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Photography Question 
Mike Cascagnette

how metering works?

this is probably a dumb question but, how does the function of metering work? I have never used it,I just use the point and shoot method,but I need to go beyond that method and get a clue.Thanks in advance.

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6/20/2003 12:26:23 PM

Laljit S. Sidhu
BetterPhoto Member Since: 8/13/2002
  Metering is the method by which a photographer, through his equipment, measures the quality of light in order to determine proper exposure.

Basically, you have two types of light meters - reflective and incident. A reflective light meter (found in most cameras), reads the light that is being bounced of the subject and "reflected" back at the meter (the camera in this case); An incident light meter (in the form of a handheld meter) takes a reading of the light that is actually falling on the subject. These are the types of meters that have a white semi-sphere on them. Reflected light meters are the ones found in cameras and in handheld meters; incident light meters only come in the handheld version.

In both cases you will get back two values from the reading ... aperture and shutter speed. Aperture readings are often in the following range: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, and so on. Each of these is referred to as a "stop" and more, specifically an "f/stop" when discussing aperture values. The technical meaning of these numbers is a little beyond the scope of what you asked in this question and a previous one. But know that this is one of the values provided by a meter.

In conjunction with aperture, the meter will also give you a shutter speed reading. These can vary from 1 sec or slower all the way to 1/8000 of a second (depdending on the maximum shutter speed of your camera). These are also measured in "stops"

Both the aperture and shutter speed value control the amount of light that comes into the camera and records on film. The aperture value tells the camera how wide to make the hole and the shutter speed tells the camera how long to keep it open. So, the wider the hole and the longer it is kept open the more light will come in. These values are based on the meter reading taken by the reflected or incident meter. With experience and a little knowledge, a photographer can learn to adjust both the aperture and shutter speed to get various effects. Most cameras have different shooting modes (see below) in order to allow for easier control of these values.

Some of the more advanced cameras have
different types of metering modes such as the following

Evaluative/Matrix: This mode is the most sophisticated and probably one of the most accurate in most situations. This mode evaluates the ENTIRE scene, gives weight to different parts of it, and uses sophisticated algorithms in the camera's computer to determine the necessary aperture and shutter speed to render the scene on film.

Partial: This mode takes takes a reading from the center of the viewfinder ... on the EOS 3, this is about 8.5% of the image seen.

Spot: This mode is used to meter of a particular part of the scene; i.e., a small "spot" in the scene.

Finally, some other cameras have other modes, such as center weighted (which is similar to partial, but takes into account the entire scene with emphasis on the center, versus just using the center) and multispot (which allows you to take different spot reading to find your own average values)

In general, for most photography, evaluative/matrix metering works great; For a little more control of the values, most photographers use spot metering.

Depending on the type of camera one has, there are a number of different shooting modes available to photographers.

It is important to know that shooting modes are INDEPENDENT of metering modes; how you meter a scene (spot v. evaluative) has nothing to do with how you shoot it.

The shooting modes are as follows:

Program (p): In this mode, the camera sets the aperture and shutter speed based on the values given by the meter. Sometimes, a camera will allow you to shift these values to another equivalent for creative effect by a "Program Shift".

Aperture Priority (Av): In this mode, you set the aperture and the camera sets the shutter speed that the meter determined is needed for a correct exposure at that aperture value.

Shutter Priority (Tv): The Tv comes for "Time Value" and in this mode, you set the shutter speed and the camera sets the aperture value that the meter determined is needed for a correct exposure at that shutter speed.

Manual: In manual mode, you set both the aperture value and the shutter speed and the camera's meter is used to determine whether the values you are setting will result in a correct exposure.

There are other modes as well, such as bulb mode (used for really long shutter speeds), depth of field mode (used for sophisticated management of aperture), and a various Programmed modes for specific situations (action, night, portrait, etc.) These Programmed modes are often found on more consumer oriented cameras and are slightly more sophisticated versions of the Program mode. In these modes, the camera sets the aperture and shutter speed values for specific situations.

Hope this long winded response helps somewhat.

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6/21/2003 9:39:52 AM

Laljit S. Sidhu
BetterPhoto Member Since: 8/13/2002
  Oh ... and in case my response didn't help with your question. I would recommend looking at the following:

These are great sites with useful information on photography

I would also consider picking up a copy of a Magic Lantern Series guide to your camera. These are excellent books that go beyond the manual that comes with the camera.

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6/21/2003 9:51:01 AM

Mike Cascagnette   Thanks, you have answered my question and a few others as well.
Thanks again.

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6/23/2003 5:56:37 AM

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