parallax and rifle scopes
Drexel Hallaway
I'm a novice about optics -- I excelled at Physics, but didn't cover the
optics section. ;-) A hobby with which I recently renewed acquaintance is
target shooting and hunting. I've searched the net for information about
the following issue, to no avail....
A reasonably well made rifle-scope supposedly comes from its manufacturer
with its reticle (crosshairs) positioned somewhere along its optical center
/ axis. In a proper installation, the scope is mounted and/or adjusted so
that its axis (defined by the reticle) is -- as nearly as possible --
coplanar with that of the rifle's barrel, and so that both axes cross to
form the desired relationship between bullet trajectory and "line of sight."
Confirming / adjusting this is often casually called "sighting in."
The best casual guidance I've seen for installation usually suggests that
the preponderance of any needed alignment adjustments be done with some
combination of shimming the mount (for elevation adjustments) and adjusting
the mount's windage screws (if present, for lateral adjustments). Doing so
"keeps the scope reticle at or near its optical center." In extreme
cases -- if this is not observed, a badly mounted scope needs substantial
adjustments to be made on the *scope* -- via elevation and windage
adjustment knobs on the scope itself. That those knobs are there for that
very purpose, is clear -- they move either the reticle WRT the image passed
through the scope, or the image WRT to the reticle. In either case, the
effect is to change the relationship between the line of sight and the rifle
barrel.
What seems the subject of some controversy is whether moving the reticle /
inner lens tube (as the case may be) from the optical center/axis of the
ocular and objective lenses (the outer pair) has adverse effects. Some say
that doing so introduces more parallax error. If this makes any difference,
the wise scope installer should avoid using the adjustment knobs, instead
opting for more tedious mount adjustments, keeping the reticle and inner
lens tube at the optical center of the scope.
Hunting-rifle scopes typically come from the factory with their parallax
"minimized at X yards" -- where X is typically in the 100 - 200 range. Some
scopes allow the marksman to "focus" of the objective lens, which apparently
allows the image to focus in the same plane as that of the reticle. If such
is done perfectly, parallax is supposedly zero for that distance -- i.e.,
the reticle image doesn't move across the target just because you move your
eye slightly WRT the assumed immobile scope/rifle. When parallax is
present, it becomes more or less important how one holds the rifle --
particularly his eye WRT the scope! Ideally one minimizes the problem.
Any optical experts interested in weighing in on this issue? Or, does
anyone know where this sort of theory is illuminated -- either the science
of scope design, or the optics that would directly apply? I'm a curious
soul, and a little obsessive, as is perhaps apparent. ;-)
Drexel
Greatest Prime
dre...@cs.columbia.edu wrote on 2/16/02 9:33 AM:
> Any optical experts interested in weighing in on this issue? Or, does
> anyone know where this sort of theory is illuminated -- either the science
> of scope design, or the optics that would directly apply? I'm a curious
> soul, and a little obsessive, as is perhaps apparent. ;-)
I am not.
Reading shooting books should give you the requested information. Even if
you do not read them, read a few simple optics books. There is nothing
magical about sighting parallex. Any high school physics and math whiz
should be able to solve the problems without outside help.
Bill
Mark W. Lund
up enough optics in a physics class to get an answer! The answer depends on the
exact design of your rifescope. To save time and money
most optics are designed with all kinds of compromises.
(Actually all optics are designed with all kinds of compromises).
One of the things you design a rifle scope for is good imagery and
low distortion in the center of the field of view. So keeping
the crosshairs and the relay lens along the center will
give you best results. The cheaper the scope the happier
you will be with everything lined up along the optic axis.
However, shimming and wedging is a pain, which is why
the scope manufacturer gives you the option of tweaking
the optics. If you are wildly off center the field curvature
of the objective and eyepiece will conspire to keep you
from having the reticule and target in focus at the same time.
The image might also pick up some astigmatism if the
relay lens is decentered too much. Also, the crosshairs
and the target might stop tracking with each other
as you move your eye across the exit pupil.
I haven't designed any riflescopes, but I suspect that
some are designed better than others, and I suspect
that one of the design criteria is how far you can
adjust it internally and still be useful. I also suspect
that what was true for 1940's scopes is not necessarily
true for 2002 scopes, since Doug and Jim wrote all that
fancy design software.
The bottom line is, if you can "sight" the scope to the rifle and
your are happy with the results, you are fine. If you
find the thing harder to use because of image problems
you should shim and wedge.
best regards
mark
Drexel Hallaway wrote:
--
----------------------------------------------------------------------
Mark W. Lund, PhD ** Custom Battery Chargers
CEO ** Custom Power Supplies
PowerStream Technology ** Custom UPS
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Brigham Young University Alumni e-mail: lu...@xray.byu.edu
Acme Optics
I'll answer your question differently.
With few exceptions, people can not shoot as well as their rifle/sight
system is capable of.
A rifle sight is adjusted so that it "aims" at a point down range
through which the bullet passes on its parabolic trajectory.
Since the sight is not coaxially aligned with the barrel, there will
always be parallax.
A good zoom scope, sighted in at one zoom position for a specific
range should not change point of aim when zoomed. If it does, tested
in a bench rest, take it back and get you money back.
The best shooters become "one" with their guns just as race car
drivers "put on" their car and go to work. Thus spend as much time
shooting and becoming a part of the weapon.
Carlos Hathcock shot right down the bore of a VC scope and into the
head of the sniper. That is the shot you see in Prvt. Ryan. He did it
with a Remmingtom 700 using Government issued Lake City issued 7.62x51
and a Redfield scope circa 1960's.
With $1000.00 of Remmington Police 700 and $1000.00 worth of almost
any of the top line scopes he could out shoot that today if he lived
with the gun long enough so don't get tied up in the pieces and go to
the range and get good.
Acme Optics
Drexel Hallaway
"Acme Optics" <acmeo...@earthlink.net> wrote in message
news:puht6u4gqm5efh10k...@4ax.com...
<snip>
> With few exceptions, people can not shoot as well as their rifle/sight
> system is capable of.
No doubt. I profess no great expertise -- I'm just trying to remove
systemic factors that might cause problems for an inconsistent shooter.
And, this issue also addresses the debate in some sectors over whether
"sighting in" is a personal thing -- which may have to be redone for another
shooter who addresses the scope differently. You hear such lore here and
there....
> A rifle sight is adjusted so that it "aims" at a point down range
> through which the bullet passes on its parabolic trajectory.
No doubt -- this is basic.
> Since the sight is not coaxially aligned with the barrel, there will
> always be parallax.
I think we're talking about two different kinds of parallax. Or are they?
One is the sort of parallax that causes an object to seem in a different
position depending on often large displacements in point of view --
measuring stellar distances by observed-angle differences over known
distances along the earth, etc.
Another is the one that concerns apparent positional relationships caused by
the optics within scopes -- the notion that the target-reticle alignment
might vary depending on one's eye position in the "exit pupil." In the
extreme, you can take, say a 9X scope, constrain it within a stable mount of
some sort, and focus it on the wall across the room. Moving your head back
and forth can "move the crosshairs" on the wall by as much as an inch or two
over, say, 15 feet. The first time I saw this, I freaked, and wondered --
if such error crept in over such a short range, what happened at long ones?!
Happily, though, scope manufacturers optimize their products to minimize
this parallax at longer ranges -- at the expense of short ones. That same
scope can be pointed at a 1.5" fence-post 200-300 yards away, and the
crosshairs will not move from that post despite how far the eye moves across
the "exit pupil" -- at least when the scope's reticle is at optical center.
My question goes to whether it wanders more when away from optical
center....
I'm guessing, by the way, though no expert, that even were there no gravity,
and the scope axis were allowed to be parallel to the bore -- or even
coaxial, for a thought experiment -- there would be parallax issues of this
second sort....
