From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Testing Scopes.
Organization: Hewlett-Packard Fort Collins Site

Gary Coffman (gary@ke4zv.atl.ga.us) wrote:

: I disagree. There are variables which closely return to the same POI
                                        ^^^^^^^
: when returned to the same power setting.

That word `closely' is what my comments were about.  If the return only
closely, that's not the same as `exact.'

: ......but as long
: as they give the same POI at a given zoom setting (+/- some acceptable
                                                     ^^^^^^^^^^^^^^^^^^^
: tolerance), they are repeatable.
  ^^^^^^^^^
Again, if there's `+/- some acceptable tolerance,' they are not repeatable.

: Several good variables I've tested using Bart's method return to the same
: POI within 1/8th MOA or less when put to a given power. That should
: satisfy even the most particular varminter.

That depends on how far the particular varmiter wants to miss his point of
aim, or come close to it.

:  Leupolds hold the same POI
: over their zoom range better than other scopes,

Some of the European scopes have 1/4th to 1/3rd the errors that Leupold
variables do.

: Returning
: to power from different directions can result in two different POIs for
: that power setting.

Good point.  But here's something you've not considered, at least as far
as your comments reflect.  You note that if zoomed to a given power from
both directions, there is more error than if that power is zoomed to
from only one direction.  That test shows you that there is some very
small amount of tolerance (mechanical slop) in the two lens groups that
move different amounts for each when the zoom ring is moved.  Those
helical grooves in the tube that's connected to the zoom ring move pins
connected to the two lens groups whose distance between them, and their
distance in front of the reticule, changes with power; what they do is
change the focal length of the objective lens system which included both
the front lens and those two lens groups.  Moving the zoom ring the same
direction to a given power eliminates virtually all the lost motion or
slack between those mechanical parts; there they sit with very good
repeatability of their image forming precision on the reticule.

Now you load and shoot the rifle.  All that shock has just displaced those
two lens groups.  No longer are they in the same place they were before.
Which means the target's image is now not gonna be focused the same.  As
the objective lens system now has all the pre-shooting positioning of its
lens groups vibrated into another set of positions, the optical axis of
the objective lens system is not aligned the same as it was before you
fired that shot.  And those two zooming lens groups settle down in another
postion after the next and subsequent shot.  If you've noted a repeatability
error of about 1/8th MOA, that dimension will be added to the groups the
rifle is capable of shooting.  Which means one will probably shoot smaller
groups with a variable if it's zoomed to max power from low power before
shooting each shot.

Because of the clearances needed for mass production tolerances, zero
tolerance is not possible for zoom lens groups.  If production tolerances
were made much smaller, then the scope would cost more.  But that's what
some European scope makers do; their optics are equal to the others, but
mechanically, they have smaller dimensional tolerances in their zoom
mechanics.  And those added costs get passed onto the customer.

Had you thought of this?

BB