Very simply, the proposal represents a camera is a sealed box with a hole. In this box, there is a film or electronic sensor. As long as the hole is closed, there can be no light come from the film or sensor. Is there too much light then hits the photo is overexposed, there is too little light then hits the photo underexposed. The camera will therefore ensure (or the camera offers you the opportunity) to the hole just long enough to open and close on time.
There are two ways to view what the camera comes to a minimum. The size of the hole can be adjusted, and the length of time in which the hole is open can be adjusted. The size of the hole or aperture is called (in English) aperture. The difference between a random-number and aperture of the foregoing, or successive number (for example, the difference between f5.6 and f4) is called a f-stop. Figure 1.1 shows a lens, and in which some numbers are visible. These numbers are the f-stops. The smaller the number, the larger the aperture. In other words, f 2.8 lets more light into the camera than f 22.
The second way to limit the amount of light is the duration in which the hole is open to a minimum. The period is called the shutter speed or shutter. Figure 1.2 shows the display of a Nikon D200 when the aperture and shutter speed can be read. The number 100 stands for 1/100th of a second. The larger the number, the shorter the time that the shutter is open.
The combination of aperture and shutter speed determines the amount of light. If one of the two doubled, and the other half, than the amount of light will remain the same. The amount of light such as when the camera is set to f5.6 aperture and shutter 125 and when the camera is set to f8 aperture and shutter 60. Although the exposure is equal, the result is different indeed. Read more about this subject in the aperture and shutter speed.
Many SLR cameras offer different methods to measure light. The three methods you'll use most often are: central measurement, multiple exposure and spot metering. Central Measurement is most commonly used. It measures the quantity of light of about 60% of the image. In general, this is a reliable method to measure light. Evaluative metering is intended for difficult lighting situations. The quantity of light is divided between multiple zones of the image is measured. The different zones are then analyzed by the camera, taking into account the extreme light and dark areas in the image. Spot metering finally, a small amount, usually 1%, of the image is used to adjust the light quantity to be measured. Spot metering can be very useful if you put a person in snowy landscape photographs. The light snow, the camera key measurement in confusion to. The amount of light using spot metering of the face of the person measuring the camera gets confused by the surroundings.
As mentioned, a bright environment, such as a snowy landscape or a desert in the sun, the metering of a camera a lot to upset. The same goes for a dark environment. In the first case, your camera will tend to under expose the photo, so you have snow will look gray in the second case, your camera will overexpose the picture correctly.
There are a number of methods in order to prevent this problem. Earlier it was talked about spot metering. In the example in Figure 1.3 is the perfect spot metering solution. The barn has an excellent, neutral color to the amount of light using spot metering to measure. Another method is the measurement of the camera simply manually. Measure the camera in a bright environment such as a shutter speed of 1/250 with an aperture of f5.6, you can manually set the camera at a shutter speed of 1/125 (1 stop) or 1/60 (2 stops) at the same aperture. In a dark environment would you adjust shutter speed in the opposite direction, thus 1/250 to 1/500 or 1/1000. Finally you would for the measurement of the amount of light able to make use of an 18% gray card, which the subject holds for you. A gray card you can at any photo shop and you can easily carry.
e-DigiCam
