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----------------- Optics Primer - Focusing by Lowering Focal Length

--------------------------------------------------------- By Bill Claff

Here is a Nikon 105mm macro lens at infinity focus:

From the patent figure we can expect lens groups G2 and G3 to move toward the center as we focus closer than infinity.

And we see that the lens does not move away from the image plane as we focus to 0.5x:

Or when we focus to 1x:

But lens groups G2 and G3 move toward the center, as expected, and the focal point moves away from the image plane.

The Optical Bench reports that focal length drops from 104mm at infinity to 94mm at 0.5x and 75mm at 1x
This is entirely consistent with the fact that I have measured 73.5mm at closest focus for my 105mm micro-nikkor.

Here's a more complete view of the lens at 1x: