Alley in Pisa, Italy

by Lars Kotthoff

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I've heard that focal length of some lenses will become noticeably longer when focusing to a close distance, an effect called "focus breathing". Since f-number is focal length divided by diameter of physical aperture and aperture size does not change, it seems logical to conclude that such lens should become slower when focusing close.

Is that really so, or is there something I'm overlooking?

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You mean "slower," not "faster," right? –  whuber Nov 25 '11 at 19:15
    
@whuber Ah, yes, of course! –  Imre Nov 25 '11 at 19:17

1 Answer 1

up vote 5 down vote accepted

That is true, and very noticeable in macro lenses. For example a Nikon 105mm f/2.8 VR (at infinity) is f/4.8 at it's closest focus distance of 30cm or so.

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