called Evaluative), Center-weighted and Spot metering. Each of these is applicable in particular situations, but do not rely on just one of these modes to do it all for you.
Evaluative Metering
Evaluative Metering
In this metering mode, the meter divides the scene up into a grid and analyzes each segment for highlight and shadow (bright and dark) information. Once that data is collected, it calculates the average value and bases the exposure on that average. Keep in mind that all cameras do not have the same number of areas within the frame. Also, they don’t all calculate the average value for exposure the same way. Manufacturers use complex formulas to arrive at exposure values. Therefore, it is important that you understand how your camera behaves in various situations and learn when to trust it, and when not to trust it.
Many of the newer DSLR’s not only average the grid, but place additional emphasis on the focusing points that are in use during that particular image capture.
In the next series of images, matrix metering was used to set the exposure. Two foam-core boards, one white and the other black, were placed side by side in the same light.
For the first image, the camera read the exposure while it was pointed in the middle of the white and black boards. The camera read all the white and the black and came to a reasonable conclusion by averaging the exposure.
Evaluative Metering – centered between the white and black boards
Metered off of the white board
The next image was shot with the camera meter reading taken from the white board. The white was captured as grey and the black a darker grey. This is because the camera tries to make everything neutral grey or 18%.
Metered off of the black board
In the third image, the camera read the exposure from the black board. The resulting image is overexposed making the whites too bright, and a dark grey where it should have been black.
Center-Weighted Metering
Center-Weighted metering
This method of metering lays the most importance on the central portion of the frame, which can be as much as 75% or more, while little or no importance is given to the corners of the frame. Many pro DSLR’s will allow you to adjust the diameter of the center weight area.
This mode of metering is preferred by many photographers and has a reasonable level of accuracy. Also keep in mind that most subjects tend to be centrally placed when framing. You then obtain the exposure and recompose before capturing the image.
Spot Metering
Spot metering
This mode measures the light from a very small part of the scene only. The area measured is typically the center of the image, using approximately a 3 to 7 degree range of measurement. It is usually less than 5% of the area of the frame. With most of the mid to upper end DSLR’s, you can arrange the placement of the spot within the frame so as to identify where you want the reading captured (usually it follows where you focus).
This is a very accurate metering mode. It will provide precise readings from small areas of your scene and is most effective in high contrast situations.
The same white and blackboards were photographed once again using spot metering. As you can see in the images below, the same problem exists. Even the spot meter was fooled.
Spot metered on the black (left image) and then Spot metered on the white board (right image)
To obtain a correct exposure so the camera is not fooled, a spot meter reading was taken using a grey card placed in the same light as the black and white boards. This exposure setting based on the grey card was used to photograph the two boards. The image below shows good exposure.
Exposure reading using a grey card
How do you switch metering modes?
The Metering icon is represented by an eye-shaped graphic within a rectangle. Your camera’s metering system may have three or more metering modes and the display will change based on which mode is selected.
Which Metering Mode should you use and when?
Matrix metering
Matrix metering works well for scenes that are evenly lit. It can be used as a go-to method for grab shots. Even though the camera meter may fail you, these meters are highly sophisticated, computer controlled devices and can be relied upon for general photography. You can leave your camera on this mode and use it as a means of exposure education.
Center-Weighted metering
Use this for any scene where you want the primary subject to be correctly exposed while the rest of the image can be generally ignored for proper exposure. This is ideal for people and pet portrait photography, Still life and some product photography.
Center-Weighted is much more consistent and predictable compared to matrix metering. Use it wisely to control where the camera will be measuring the scene and those areas where the lighting does not play a key role in your composition.
Use this mode for outdoor portraits, high contrast scenes, product and food photography to name a few.
Spot metering
Spot metering gives the maximum accuracy and exposure control. This is ideal for back-lit subjects, close-up and macro photography. It can be used to read the brightest and darkest zones for landscapes. You could not do lunar photography without this mode. Remember to use this mode any time it is important to correctly expose for a subject that does not fill the frame.
Spot metering mode works exceptionally well in situations where your primary subject is much lighter or much darker than its surroundings.
Exposure compensation
In a number of situations, you will need exposure compensation to get the right exposure, irrespective of the metering mode you select. Scenics with a lot of snow cover will be under exposed and will require an increment of +1 or more stops of exposure to make the snow look white.
Conversely, a black furry bear or a person wearing very dark clothing will be overexposed and will require negative exposure compensation of -1 or more stops.
So which mode should you use?
The answer is – it depends on the subject, the light direction, etc. Choose matrix or evaluative metering for evenly lit scenes. Use Center-weighted for scenes that have high contrast and where you want the primary subject to be properly exposed. Use Spot metering for back-lit subjects until you get very familiar with metering, use the spot meter for back lit subjects.
In conclusion, for exposure accuracy an incident light meter can be helpful as your camera’s light meter is easily fooled. But learning how your camera meter works will help you get better, more accurate exposures as well.
