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Old Jan 27, 2009, 4:30 AM   #1
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I am confused about lens speed and low light. from what i understand, the longer the shutter stays open the more light comes in which is good for low light situations. what does a lens speed have to do with the shutter staying open longer? seems like a fast lens speed would make the shutter close faster keep less light from coming in.
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Old Jan 27, 2009, 6:43 AM   #2
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There are three factors that affect exposure:
  • Aperture - The amount of light that passes through the lens to the image sensor [/*]
  • Shutter Speed - The length of time the image sensor is exposed to light [/*]
  • ISO Setting - The sensitivity of the image sensor to light.
A fast lens has a large aperture, so it lets in more light, so you can use a faster shutter speed and still get a properly exposed image.

You can adjust any or allof these to obtain a proper exposure. If you halve the shutter speed (from, say, 1/30 to 1/60) you can make up for the loss of light by doubling the aperture ( from, say, f/4.0 to f/2.8 )or doubling the ISO Setting ( from, say, 400 to 800 ).
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Old Jan 27, 2009, 7:16 AM   #3
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A "fast" lens is one with a large maximum aperture, the larger aperture allows you to get faster shutter speeds.
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