Camera Exposure: An Introduction
Built-in camera meters are precise and typically give great results, however there are times when you will want to over- or under-expose your image. A good exposure accurately renders an image that represents what you saw through the viewfinder. When you have too much light, the image will be overexposed and with too little light, the image will be underexposed. Correct exposure is a combination of three elements: film speed, aperture, and shutter speed.
Film speed is set and stays constant. Since light is always changing, there is no single correct exposure for the film you choose to use. The speed of your film, both regular and digital, is represented by the asa/iso number; the higher the number, the faster the film. In low light situations or when you need to freeze the action of moving subjects, you want higher film speeds; in brightly lit conditions, you want lower asa/iso. You don't want to use a high asa/iso all the time, because the higher the film speed the more grain (noise if it is digital) you will see on your final image.
The shutter speed and aperture are not constant. You change them in different combinations to achieve the result you want. Proper exposure makes the photograph, and understanding how to manipulate the exposure enables you to make creative photographs. Even with digital film, proper exposure matters. You can bring out details in dark areas and soften highlights in light areas with Photoshop techniques, but you can't make a properly exposed image from one that was taken wrong to begin with.
The opening of the diaphragm in your lens allows light to pass through the lens onto the film. The size of the opening is called the aperture, which is controlled by the aperture ring or dial. The smaller the aperture of the lens, the greater the depth of field will be. Depth of field is the area of the photograph that is perceived to be in focus. It changes depending on the size of the lens being used. The aperture is broken down into numbers called stops, or F-stops. These numbers are in one-stop increments, typically 2.8, 4, 5.6, 8, 11, 16, 22, and 32. Each one-stop increment lets in exactly twice as much light as the one before it. Therefore, F 8 lets in twice as much light as F 11 and half as much light as F 5.6.
A small opening is needed with a high F-stop to make the image. The bigger number refers to a smaller opening, and the smaller number refers to a bigger opening. The reason for using one aperture over another will be determined by the amount of depth of field you want or the desired shutter speed you want to use.
If you want greater depth of field, as in a scenic photograph where you need everything in focus, you need a small aperture such as F22. If you want to isolate the subject from the background, and only have the subject in focus, choose a large aperture such as F 5.6 or smaller.
The aperture determines how much light passes through the opening of the lens. Think of the shutter as the door and shutter speed as how long the door stays open. When you make a photograph, the shutter opens for the film to be exposed. The amount of time the shutter should be open is determined by the size of the aperture on the lens.
Differences in shutter speeds are measured in one-stop increments. Until you reach exposure times of one second or more, the numbers of your shutter speed refer to fractions of a second, such as 1/30, 1/60, 1/125, 1/250, 1/500, or 1/1000 of a sceond. You chose one shutter speed over another depending whether you want motion in the image frozen (stopped) or blurred (moving). When photographing a waterfall, you may want to freeze (stop) the scene to see every drop of water falling, even though it is in motion. Use a fast shutter speed, like 1/500. If you want to show the movement of the water and give it a more flowing (blurred) feeling to the image, use a slower shutter speed, like 1/30. You decide if you want more or less depth of field and frozen or flowing motion.
When the meter in your camera gives you an exposure for the photograph as (aperture) F 8 at (shutter speed) 1/250 of a second, you can change either the aperture or the shutter speed as long as you change the respective other to match your change. Although you can make less than full stop adjustments, for the sake of understanding the explanation is in terms of one-stop increments. If you increase the aperture one stop, you decrease the shutter speed one stop as well. For example, F 8 at 1/250 will give you the same exposure as F11 at 1/125, or F5.6 at 1/500. If you want more depth of field, you would favor a smaller F-stop, while if you wanted to freeze the motion, you would favor a faster shutter speed.
If you are making a photograph that requires a large depth of field, you will use a slow shutter speed. Handholding your camera for longer exposures gives poor results, so use a tripod or other support to steady your camera. The general rule of thumb for handholding your camera is the reciprocal of the length of the lens. If you are using a 200mm lens, the slowest shutter speed you should handhold your lens is 1/250 of a second. (Use 1/250 because it is the closest shutter speed to 1/200.) This is the general rule, however you may be steadier than most or a little shakier than others. With lenses that reduce vibration and stabilize cameras, you can handhold at slower shutter speeds. The old rule is becoming older and maybe obsolete, but always use some type of steadying device if you can; handholding is the least favorable way to photograph.
Don't be fooled
The meter in a camera is designed to average the exposure needed for a photograph. It does not see the world in color, but rather on a gray scale that assumes the entire world is about 18% gray. Most of the time this is fine, but on occasions there are enough very light or very dark areas in the photograph to throw your meter off and give you a poorly exposed photo. If you make a photograph with lots of white, for instance a white polar bear on white snow, your camera meter will see it as all gray and give you an underexposed photo, since gray is darker than white. In this case, you need to increase the exposure by one or more stops to properly expose the photo. If you make a close-up photo of a black cape buffalo on safari in Africa, you need to underexpose, because your camera meter sees it as lighter than it really is.
If you are unsure, bracket the exposures to ensure that some of the photos will be properly exposed. Bracketing is when you make one exposure at the setting your camera suggests and another one or more at exposures either over or under what your meter suggests. You might change your exposure by 1/3 to I stop depending on the situation. Bracketing is like an insurance policy; if you spend money and time to go somewhere and photograph, you might as well take a few extra frames to ensure good results. When you use digital images, you can check your monitor and histogram to make sure your exposure is on the mark.