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Re: Fast lens?
Fast lenses are those that allow a greater amount of light to pass through them than 'ordinary' lenses. The lower the f number, (that is the bigger the apature) the faster the lens.
The actual f number will vary depending on the focal length of the lens, and if it is a prime or zoom lens. Fast primes will be f2.8 or under up to about 85mm, around f4 at 300mm. Standard or wide angle zooms can get closer to the primes than telephoto zooms.
The downside is that fast lenses are physically bigger and heavier than 'ordinary' lenses, which is why there are more and more lenses with stabilization available. This effectively enables an 'ordinary' lens to be used with slower shutter speeds which has the same effect as a fast lens using faster shutter speeds.
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Graham
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