What Thom Hogan means is that the focusing ring will turn just a
little past the infinity point.  He implies the reason why in the
words "under normal temperature conditions".

If the lens barrel expands or contracts due to extreme temperatures,
it is sometimes necessary to allow the focusing ring to operate
outside the normal range of indicated distance to obtain correct focus
in those conditions.  If this allowance was not made, you might not be
able to obtain correct focus on a distant object, so that makes it a
good thing rather than a bad thing.  However, it can be a little
confusing until you realise what's going on.

I think it applies to more AF Nikkors than just the ED lenses.

Hysteresis is left to account for shooting across temperature variations
(mechanical expansion/contraction with temp).  So, when focusing at
infinity with longer focal lengths and at large apertures (low f/
numbers) it is important to focus precisely and not simply turn to the
limit.

At smaller apertures and shorter focal lengths (say less than 85mm) this
is less critical.

But always a good habit.

Cheers,
Alan.

Lenses with ED elements can change focal length slightly
with temperature changes, requiring focusing mounts that
can accommodate these changes at infinity focus. This is
neither a good thing nor a bad thing - it just is, and for
correct focus, mechanical allowances must be made in the
focus mount...

       * = object

       -- / \  = light rays
         /   \

       |
       | = lens
       |

If you focus                               well
 very close:    at infinity:         past infinity

      |          --------|             --__    |
     /|                  |                 --__|
    / |          --------|                     |
   *--|                  |             --------|
    \ |          --------|                   __|
     \|                  |               __--  |
      |          --------|             --      |
 
            the object is so         Rays come from 'past'
            far away that light      parallel.
            arrives as parallel
            rays

Mixed blessings, but 99% good.

If you want to focus to infinity (or whatever reasonable distance
happens to be negible to the infinity focus for your focal length
and wide-open aperture) by simply twisting the focus ring till
you reach the hard stop, you loose something.

If you use an AF, it doesn't hurt you.

If your lens changes length due to e.g. temperature (or your hard
stop is off, or the distance to the sensor is off a tiny bit),
what "should be" infinity isn't --- and if you have bad luck, you
cannot focus to infinity, because it is past the had stop.  D'oh.
This is especially troublesome with long lenses that differenciate
between 20m, 100m, (or even 500m) and infinity ...

So there you have your brand new Canon 600mm f/4 lens for USD
$7,600.00 (B+H), and you cannot focus to infinity because of
the temperature?  Guess who'd be crying bloody murder!  And
that's why it can focus past infinity.

-Wolfgang

It really just means to focus past the infinity *mark* -- you can't actually
focus beyond infinity.

It's just a necessary thing with certain kinds of lenses that have small
dimensional changes due to temperature changes. With such lenses, the
infinity mark on the lens (as well as other distance markings) can only be
an approximation since the actual infinity position changes slightly with
temperature.

Neil

For most lenses, the exact position of the plane of focus depends on the
"color" of the light.  Technically this is covered by what is called
"chromatic aberration".  But there's another situation in which the plane of
focus shifts from that with visible light and that's with IR (infra-red).
IIRC the historical purpose of the focus-past-infinity was to allow focusing
with IR film.  That there are additional physical reasons related to AF and
thermal expansion are only an additional requirement for the feature.

   Norm