> A good zoom scope, sighted in at one zoom position for a specific
> range should not change point of aim when zoomed. If it does, tested
> in a bench rest, take it back and get you money back.
No doubt. But this isn't my question.
I appreciate your (snipped in my reply) exhortation to invest time and
become one with the gun, and all that. I have a theoretical interest in
this issue, though, and would like to learn.
Thanks for your input!
Drexel
Drexel Hallaway
"Greatest Prime" <Fish...@mediaone.net> wrote in message
news:B893FF11.EF8B%Fish...@mediaone.net...
> in article PJwb8.11716$M_2.1...@typhoon.nyc.rr.com, Drexel Hallaway at
> dre...@cs.columbia.edu wrote on 2/16/02 9:33 AM:
>
> > Any optical experts interested in weighing in on this issue? Or, does
> > anyone know where this sort of theory is illuminated -- either the
science
> > of scope design, or the optics that would directly apply? I'm a curious
> > soul, and a little obsessive, as is perhaps apparent. ;-)
>
> I am not.
>
> Reading shooting books should give you the requested information.
Believe it or not, finding really solid information about such issues seems
outside the scope of most books on, say, hunting rifles. OTOH, to be fair,
I haven't read any books that specialize in the arena of the sniper, or
whatever....
>Even if
> you do not read them, read a few simple optics books. There is nothing
> magical about sighting parallex. Any high school physics and math whiz
> should be able to solve the problems without outside help.
Recommendations? I didn't mean to seem intellectually lazy! I *was* a high
school physics and math whiz -- just have little exposure to optical issues.
;-) And, I'd expect some good papers to be posted around the web, as
well....
Thanks for your reply!
Drexel
Drexel Hallaway
"Mark W. Lund" <ml...@powerstream.com> wrote in message
news:3C6EC5AC...@powerstream.com...
> This is an excellent question.
Thanks! It's somewhat a relief to enter an arena like this and find that at
least a couple of veteran participants don't consider one's issue
overwhelmingly callow! ;-)
>You wouldn't have picked
> up enough optics in a physics class to get an answer! The answer depends
on the
> exact design of your rifescope. To save time and money
> most optics are designed with all kinds of compromises.
> (Actually all optics are designed with all kinds of compromises).
> One of the things you design a rifle scope for is good imagery and
> low distortion in the center of the field of view. So keeping
> the crosshairs and the relay lens along the center will
> give you best results. The cheaper the scope the happier
> you will be with everything lined up along the optic axis.
I was somehow suspecting.... Where might I find papers (or whatever) about
scope design -- I didn't know the term "relay lens," for example? Seeing
some diagrams and some discussion of related issues would be enlightening.
All the scope manufacturer's websites have to offer is a lot of hype, and
simple definitions of the terms with which they describe their particular
products....
> However, shimming and wedging is a pain, which is why
> the scope manufacturer gives you the option of tweaking
> the optics. If you are wildly off center the field curvature
> of the objective and eyepiece will conspire to keep you
> from having the reticule and target in focus at the same time.
...which is the defining issue in this sort of parallax, right? The reticle
not being in the plane of focus of the target -- though I don't have as
precise a sense of what this means as I'd like....
> The image might also pick up some astigmatism if the
> relay lens is decentered too much. Also, the crosshairs
> and the target might stop tracking with each other
> as you move your eye across the exit pupil.
I think this is the issue. Scope manufacturers seem to call this
"parallax."
Thanks!
Drexel
Lou Boyd
For my long range rifles I purpose put in 40 minutes of wedge between
the bore angle and the centerline of the scope. It does no harm at all
with the reticle adjusted to about the top 1/4 of the field when
shooting at 200 yards and at the bottom 1/4 of the field when shooting
at 2000 yards. The scope has a focusable objective so parallax is no
problem at any range. On a quality scope the focus marks will be
accurate and a pocket laser rangefinder gives the distance with more
than adequat accuracy. If you don't know the distance well enough to
focus the scope you aren't going to shoot anywhere near the target
anyway.
