Here's another lens guide by CameraLabs, this time for the Sony Alpha (Minolta for us old people) mount:
 
"Anyone shooting action or working in low light will also appreciate a lens with a larger aperture, indicated by a small f-number, such as f2.8. These may make the lens bigger, heavier and more expensive than models with average apertures, but have the major advantage of gathering more light. This allows

quicker shutter speeds to be selected, which in turn enable you better freeze action or reduce camera-shake without having to increase your camera's sensitivity and compromise the image quality. Smaller f-numbers also allow you to achieve greater blurring on backgrounds, which is desirable on many action, wildlife or distant portrait shots."
 
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