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>>142012 At its most basic, focal length is the length from the lens to the focal plane. The physical length isn't really equal to the focal length anymore, thanks to more complex multiple-lens optics, but there's still the basic relationship there. When you give your camera some extra macro ability by adding in an extension tube, you're increasing the focal length (thereby increasing the magnification) while maintaining the same minimum focus distance.
Dedicated macro lenses basically do the same thing, just with the "extension tube" built into the lens--either by physically extending the front element much much farther than non-macro lenses, or by doing the same using floating elements inside the lens barrel.
The same is true for regular lenses, actually. I've got a fixed-lens film SLR where you can physically see the field of view changing when it focuses because focusing changes the focal length slightly. With most lenses out there, though, the change in focal length when you focus is small enough that it doesn't make a noticeable difference (or maybe it's engineered away. Like the maximum aperture of the lens might actually be f/1.75 when focused at infinity but they limit you to f/1.8 to match its maximum aperture at its minimum focus distance)
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