The way I understand it is infinity focus is focusing on an object that is - for all intents and purposes, an infinite distance away. Geometrically, this corresponds to the focal point for parallel rays of light.
If you think about the light traveling from everything in your cameras field of view to the sensor, objects that are closer to you have a wide angle that light has to be focused onto a point to be "in focus." Objects that are extremely far away basically have all of the rays of light moving parallel towards your camera by the time they get to you.
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Canon EOS (500D) T1i, PowerShot D10
EF 50mm f/1.8 II, EF 75-300mm f/4-5.6 III, EF 24-105mm f/4 L, Tamron 90mm f/2.8 Macro
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