As with modern "point and shoot" film cameras, most digital cameras have auto exposure settings. The cameras sense the brightness of the light in the direction the lens is pointing, and automatically adjust for optimum image quality as determined by the camera. For most images, the photographer can rely upon the camera's auto adjust feature for excellent images. However, in certain instances manual exposure adjustments would provide better image quality.


For example, when images are being collected for use in a panorama, some areas of the panorama will be brighter than other areas. If the auto exposure is set, some images may appear darker or lighter than others, which will produce stitching lines between the images when stitched. If the camera has a manual adjustment, these stitching lines can be eliminated.

The image above shows two images which make up a panorama. The image on the left includes a daylight sky. The camera automatically adjusted for this, which made the walls of the landform look dark. When the shot on the right was taken, the light was not as intense, so the camera adjusted the exposure and lightened up the picture. Therefore, the rock wall on the right looks brighter than the same rock wall on the left. It would have been better to set the camera exposure instead of using auto exposure.

What determines the image exposure? In other words, what influences the amount of light entering the lens during imaging?

The Effect of Shutter Speed
Compare the images obtained for various camera shutter speed settings.
Setting = 1
Setting = 2
Setting = 4
Setting = 8
Setting = 15
Setting = 30
The longer the shutter is open, the brighter the resulting image. What is the relationship between the camera setting and the time the shutter is open? The movie below was taken by a video camera that records images at a rate of 30 frames per second. Step through the movie using the arrow keys to determine how many frames the shutter is open at the various settings. Compare the camera setting to the time the shutter is open (time = number of frames/30).
The inverse of the camera shutter speed settings represent the time period which the shutter is open. For example, a setting of 4 means that the shutter is open 1/4th of a second; and for a shutter setting of 60 means that the shutter is open for 1/60th of a second. A tripod is best used when the shutter speed 1/125th of a second or longer.
The Effect of Shutter Aperture
For some cameras, the size of the shutter opening can be adjusted. This is usually done with a device call a diaphragm on a Single Lens Reflex camera. If the shutter opening, or aperture, is large, more light can come into the camera for a given shutter speed. The object movie shown below illustrates the changes in the shutter opening using a lens from a SLR camera.

The aperture of a lens is usually referred to in terms of an f-number (focal ratio number). A large aperture has a small f-number (such as 1.8). A very small aperture would have a larger f-number (such as 22).

Digital cameras that allow for manual exposure settings may enable the photographer to change either the shutter speed or the aperture f-number or both. Sometimes the camera will use icons selected by the photographer which set both the shutter speed and f-stop based on what the icon represents. This is demonstrated below, looking at the display screen of the Sony Cybershot DSC-30.

Twilight Mode:

Suppresses the washed-out color of a bright subject in a dark place so that you can record the subject without losing the dark atmosphere of the surroundings.

Twilight plus mode:

Increases the effectiveness of the twilight mode function.

Landscape mode:

Focuses only on a distant subject to record landscapes, etc.

Panfocus mode:

Changes the focus quickly and simply from a close subject to a distant subject.

 

Spot light-metering mode:

Select this mode when there is backlight or when there is strong contrast between the subject and the background, etc. Position the point you want to record on the spot light-metering cross hair.

 

The Default Setting

 

Aperture priority mode:

Makes the subject stand out against an unclear background or makes both the subject and the background stand out clearly.

Shutter speed priority mode:

Records a sharp picture of a fast moving subject or the flow of motion of a moving subject.


©2000-2002 Dr. John Park for Science Junction, NC State University.
All rights reserved.
URL: http://www.ncsu.edu/sciencejunction/route/usetech/digitalcamera/expose/expose.html
Last updated 07/25/02

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