> What seems the subject of some controversy is whether moving the reticle /
> inner lens tube (as the case may be) from the optical center/axis of the
> ocular and objective lenses (the outer pair) has adverse effects. Some say
> that doing so introduces more parallax error. If this makes any difference,
> the wise scope installer should avoid using the adjustment knobs, instead
> opting for more tedious mount adjustments, keeping the reticle and inner
> lens tube at the optical center of the scope.
Whether reticle centering has any effect on an out of focus reticle
depends on where vignetting of the light path to the eye occurs. If
it's only where the light enters the eye then the position of the
reticle within the scope's field makes no difference at all. The
parallax effect is simply because the reticle is not in the image plane,
but your eye forms a field stop which can move laterally. As it does
so it tends to trim off part of the light causing an apparent movement
of the reticle with respect to the field. This only happens if the
light path maintains its directional component. A night visiion scope
for exmaple with a front projection reticle can be out of focus and have
a "fuzzy" reticle pattern, but it will show no parallax effect from
lateral eye movement. A night vision scope with a rear projection
reticle or a physical reticle not in the plane of the phosphor will show
a parallax error even though there is no change at all of the image on
the tube's cathode. It isn't magic, just simple geometry.
> Hunting-rifle scopes typically come from the factory with their parallax
> "minimized at X yards" -- where X is typically in the 100 - 200 range. Some
> scopes allow the marksman to "focus" of the objective lens, which apparently
> allows the image to focus in the same plane as that of the reticle. If such
> is done perfectly, parallax is supposedly zero for that distance -- i.e.,
> the reticle image doesn't move across the target just because you move your
> eye slightly WRT the assumed immobile scope/rifle. When parallax is
> present, it becomes more or less important how one holds the rifle --
> particularly his eye WRT the scope! Ideally one minimizes the problem.
That why scopes made for precision shooting have adjustable focus
objective lenses. That's besides the eyepiece focus which just corrects
for your particular eye problems. If you care about shooting to a point
smaller than about 1/4 the diameter of the objective lens at variable
distances you simply need to purchase a scope an adjustable objective
and which also holds its alignment under recoil. There are several
good brands/models available for well under $1000.
> Any optical experts interested in weighing in on this issue? Or, does
> anyone know where this sort of theory is illuminated -- either the science
> of scope design, or the optics that would directly apply? I'm a curious
> soul, and a little obsessive, as is perhaps apparent. ;-)
I won't call myself an optical expert but I build astronomical
instruments as a profession and am also an avid long range target
shooter. No, it's not obsessive to consider parallax, but it's not a
big deal either. If you want to eliminate parallax error just buy an
appropriate scope and use it correctly. Focus it for the particular
target distance. If you're hunting deer at 150 yards you don't want to
spend 5 seconds screwing with the focus while the deer walks over the
next hill nor do you care if the shot is an inch off because you
didn't. If you try benchrest shooting at 1000 yards, or even 200 yards,
you'll find you won't be competitive if the scope's focus is fixed at
100 yards.
--
Lou Boyd
Fairborn Observatory
Drexel Hallaway
Point well taken about the adjustable-objective solution many scopes offer!
Given your practice of putting 40' shims under your scope mounts for
long-range rifles, do you find any issue with more of a tendency toward
parallax when shooting with the reticle in the top or bottom quarter of the
field? I mean, I knew that the parallax was supposed to go away (at least
in the center of the field) when the objective focus was corrected. OTOH, I
wasn't sure you could depend on this being valid very far from the
center....
Thanks!
Drexel
"Lou Boyd" <bo...@wh.fairobs.org> wrote in message
news:3C6EDBB2...@wh.fairobs.org...
