Posted by: admin 4月 17th, 2012
By definition, the camera is a mechanical device made to control
the way in which light is recorded on film.
And built into the lens and camera body are two controls which make this function possible.
Firstly, there is the diaphragm in the lens which can be adjusted to vary the size of the aperture or lens opening. This allows a measured amount of light to pass through to the film. A large aperture allows a large amount of light, while a narrow aperture passes a small amount.
The size of the aperture also has an optical effect on the image, because it determines how much depth-of-field (the zone of sharp focus around the subject) can be recorded. As the aperture decreases in size, a greater range of focus between background and foreground becomes sharp, while a large aperture will give a smaller depth of field, where less of the background and foreground is in sharp focus.
The shutter -the second mechanical device which controls the recording of light - is normally built into the camera body, and is basically a timing device connected to a mechanism for exposing the film to light.
There are two main types of shutters - the leaf shutter which consists of several small, overlapping metal blades, and the focal-plane shutter which is placed in front of the film. Most cameras are now manufactured with focal plane shutters, partly because they allow very fast shutter speeds to be attained.
The shutter speed is the amount of time the shutter remains open, that is, the time in which film is exposed to light. In manually adjustable cameras, shutter speeds usually range from one second onwards in progressively shorter times. These times are halved at each interval such as half a second, a quarter of a second, an eighth of a second, and so on.
Until recently, the fastest attainable shutter speed was l/1000th sec or 1/2,000th sec. But new technology applied to the making of camera shutters has enabled speeds of 1/4,000th sec and an incredible 1/8,000th sec to be reached.
The B shutter setting provided in the shutter speed control allows long exposures of more than a second to be made. The B stands for Bulb, a left-over term from the days when the shutter was released by squeezing a rubber bulb connected to the camera. Nowadays, the B setting as well as the shutter release control can be released by attaching a cable release to the shutter control.
The shutter speed allows a photographer to control the apparent motion of a moving object in the picture. For example, a slow shutter speed of 1/15 sec or I/8th sec will record the slightly blurred image of a speeding runner or a show diver in mid air, thus conveying a sense of motion in the picture, while a fast shutter speed of 1/250 sec or 1/500 sec will capture a much sharper image of the runner or diver frozen in motion,
There are two different ways in which the same amount of light can he delivered to the film to give a correct exposure. These can also be combined to
produce interesting and varied results according to the way in which the photographer wishes to interpret the scene.
For instance, a small aperture of f/ 16 and a slow shutter speed of 1/30th sec or 1/I5th sec will help to produce a photograph of a fast fading sunset and a panoramic view of Hongkong with considerable depth of field where everything in the foreground and the background is in sharp focus.
A large aperture of f/2 and a faster shutter speed of 1/500th sec or 1/250th sec will produce a picture where the setting sun and background areas will be in softer focus, while only the properly focused areas will appear sharp.
For manual settings of exposure and aperture controls, the photographer should first determine whether the aperture and its depth of field or the shutter speed is more important for the effect he wants to achieve. But it must be remembered that the shutter speed works in concert with the aperture; change one and the other must also be changed.
Unlike manual controls, automatic cameras adjust exposures by varying the settings continuously.
Some cameras set both the shutter speed and the aperture, while others allow the photographer to have control over either the aperture or the shutter speed while the camera automatically sets the other during exposure.
In aperture priority mode the photographer sets the aperture and determines the depth of field in the picture, while the camera sets the shutter speeds.
So in dim lighting conditions, or when the least depth of field is wanted in the picture, set the lens aperture to its widest setting.
In strong lighting conditions, or where great depth of field is required, use a smaller aperture and the camera will adjust the shutter speed accordingly. field is required, use a smaller aperture and the camera will adjust the shutter speed accordingly.
But care should be taken when photographing action scenes with an aperture priority mode to ensure that the shutter speed set by the camera will stop the action without too much blurring. If the speed drops too low, open the aperture as required until the camera sets a more suitable faster shutter speed.
With shutter priority mode, the photographer sets the shutter speed and the camera determines the aperture. So when the shutter speed is changed or the light level alters, the aperture will automatically change to suit it.
Always check the aperture read out when using the shutter priority mode before shooting to make sure there is enough depth of field, if the aperture is too large, lower the shutter speed until the camera sets a more appropriate lens opening.
Fully programmed exposures respond to changes in light levels by adjusting both the shutter speed and the lens aperture, either simultaneously or alternatively.
Most programmed exposure modes are based on a compromise between an adequate shutter speed to freeze movement and a sufficient aperture size to give reasonable depth of field in the picture.
So the photographer may wish to use a high speed film to help keep shutter speeds from dipping too low when taking pictures of action scenes or low light photographs.
Programmed exposure modes are best for casual photography when exposure settings and depth of field are not too important.