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Zoom Ring on a LensZoom ring on a zoom lens barrel is mainly for changing focal length of the lens. In one touch zoom lens design, this ring is for focusing. Focal length of the lens is changed by pumping in or out the entire barrel. Be sure of this ring for its clutch force. Clutch force will keep the ring in its position and there is less chance of any accidental shift. This is an important consideration, while purchasing a used lens, as a used one ring may loose its clutch force and become loose.
Some zoom lens has internal mechanical movement, which is a better design. On rotation of the zoom ring, focal length will change but the physical size of the lens remains the same. This type of design gives dust proof seal and internal parts and elements of the lens are safe from foreign particles. If the lens has an auto focus mode, then be sure for the ring, as some lens has extra loose ring to reduce mechanical load on the electric motor. In auto mode, it will work perfectly but when we shift to manual focusing, there is no clutch force in the ring. This may lead to accidental shift of focus.
A good designed ring has a rubber tube with grip on it. If the ring is all metal, then it has knurling on it for a better grip. As a lens is normally hold from the ring, this section of the lens reveals how much a lens is used. Inspect this part when purchasing a used lens. Read about this: Aperture of lens: What we should check in aperture setting of a camera lens Built, finish and weight of a lens: Why finishing and weight of a lens is important Focusing mechanism of a lens: Different type of focusing mechanism of a camera lens Lens faults: Various types of faults a lens can have, and how to check it Other information on the lens: Indication and information given on a lens barrel Range of focal length: Range of zoom lens in terms of focal length Sharpness of lens: Resolution and lens contrast - how to test it
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