In the segment on exposure, photo enthusiasts learnt about Canon's built-in multiple metering systems for its EOS AF 35mm SLR cameras and the right way to use them to achieve correct exposures.Metering modes like the multi-segment Evaluative, Partial and Spot, have their own unique way of reading the reflected light off the subject and then calculate the exposure based on the metered light readings.Most photo enthusiasts will not face any problem understanding what the various metering systems are all about, more so if they are already Canon EOS camera users. But the newcomers, who have just started to learn about photography, may not understand them and know very little about what exactly metering is in photographic term.
Photography is all about using light. You use light to make 
photographs via the lighting available. The lighting can be from any sources like natural light, available light, artificial light or from studio strobes and flash guns.You need light to expose the film and to record the image you have visualised. The camera body controls the timing for the light to expose the film via the shutter button which triggers open the shutter curtains. And the camera lens controls both the aperture and the image quality by determining the amount of light required to enter and expose the film through the optical elements that also form and focus the image.
photographs via the lighting available. The lighting can be from any sources like natural light, available light, artificial light or from studio strobes and flash guns.You need light to expose the film and to record the image you have visualised. The camera body controls the timing for the light to expose the film via the shutter button which triggers open the shutter curtains. And the camera lens controls both the aperture and the image quality by determining the amount of light required to enter and expose the film through the optical elements that also form and focus the image.But how much light does one need in order to make a photograph? That will be determined by the camera's built-in metering systems or by using a hand-held incident light meter which is used to measure the amount of light that is reflected off or fell on the subject.Metering, then, is about measuring the existing levels of lighting so the right shutter speed or aperture values can be set to obtain the correct exposures - to let in the right amount of light needed to strike the film and form the image.
A meter reading can be something like this: 1/250 sec. at f/11 or 1/500 sec. at f/8.0. It has to be noted that to get the right meter reading for a given exposure, the camera's built-in metering system have to know what type of film speed is in use.For example, if an ISO 100 film is used, the metering system will calculate the exposure settings based on the current lighting situation to ensure the right amount of light needed to expose the film.If an ISO 100 film is used to photograph a scene at dusk, the camera's built-in meter will recommend an exposure of 1/60 sec. at f/4.0 in order to get the correct exposure value. If the film used is an ISO 400 film for the same scene, the camera will then recommend the settings to be at 1/60 sec. at f/8.0. This too, will depend on which shooting mode the camera is set.All EOS cameras are designed to base their exposure readings on the shutter speed set by the photographer when used in the Manual Exposure or Shutter-Priority AE modes. If the camera is used in the Aperture-Priority AE mode, the exposure readings will be based on the aperture set by the photographer. The exposure will then be at f/4.0 with a shutter speed of 1/250 sec for the dusk scene when shooting with an ISO 400 film.
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