How Digital Cameras Choose Aperture and Shutter Speed

May 21, 2010

You might ask: How do digital cameras choose aperture and shutter speeds for photographs?

Usually when you apply pressure to the camera’s shutter release, the camera’s microcomputer samples the scene in front of the lens and decides how much light is needed to adequately expose the scene. With most digital cameras the camera selects the shutter speed and aperture combination that is just sufficient enough to get the picture taken.

You might wonder, how does the camera make its choice? When you consider how many shutter speed/aperture settings that work, how does the camera know which one to choose? Add to that with the same ISO setting you can have different aperture and shutter speed combinations.

Point and Shoot Digital Camera

Point and Shoot Digital Camera

Typically, the digital 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.

Most cameras will 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 situation and the ISO setting.

However, that isn’t what you always want the camera to do. Sometimes you might want to choose a slower shutter speed, over expose the image, under expose the image, or perhaps base the exposure on a particular portion of the picture. You should investigate your camera and look for the controls that let you decide the shutter speed and aperture.

One thing you should be aware of is that few digital cameras have a real physical shutter. If you remember, the 35mm cameras actually had a physical barrier that blocked light from entering the chamber where the film was stored. This mechanism was called a shutter blade which could move lightning fast, and was able to deliver shutter speeds as fast as 1/8000 of the second!

Most digital cameras don’t have a real physical shutter. So, if there is no shutter blade, how is the picture actually taken by the digital camera? The CCD is simply turned on long enough to expose the picture. Since the CCD is an electronic component that acts like film, it can be actuated electronically for whatever exposure time you may need.

The most basic manual exposure control, you can exert over your camera and wall setting both the aperture and shutter speed. Some digital cameras allow you to set these controls as if you had a fully automatic 35mm SLR camera. There are two types of cameras that may allow you to do this:

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