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Focus ring issue


steve_stark1

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<p>I have 3 Nikon lenses that the focusing ring does not stop once you have passed infinity. It will continue to move in a clockwise direction as long as you keep turning it. 2 of the lenses( AF-S NIKKOR 70-300mm f4.5-5.6 G & AF-S NIKKOR 18-200mm f3.5-5.6 G ED) came with the initial purchase of the camera.....a Nikon D300. The other lens I recently purchased from B&H. That is a AF-S NIKKOR 10-24mm f3.5-4.5G ED. Is this something I should be concerned about or not. The photographs are all sharp and it does not seem to be an issue other than I'm constantly checking the focus distance indicator on the lens.</p>
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<p>AF-<strong>S</strong> lenses have what essentially amounts to a "clutch" mechanism that couples the manual focusing ring to the focusing helicoid. When you reach the limit of the focusing helicoid travel (either direction), the manual focus ring will "slip" if you continue to turn it. That is COMPLETELY NORMAL behavior.<br /> -</p>

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<p><em>"...this does not happen on my other Nikon lenses."</em></p>

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<p>If your other lenses are AF or AF-D (i.e. "screwdriver" driven), that is because the manual focusing ring on those lenses is mechanical attached with fasteners directly to the focusing helicoid. When the helicoid hits the limit stops, the focusing ring stops.</p>

<p>As an aside, with most (perhaps all) AF-S zooms, you can not rely on the focusing end stops to manually set sharp infinity focus. They will focus "beyond infinity" before they hit the internal stop, so you need to set infinity focus via the viewfinder focusing screen or via Live View.</p>

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<p>Some AF and AF-D Nikkor lenses also have this property. Some do not having a hard stop at infinity due to the focusing mechanism. Many AF and manual focus telephotos have the need to control for expansion of their ED or fluorite glass elements in hot weather. This causes the need to focus beyond the marked infinity at high temperature to get the lens to actually be in focus at optical infinity, because the focal length/focus point actually changes slightly with increased temperature, and the lens is designed allows the focusing mechanism or helicoid to shorten the lens to compensate.</p>
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