Let’s suppose that we are outside on a beautiful summer day trying to take a picture of the family dog. We are using a film camera. We finally get the dog to lie still. You point the camera at him. What happens? Light goes into the camera lens and hits the shutter. In other words, nothing happens yet. Now let’s say that the dog looks really cute and you decide to snap a picture. What happens? When you press the button, the shutter opens for a very short period of time. A small amount of light passes through and hits the film at the back of the camera. This creates an upside-down and reversed (反向的)image on the film.

　　When you finish the roll of the film, you can take it to the photo shop to develop it and you will have a great picture of your dog!

　　Cameras come with different lens lengths. Why does it matter? Many small cameras have shorter focal lengths, which means that there is a small distance between the lens and the place where the light focuses at the back of the camera. This gives you a large view of the area you are taking a picture of. Lenses with a long focal length show a smaller area but allow you to focus on distant objects and make them bigger. They are often called telephoto lenses. A good example of a long focus lens is one that is used by sports photographers to get photos of football players as if they were standing right beside them.

　　Digital cameras

　　In digital cameras, the light falls not on film but onto a sensor (传感器) called a CCD (Charge Coupled Device). This digitally converts(转变) light and color into a digital information or pixels (象素). The CCD is the heart of any digital camera and usually the most expensive part ---- depending on how good it is.

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