Questions like "How close can I focus with this lens on my camera (with max bellows extension X)?", or "What magnification will this lens give at extension Y?" are easily answered with basic optical equations. One source for the equations is the Lens Tutorial at http://www.photo.net/learn/optics/lensTutorial. The equations are in the first section, "Object distance, image distance, and magnification" Be sure to use the same units (e.g., mm) for all distances and the focal length.
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