Exposure control and the use of exposure meters.
lightmeter

Exposure refers to the amount of light that hits the film while taking a picture. Exposure is directly related to the physics concept of illumination. Illumination refers to the amount of light that hits a surface. In fact to measure the required exposure for a picture an exposure meter measures the illumination. The most common type uses a light-sensitive selenium cell to accomplish this. Light either reflected from the subject in the case of a reflected-light meter or direct from the source in the case of an incident-light meter strikes the light-sensitive selenium cell. The light energy is converted into minute amounts of electrical energy which travel through a highly sensitive s microammeter. The microammeter causes a pointer to move across a scale.  The stronger the current passing through the microammeter, the more the pointer moves.  Refinements may be added to the meter to make it more useful-various dials and scales, grids, baffles, light-collecting disks or spheres, etc.  But the light-sensitive cell remains the heart of the meter.







The exposure can be changed in two ways.

1. Increasing or decreasing the aperature - Enlargening the aperature allows more light into the camera and increases exposure.
2. Increasing or decreasing shutter speed - Slowing the shutter speed allows light to hit the film over a longer period of time and increases exposure.

Look at the two pictures to the right to see pictures with two different levels of exposure. As you can see pictures with a higher level of exposure will be lighter than those with lower exposure because more light hits the film. This can be either desirable or undesirable depending on the existing lighting conditions.

The equation for exposure is
shutter speed + aperature = exposure

 The correct exposure is the one that gives you the effect you want. Different combinations of shutter speed and aperture can produce the same exposure-but not the same visual effect.  So keep in mind the effect you want  shallow depth of field or maximum sharpness, stopped action or blurred action – when you set your camera controls.
normal
Lower Exposure Level 

 over
Higher Exposure Level

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