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Exposure Control

Exposure Controls
Choosing the Best Combination of f-Number And Shutter Speed
Selecting the f-number
Selecting the Shutter Speed
Depth of Field
Depth-of Field/ Hyperfocal Distance

Exposure Controls

The two controls on adjustable cameras that regulate the amount of light reaching the film are shutter speed and lens opening (also called aperture or f/stop). Setting these two controls correctly lets you take properly exposed pictures. With manual cameras, you adjust the shutter speed and aperture controls until the camera's meter indicates you have set the proper exposure. 

Automatic cameras, on the other hand, adjust the shutter speed or lens opening (or both) automatically, after determining an optimum exposure setting. Automatic cameras equipped to handle film with DX-encoding designations even set themselves for the speed of the film you're using. 

Whether your camera uses a built-in meter to guide you in setting aperture and shutter speed or sets them itself, you should understand the basic premise behind shutter speed and aperture to gain greater control over image quality. The shutter speed controls the length of time the film is exposed to light. Shutter speeds are indicated by the numbers 1, 2, 4, 8,15, 30, 60, 125, 250, 500, 1000, and 2000. The speeds may be marked on a dial or shown on a liquid crystal display (LCD) panel atop the camera or in the viewfinder. Your camera may not have all of these speeds. The numbers represent fractions of a second (except 1 second) and mean 1/2, 1/4, 1/8, 1/15 second, and so on. You can use the B setting to make time exposures-the shutter will stay open as long as you press the shutter release. For more precise control of time exposures, some advanced cameras allow you to set electronically timed shutter speeds of up to several minutes. 

The lens openings on cameras are indicated by f-numbers The larger the f-number, the smaller the lens opening. Each smaller (size) lens opening marked on the lens opening scale lets in one-half the amount of light as the preceding opening. If you change from a small lens opening to the next larger one, the lens will let in twice as much light. On some camera lenses, the maximum lens opening may not let in twice as much light as the next smaller opening. You can also set the lens opening between the marked settings on the lens for finer changes in exposure. 

Changing from one shutter speed to a speed that is twice as fast, for example 1/60 to 1/125 second, allows the light to strike the film for half as long; therefore half as much light reaches the film. Changing to a shutter speed that holds the shutter open twice as long, for example 1/60 to 1/30 second, lets twice as much light strike the film. 

The size of the lens opening on your camera is the other factor that controls the amount of light that reaches the film. The different sizes of lens openings are indicated by f-numbers. These numbers form a series, such as 1.4, 2, 2.8, 5.6, 8, 11, 16, and 22, marked on the camera lens or shown on an LCD panel. The smallest f- number refers to the biggest opening. The largest f-number is the smallest lens opening. 

When you change from one lens opening to the nearest number, you're adjusting the lens by 1 stop. If you move the setting to the next larger one, for example f/11 to f/8, the area of the opening is doubled, so you expose the film to twice as much light. Changing from one lens opening to the next smaller one, for example f/11 to f/16, cuts the light by half. 

Automatic-exposure cameras dominate the camera market. Electronic sensors and microprocessors have not only taken the guesswork out of correct exposure but the labor as well. The camera sets the shutter speed and aperture the moment you press the shutter release. Cameras that measure the light reflecting off of the film itself can even adjust these settings as the exposure is occurring. 

Choosing the Best Combination of
f-Number And Shutter Speed

There are many combinations of shutter speed and lens opening that will allow the same amount of light to reach the film for proper exposure. These are known as equivalent exposures. If you change from one shutter speed to the next higher speed, this lets half as much light expose the film. You should keep the total amount of light-the exposure-the same by opening the lens to the next larger lens opening. It also works the other way around. If you change to the next slower shutter speed which lets in twice as much light, you should use the next smaller lens opening to let in the same amount of light as before. 

Besides obtaining the proper exposure, you might want to use a particular combination of lens opening and shutter speed for three good reasons:

               1. To reduce the effects of camera motion. A good,
                   general-purpose shutter speed to achieve this is
                   1/125 second. A higher shutter speed of 1/250
                   second may even produce sharper pictures. With
                   telephoto lenses even higher shutter speeds may
                   be necessary.

               2. To stop action. A shutter speed of 1/125 second
                   helps stop the action of someone walking, for
                   instance. However, there may be times when you
                   want to use a higher shutter speed to stop fast
                   action, such as a person running.

               3. To control depth of field. By using a small or a
                   large lens opening with the appropriate shutter
                   speed to maintain the correct exposure, you can
                   increase or decrease the range of sharp focus, or
                   the depth of field.

