Why do lenses have a minimum focusing distance?
Asked 9/15/2014
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Why can’t a lens focus arbitrarily close to a subject? I understand that some lenses have a macro mode or focus-range switch that lets them focus closer, but why isn’t that available all the time? What limits how close a lens can focus?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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A given lens's optical formula can shift groups of elements in the lens around only so much to achieve focus. If you wear corrective glasses or have a magnifying glass, you can simply see how this works: you need the corrective/magnifying glass some specific distance from your eye for it to be able to focus on your subject. Move the corrective or magnifying glass back and forth to subjects of varying distance and you'll find there's a close-up distance where you just can't get the glass any closer to your eye and achieve focus -- it just doesn't work.
Originally by user8473. Source · Licensed CC BY-SA 4.0
user8473
12y ago
0
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A lens has a minimum focusing distance because its optics and mechanics only allow so much adjustment. To focus on closer subjects, the lens elements generally need to move farther from the sensor/film. That requires more extension and more travel in the focusing system.
Designers also optimize lenses for certain subject distances. Most non-macro lenses are corrected primarily for normal-to-distant use, where most people shoot. If those lenses were allowed to focus much closer, image quality could drop because aberration corrections are no longer ideal at that distance.
So the close-focus limit is a tradeoff among optical performance, size, weight, complexity, and cost.
Macro lenses are designed to work better at short distances, and some close-up-specialized lenses may not even focus to infinity. A macro switch or focus-range limiter changes how the lens operates over different distance ranges, helping it focus closer or focus more conveniently/quickly within a chosen range.
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