Bob May
considered to be at infinity. This error of focus that happens is pretty
well accomodated by the eye. Unfortunately, the change of focus isn't
reflected in the position of the reticule so as a result, when the distance
to the target is other than when what it's set for, as you move the eye back
and forth across the exit pupil, the reticule moves it's position even
though the scope hasn't moved it's point. This reticule being not at the
focus of the target's image is what causes parallax.
The next thing you have to remember is that the further off from the center
of the scope you get the reticule, the further you get from that correct
focus position (the focus plane is always curved with simpler lens systems -
it's the Petzval curvature of the lens and more complex lens systems like
35mm cameras spend some of their design making the curvature as flat as
possible) and thus the larger parallax movement you get.
Thus, it is wise to adjust the scope so that it points in the right
direction first and then trim the last little bit with the knobs. Making
the mount point in the right direction is a mark of a quality installation
of a scope. I might note that shims won't hold as well as trimming the
surface will when doing this work.
--
Bob May
Imagine the terrorist's fun when they realize that their 72 "nubile virgins"
are all lesbians and cranky from it being that time of the month!
Lou Boyd
>
> Lou,
>
> Point well taken about the adjustable-objective solution many scopes offer!
> Given your practice of putting 40' shims under your scope mounts for
> long-range rifles, do you find any issue with more of a tendency toward
> parallax when shooting with the reticle in the top or bottom quarter of the
> field? I mean, I knew that the parallax was supposed to go away (at least
> in the center of the field) when the objective focus was corrected. OTOH, I
> wasn't sure you could depend on this being valid very far from the
> center....
The location of the reticle in the field makes no difference if the
plane of the image of the target matches the plane of the reticle. The
test (with the rifle sitting on a rest) is to move your eye from side to
side. If you can't see apparent movement between the reticle and the
target the parallax error can't be large then what you can resolve
though the scope. There are plenty of other larger sources of error.
On a junky scope the image plane might not be flat or the adjustment
markings may not match the actual distance.
--
Lou Boyd
Drexel Hallaway
"Bob May" <bob...@nethere.com> wrote in message
news:u6udop2...@corp.supernews.com...
<snip>
> The next thing you have to remember is that the further off from the
center
> of the scope you get the reticule, the further you get from that correct
> focus position (the focus plane is always curved with simpler lens
systems -
> it's the Petzval curvature of the lens and more complex lens systems like
> 35mm cameras spend some of their design making the curvature as flat as
> possible) and thus the larger parallax movement you get.
Ah. It is just this "field curvature," then -- the focus *manifold* or
surface not being planar -- that can exacerbate the problem. It sounds like
the parallax adjustment for a reticle off the optical center would be
different than that for a centered one. I've never seen any numerical
functions to indicate how much this effect contributes, but this is the sort
of thing I suspected....
> Thus, it is wise to adjust the scope so that it points in the right
> direction first and then trim the last little bit with the knobs. Making
> the mount point in the right direction is a mark of a quality installation
> of a scope. I might note that shims won't hold as well as trimming the
> surface will when doing this work.
Trimming the bottom surface of the front of the mount, or trimming the
receiver under it? In either case, with what? I'm not a machinist with
equipment to do some of these things....
Thanks!
Drexel
Greatest Prime
ml...@powerstream.com wrote on 2/16/02 12:48 PM:
> This is an excellent question. You wouldn't have picked
> up enough optics in a physics class to get an answer! The answer depends on
> the
> exact design of your rifescope. To save time and money
> most optics are designed with all kinds of compromises.
> (Actually all optics are designed with all kinds of compromises).
> One of the things you design a rifle scope for is good imagery and
> low distortion in the center of the field of view. So keeping
> the crosshairs and the relay lens along the center will
> give you best results. The cheaper the scope the happier
> you will be with everything lined up along the optic axis.
>
> However, shimming and wedging is a pain, which is why
> the scope manufacturer gives you the option of tweaking
> the optics. If you are wildly off center the field curvature
> of the objective and eyepiece will conspire to keep you
> from having the reticule and target in focus at the same time.