Selecting the f-number

f/2-Good for obtaining enough exposure in poor lighting conditions, such as existing light. Minimum depth of field-very shallow. Poorest image quality for specific lens. 

f/2.8-Good for obtaining enough exposure in poor lighting. Shallow depth of field. Helpful to throw background out of focus to concentrate attention on subject. Good image quality.

f/4 and f/5.6-Best image quality for specific lens. Better depth of field than with larger lens openings. Good for limited distance range of sharp focus. Good for obtaining proper exposure when lighting conditions are less than optimum, such as on cloudy days or in the shade. 

f/8-Moderate depth of field. Good all around lens opening to use for outdoor daylight pictures. Excellent image quality. 

f/11-Great depth of field. Good all around lens opening to use for outdoor daylight conditions. Excellent image quality. 

f/16-Very slight loss of sharpness due to optical effects. When maximum depth of field is important, the benefits from increased depth of field with this lens opening outweigh the disadvantages from an almost imperceptible loss in sharpness. 

Selecting the Shutter Speed

B (Bulb)-Use camera support, such as a tripod. Shutter remains open as long as shutter release is depressed. Good for obtaining great depth of field with small lens openings in outdoor night scenes, for photographing fireworks and lightning, and for recording streak patterns from moving lights at night, such as automobile traffic. Long exposures can cause an overall color cast with color films.

1 second and 1/2 second- Use camera support, such as a tripod. Good for obtaining great depth of field with small lens openings and enough exposure under dim lighting conditions, such as existing light or photolamps. Good for photographing inanimate objects and stationary subjects. These shutter speeds can cause a very slight color cast with some color films.

1/4 second-Use camera support. Maximum slow shutter speed for portraits of adults. Good for obtaining great depth of field with small lens openings and enough exposure under dim lighting conditions. Good for stationary subjects.

1/8 second-Use camera support. Better shutter speed than 1/4 second for photographing adults at close range. Good for obtaining great depth of field with small lens openings and enough exposure under dim lighting conditions. Good for stationary subjects.

1/15 second-Use camera support. Some people can handhold their camera using this shutter speed with a normal or wide-angle lens on the camera. This is possible if the camera is held very steady during the exposure. Good for obtaining increased depth of field with small lens openings and enough exposure under dim lighting conditions, such as existing light.

1/30 second-Slowest recommended shutter speed for handholding your camera with a normal or wide-angle lens. Camera must be held very steady for sharp pictures. Good all around shutter speed for existing-light photography. Good for obtaining increased depth of field with small lens openings on cloudy days or in the shade.

1/60 second-Good shutter speed to use for daylight pictures outdoors when the lighting conditions are less than ideal, such as on cloudy days, in the shade, or for backlighted subjects. Useful shutter speed for increasing depth of field with a smaller lens opening. Also, good shutter speed to use for brighter existing-light scenes. Less chance of camera motion spoiling the picture than with 1/30 second. Recommended shutter speed* for electronic flash with many SLR cameras.

1/125 second-Best all around shutter speed to use for outdoor daylight pictures. Produces good depth of field with medium to small lens openings under bright lighting conditions, minimizes the effects from slight camera, and stops some moderate kinds of action, such as people walking, children playing, or babies not holding still. This is the minimum safe shutter speed for handholding your camera with a short telephoto lens, such as those shorter in focal length than 105mm. Recommended shutter speed* for electronic flash with some SLR cameras.

1/250 second-Good for stopping moderate fast action like runners, swimmers, bicyclists at a medium speed, running horses at a distance, parades, running children, sailboats, or baseball and football players moving at a medium pace. Good all around shutter speed for outdoor daylight pictures when you don't require great depth of field and you want to stop some action. Helps minimize the effects of camera motion. Good shutter speed to use for handholding your camera with a telephoto lens up to 250mm in focal length.

1/500 second-Good for stopping fast action like fast moving runners, running horses at a medium distance, divers, fast moving bicyclists, moving cars in traffic, or basketball players. A good shutter speed to use for stopping all but the fastest kinds of action. Gives better depth of field with the appropriate lens opening than 1/1000 second. Excellent shutter speed to use with telephoto lenses. Good for lenses up to 400mm in focal length with a handheld camera.

1/1000 second-Good shutter speed for stopping fast action like race cars, motorcycles, airplanes, speedboats, field and track events, tennis players, skiers and golfers, for example. This shutter speed gives little depth of field because it requires a large lens opening. Excellent shutter speed to use with long telephoto lenses up to 400mm in focal length with a handheld camera.

1/2000 second-Best shutter speed for stopping fast action like motor sports, racquet games, and other endeavors where movement may be quicker than the eye. This shutter speed requires the largest lens opening and gives the least depth of field. Outstanding shutter speed for use with long telephoto lenses up to 400mm in focal length with a handheld camera.

Note: It's important to hold your camera steady for all the shutter speeds recommended for handholding. You can also use slower shutter speeds than those mentioned for telephoto lenses when you put your camera on a firm support like a tripod. If in doubt about stopping the action, use the highest shutter speed you can for the conditions. 

*See your camera manual for recommended shutter speeds for flash pictures.

Depth of Field

Depth of field is the distance range within which objects in a picture look sharp. As you gain a sound understanding of depth of field, you can use it as a very effective control for making better pictures. 

What are the primary factors affecting depth of field? Depth of field varies with the size of the lens opening, the distance of the subject focused upon, and the focal length of the lens. Depth of field becomes greater as

              1. the size of the lens opening decreases, the subject
                  distance increases.

