Bellows Extension

Custom Search

FACTORS THAT AFFECT EXPOSURE

Basic Photography

LIGHT METER OPERATION

When copy cameras or view cameras are used, many subjects are photographed at very close distances. When you are photographing at these close distances, it is not uncommon for the bellows of these cameras to extend beyond one focal length. The farther the bellows are extended, the larger the image size produced at the film plane. When a 1:1 subject to image ratio (on the film plane) is needed, the bellows are extended to two times the focal length of the lens; for example, when a 6-inch lens is used to produce a 1:1 ratio, the bellows are extended to 12 inches. The distance the bellows are extended is determined by measuring the distance from the optical center of the lens to the film plane.

When the bellows are extended beyond one focal length, an exposure compensation is needed. Because light must travel a greater distance, some of the intensity is lost. This loss of light intensity must be compensated for by opening up the aperture or increasing the exposure time. There are two formulas used to adjust the exposure when the bellows are extended.

Generally, the exposure time is extended to compensate for bellows extension, because view cameras and copy cameras are mounted securely and the critical aperture is used to produce the sharpest image. To adjust the exposure time, use the following formula:

Where: BE = Bellows extension

FL = Lens focal length

T = Indicated exposure time

EXAMPLE: You are photographing a document with a camera that has a 5-inch lens and the bellows are extended 7 inches. Your light meter indicated an exposure of 1/30 second at f/4. The new exposure time is determined as follows:

To adjust the aperture, use the following formula:

EXAMPLE: A 4-inch lens is extended to 4 inches beyond one focal length. The original camera settings are 5 seconds at f/11. Using the above formula, the problem is solved as follows:

f/16 RULE

You should use a light meter for most of the photographs that you take in the fleet. These light meters are either built into the camera or are separate hand-held models; however, there may be times when your light meter does not operate properly, or you do not have time to use it in order to grab an awesome shot. The f/16 rule of exposure allows you to determine basic camera exposure settings for both black-and-white and color photography without the aid of electronic devices.

The f/16 rule states: The basic exposure for an average subject in bright frontal sunlight is

Therefore, to calculate the BASIC exposure for bright, sunny conditions, set f/16 on the camera lens and use the IS0 speed of the film for the shutter speed; for example, when you use IS0 125 film, set the shutter speed at 1/125 second and the lens aperture at f/16. For IS0 64 film, set the shutter speed at 1/60 second and the lens aperture at f/16, and so on. When the camera does not have a shutter speed corresponding to the IS0 of the film, use the shutter speed that is closest to the IS0 of the film.

The f/16 rule is based on the correct exposure for an average subject under bright, sunny conditions. If the sun goes behind a cloud, however, then the lighting on the subject is decreased and you must change the basic exposure. The aperture settings for different daylight intensities are as follows:

Bright sun on light sand or snow-f/22

Bright sun-f/16

Cloudy bright-f/11

Cloudy-f/8

Heavy overcast or open shade-f/5.6

For each of these different daylight intensity situations, you begin with the ISO speed to determine the shutter speed, set the aperture to f/16, and open up or stop down the aperture for the lighting conditions.

After calculating the exposure, you can change the setting to any equivalent exposure; for example, if you determine the required exposure to be 1/500 second at f/5.6 but you wish to use a small aperture for greater depth of field, you can change the setting to 1/60 second at f/16.

Remember, the f/16 rule provides you with a basic exposure for front-lighted subjects only. When the subjects are side-lighted or back-lighted, you must double or quadruple the exposure, respectively.

Because many cameras are fully automatic, you may wonder why you need to know basic exposure. There are three good reasons for knowing and understanding the basic principles of exposure. First, you want to control the depth of field and stop action instead of the camera controlling it. Second, a light meter cannot think All a light meter does is respond to the light it receives. You must know when to override the camera; for example, when the subject is side-lighted or back-lighted. Third, meters are mechanical and can fail. They can be inconsistent, consistently wrong, or fail altogether. When you can workout in your head, roughly what the camera exposures should be, you will know when the camera or light meter is wrong. Knowing when a light meter is giving incorrect readings could make the difference between success or failure of an important photographic assignment.

Privacy Statement - Copyright Information. - Contact Us

The correct use of a light meter greatly increases the accuracy in determining your camera exposure. You should also understand that the incorrect use of a light meter can result in consistently unacceptable results. To assure consistently acceptable exposures, you must become thoroughly proficient with the correct operation of a light meter.

A light meter can be either built into the camera or a separate hand-held unit (fig. 4-19). Both types are sensitive instruments and should be handled with care. There is little maintenance, but they do require batteries. When you think a light meter is not working properly, have it checked by a qualified technician. Always be sure to check your equipment before leaving on an assignment. Like all camera equipment, careless handling and excessive heat and moisture limit the life of a light meter. A light meter must not be subjected to high temperatures for prolonged periods of time. Unless the light meter is designed for underwater photography, it should be protected in inclement weather.

There are two methods of measuring light with hand-held light meters. These two methods are the incident-light method and the reflected-light method.

This method requires the use of an incident-light meter. An incident-light meter has a diffusing dome that covers the photoelectric cell. When an incident-light meter reading is taken, the meter is held at the position of the subject with the photoelectric cell pointed toward the camera. The meter measures the light falling upon the scene. The incident-light method of measuring light is used extensively in motion-media photography and gives fast accurate results in all photography.

Most light meters are designed for use as either incident-light or reflected-light meters. By removing the diffusion dome from the photoelectric cell, you can use the meter to measure reflected light.

When you are taking this type of light-meter reading, the diffusing dome should be removed from the photoelectric cell and the meter pointed toward the subject.

A reflected-light meter receives and measures the light reflected from a scene within the angle of acceptance of the meter. The term angle of acceptance compares to the term angle of view of a lens. Both are predetermined during manufacturing. The angle of acceptance and the distance between the meter and the scene are the controlling factors as to how much of the reflected light from the scene is measured by the meter. When the angle of acceptance is greater than the angle of view of a lens (when using a telephoto lens for example), the meter should be moved closer to the scene.

Light meters that are built into the camera are reflected-light meters. When these meters are used, the angle of acceptance is not greater than the angle of view of the lens being used. The meter measures the light from the scene as seen by the lens.

Some reflected-light meters have angles of acceptance between 1 and 4 degrees. These meters can be used from a distance to measure the reflected light from specific objects within a scene. Exposure meters with angles of reflectance this small are called spot meters.