From a practical point of view, with normal and wider lenses, I\'m happy with infinity focus when I see a crisp image of a power transmission tower that\'s located on a hilltop about 5 km (3 mi) away from my house. For longer lenses, \"infinity\" is a lot further away and you\'re often limited by atmospheric clarity, and so it\'s not easy to do. OTOH, if you happen to have a collimator on hand (as my local Camera Repair Centre does), then you can test for infinity focus with a bench rig.
When you start to discuss \"dead-crisp\" focus (I\'d call it \"critical focus\"), then that\'s what you get at the exact point of focus. Zones of \"acceptable focus\" in the near and far ground, are related to focal length, distance, aperture and the \"circle of confusion\" (CC), and CC is related to the size and distance from which you want to view the image. google \"the ins and outs of focus\" to find a downloadable version of Harold Merklinger\'s excellent book on the topic - warning, it\'s not a trivial read.
Oct 05, 2008 at 07:56 AM
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