              2. the focal length of the lens decreases.

              3. and subject distance remains unchanged.

An object at the distance focused upon will be the sharpest thing in the picture. But image sharpness doesn't suddenly disappear at the limits shown. Points closer or farther away than the distance focused upon will be less sharp, but will look acceptably sharp to the eye throughout the depth-of-field zone. Objects close to the depth-of-field zone may appear almost sharp. But the farther an object is from this zone, the more out of focus it will appear. In looking over these illustrations you can see that there are times when accurate focusing is very important because depth of field is slight. These include times when you're using a long-focal-length lens or a large lens opening or when you are close to your subject. Of course, a combination of these factors makes accurate focusing even more important. For example, let's assume you're using a 135mm telephoto lens on your camera. If you're focused on a subject 14 feet away with a lens opening of f/4, your depth of field will extend from about 13 1/2 feet to 14 1/2 feet. This doesn't allow much room for focusing error! 

You can use depth of field as a control in your pictures. In some shots you'll want as much depth of field as possible. For example, in shooting a scenic picture you may want to include tree branches in the foreground as an interesting frame. To get both the branches and the distant scene in sharp focus, you may use a wide-angle lens and a small lens opening for great depth of field. 

In other situations you may not want so much depth of field. You may be photographing a very interesting subject. But what if the background is confusing? You can use a large lens opening, perhaps combined with a long focal-length lens, to produce shallow depth of field. The disturbing background will be out of focus so as not to detract from your subject. The shallow depth of field will help focus attention on the main subject. 

You'll probably want to have the foreground objects in sharp focus in most of your pictures. But you may want to make exceptions now and then to produce creative results. Sometimes an out-of-focus foreground can add interest, excitement, color, glamour, and intrigue to your photograph. 

If you are using a manual-exposure camera, selecting the proper aperture for creative depth of field control is easy. Most auto-exposure cameras also provide a means to manipulate the aperture/shutter-speed combination to achieve maximum or minimum depth of field. There are several options for accomplishing this. 

To achieve extensive depth of field, for instance, some automatic cameras have a special depth-of-field program mode. Once set to this mode, the camera will select a shutter speed and aperture combination that gives priority to choosing the smallest possible aperture. Similarly, many auto-exposure cameras have an aperture- priority mode that enables you to control depth by allowing you to set the specific shooting aperture. Choose a large aperture such as f/2.8 for shallow depth; or a small one such as f/11 for more depth. The camera will then choose a corresponding shutter speed for correct exposure. Or, if your auto camera allows, you can switch it to full manual and use it as a manual camera. 

Depth-of Field/ Hyperfocal Distance

What part of the scene will fall within the depth of field? You can find out by using the depth-of-field scale on your lens. If there's none on the camera or lens, see the depth-of-field tables in your camera or lens instruction manual. 

The lens depth-of-field scale not only helps you put the depth of field where you want it but also helps you get the amount of depth of field you want. If you're taking a scenic picture, for example, you'll probably want all the depth of field you can get. If you simply focus on the distant scene, you'll be focused on infinity. But that's wasting a lot of depth of field. To have the most distant object in focus and also as much foreground as possible in focus as well, you can use a technique based on the hyperfocal distance. 

To figure the hyperfocal distance, first set your lens to infinity. Then use the depth-of-field scale to read the nearest distance that will be in sharp focus at the aperture you are using. When you focus a lens on infinity, the near distance beyond which all objects are in acceptably sharp focus is the hyperfocal distance. For example, with a 50mm lens set at f/16 and focused on infinity, the near-limit indicator on the depth-of-field scale shows that all objects from 15 feet to infinity will look sharp. The hyperfocal distance is 15 feet. 

If you now refocus the lens to the hyperfocal distance (by setting the hyperfocal distance across from the focusing index), 15 feet in this example, objects from half the hyperfocal distance, 7 1/2 feet, to infinity will appear in sharp focus. Using the hyperfocal distance will always give you the greatest depth of field for that particular lens opening. As you open the lens aperture to larger openings, the hyperfocal distances get farther away and the depth of field decreases. 

With an auto-focus camera, you can either use the aperture that the camera has selected, or use an aperture-priority or full-manual exposure mode so that you can select a specific working aperture. Then with your lens focused on infinity, read the hyperfocal distance from the depth-of-field scale. Finally, switch your lens to manual focus and set it for the hyperfocal distance. Not all autofocus cameras have a depth-of-field scale or allow manual focusing. 

Very often it's beneficial to know what distance to focus your camera on to get everything sharp within a range of medium distances. This is especially helpful when you're taking pictures rapidly without enough time to consult the depth-of-field scale or if there is no such scale on your lens. As a general rule, approximately 1/3 of the depth of field is in front of the point of sharpest focus with 2/3 of the depth of field behind. As a result, you should focus on a distance 1/3 of the way from the nearest object you want sharp to the farthest. For example, for objects within a 5-to 20-foot range, you should focus on 10 feet and use the smallest lens opening that you can. This rule does not apply to very close subjects or to those at great distances from the camera, including those at infinity.

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