Table of Contents
Shutter and Exposure
Some people call the camera a time machine which brings back many forgotten moments in time. If so it will be fitting for us to call the next important component in the camera "the Time-Gate". This is the Shutter -- the component that opens and shuts to let the light through the lens the moment you press the release button on your camera. The shutter is a device in the camera which controls the length of time the light can fall on the film or the CCD image sensor. The image sensor or film in the camera needs the right quantity of light to form a picture with the proper brightness and contrast. Too much light will result in a faded picture where everything in the picture is a shade of white. Too little light will result in an overly dark picture where everything in the picture is in the shadow.
You can imagine this like cooking. If you cook your food too long or set the stove to a very hot setting your food will be overcooked and if you did not cook long enough or set the stove to a low heat you will have under-cooked food. Earlier we talked about the aperture being a device that controls the amount of light that can pass through by opening up a large opening to let in more light or a small opening to let in less light. This in our cooking example is like the heat setting of your stove such as high, medium and low and the shutter is the duration you are going to cook. If you cook with microwave oven then the aperture is the heat setting and the shutter is the timer setting of your microwave oven.
The photography term for this process is called Exposure. A picture with the proper exposure will result in vibrant colours, with many layers of details in the images, proper contrast and brightness. A picture with very dark images is the result of under exposure and a light faded picture with very little detail is an over-exposed picture. The results are actually opposite to the cooking example above. If you over-cook your food is darker but if you look at the film negative then the result is the same!
To get a correctly exposed picture the shutter has to work with the aperture to let in just the right amount of light. How do we know how much light is needed? Today we are lucky; earlier photographers had to depend on a light meter plus trial and error methods and mathematical calculations to find the proper exposure. They have put what they learned and discovered in writing and their very simplified methods are available in most of the paper boxes that come with the films we buy.
We are even more lucky as most of today's high-tech auto cameras have all these pre programmed, which will automatically adjust the shutter and aperture for the correct exposure for us. These cameras perform very well under "average" scene conditions. The exposure set by these auto cameras is almost spot on though some may be slightly over or under exposed. I use the word "average" as most auto cameras are programmed for the average conditions; they are not designed for non average settings such as a backlit scene, sunset scene, snow or desert scene, etc. Under such conditions you need to switch over to manual exposure that is if your camera has manual mode. If your camera is one of those modern wonder that has many special scenes programmed in, then switch to the built-in scene mode that matches the scene you are taking. Finally if your camera has a spot metering mode you can also use the centre spot on the view finder to spot on the part of the scene that you want to render properly exposed.
When talking about shutter we can't avoid the subject of exposure. Now let's get back to our time-gate. Like the aperture opening or F-stop, the shutter also has a set of settings. This is usually measured in second or fraction of a second and each is half or double the next one just like the aperture opening. The common settings you see in most SLR cameras are 1/2, 1/4, 1/8, 1/16, 1/30, 1/60,1/125, 1/250, 1/500, 1/1000, 1/2000 of a second. These settings work closely with the aperture setting so if you need a setting for a certain condition i.e. f-8 aperture at 1/125 second shutter speed you can alter it to use f-4 aperture at 1/500 second shutter speed and you will get exactly the same exposure result. This allows a photographer to use a variety of techniques such as wide aperture opening for a shallow depth of field, a high shutter speed to freeze an action shot of a racing car or a slow shutter speed to blur out the moving car.
In theory digital cameras don't need an electro-mechanical shutter or mechanical shutter but most digital cameras have electro-mechanical shutter and few have electronic shutter alone. The electronic shutter is normally silent and a beep or simulated shutter noise can be selected for confirmation; a simple example of a camera with electronic shutter is your web cam and the mobile phone camera. Some digital cameras have both the electro-mechnical shutter and electronic shutter working together. This type of cameras have shutter speed as high as 1/10,000 of a second.
The electro-mechanical shutter in digital cameras work differently from the film cameras. The shutter in the film cameras is always closed to keep light out and only opens to expose the film and capture the pictures. The electro-mechanical shutter in consumer digital cameras is always open to allow image preview on the LCD screen. If the shutter is closed this will not be possible. The electro-mechanical shutter is closed the moment you press the camera release button to shut out light to the CCD and perform image processing.
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