camerathunder.com

Here’s what usually happens: when you apply pressure to the camera’s shutter release, the camera’s microcomputer samples the scene in front of the lens and determines how much light is needed to adequately expose the scene. With most digicams, the camera selects a shutter speed and aperture combination that is sufficient to get the job done. But, you might be wondering, how does it choose? After all, there are a lot of shutter speed/aperture pairs that will work. To take the same properly exposed picture at ISO 100, any of these combinations should be exactly the same:
Usually, the camera uses the following logic:
The photographer wants to take a picture using the fastest available shutter speed to minimize camera shake and motion blur from objects moving inside the picture.
Though there are some exceptions, most cameras tend to choose the combination that allows for the highest available shutter speed, limited only by how small they can make the aperture given the current lighting conditions and ISO setting.
This isn’t always what you want your camera to do, though, and in fact you might sometimes want to choose a slower shutter speed, overexpose the image, underexpose it, or perhaps base the exposure on a completely different part of the picture. That’s why you might want to investigate your camera and look for controls that let you tweak the shutter speed and aperture.

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