I disagree! It is a simple problem that can be made complex. Some people
just are unwilling to let well enough alone.
I do not do much shooting now. I have a .22 rifle for which I once bought a
cheap simple scope from Sears. It was rigidly attached to the rifle barrel.
Of course, it was not adjusted initially. The adjustment was made by moving
the reticle. The position of the reticle defines a direction for the
telescope.
Where to point the telescope is another problem. Because of parallax and
gravitational drop of bullets, that is a trade-off. I will repeat that a
modest knowledge of high school physics is sufficient to do well enough.
Bill
Drexel Hallaway
What is considered "well enough" may be a point-of-view sort of thing. The
relevance of one source of error among many may be hard to evaluate when the
scale of each is unknown. It sounds as though you consider that the relevant
aspects of "sighting in" are (1) being able to calculate a bullet's
trajectory -- given a muzzle velocity (and perhaps a ballistic
coefficient) -- and (2) deciding at what ranges along the line of sight you
want this parabolic path to cross. Those are, indeed, things that should be
within the capabilities of a good student of high-school Physics.
OTOH, two observations -- *not*, perhaps letting well enough alone. ;-)
First, while I, for example, was exposed to lots of calculus, some
differential equations, and two courses in Physics, I was not prepared for
the experimental reality that, for instance, the G1 drag model confronts one
with. In high-school physics one typically studies friction between solids
(based on a constant coefficient of friction) -- which is a constant force
as long as the normal force "pressing" the objects together is constant (or
a linear function of that normal force if not). If one is exposed at all to
the notion of the drag a gas volume exerts on a moving solid (air on a
bullet), he is usually taught what seems elegant -- but only apparently
*approximates reality at rather low speeds*:
drag = 1/2 * coeff * gasDensity * projSectionalArea * projSpeed ^ 2.
In other words, he is encouraged to assume that for a given bullet under
given atmospheric conditions, drag is the product of *constant coefficients*
and the velocity raised to a *constant* power -- exactly 2. Experimental
evidence, however, expressed in things like the G1 drag model, shows that
even for a given bullet moving through a given, constant set of air
conditions, the coefficient(s) and the power to which bullet-speed is raised
are *not* constant. In the G1 drag model I have, over no less than 41 speed
intervals (ignoring "ballistic coefficient," for the moment), the coefficien
t/exponent pairs range between:
Ft/sec vel coefficient exponent
1025-1059 1.2286E-14 * vel ^ 5.25; and...
2225-2459 0.0039 * vel ^ 1.55
This is messy, inelegant stuff -- doing accurate approximations, even,
requires segmenting the calculation for each speed interval.
Second, I know that the positioning of the reticle / relay-lens(es) within
the scope determines the line of sight. My initial question went to the
issue of whether moving the reticle off the scope's optical center
introduced more parallax: If the scope is perfectly immobile WRT the target,
does moving one's eye back and forth across the exit pupil cause the reticle
to move WRT the target? If it does there is parallax; if it doesn't there is
no observable parallax.
I get the sense that Bob May raises an interesting issue about the focus
"plane" of real-world lenses not being exactly planar. Assuming this,
parallax adjustment for a centered reticle would be different than parallax
adjustment for one off center. If the scope is unadjustable (parallax- /
objective-focus-wise), the manufacturer minimizing parallax for, say 100
yards, might cause the scope to minimize parallax at 100 + X yards if the
reticle is moved off center. Whether the effect is of concern is a matter of
calculation -- I don't know how to do that yet -- and opinion. Were X above
a *large*, positive number -- say, hundreds of yards -- an off-center
reticle might introduce a *lot* more parallax at close ranges. It would be
embarrassing to miss the head of a small varmint at 40 yards -- because you
didn't hold your head just right, and the parallax of your scope (with
off-center reticle) was effectively zero at 800 yards or something, instead
of the manufacturer's original intention. I'm making up numbers here, and
haven't done the math, but you get where I'm going, I imagine....
For me it's not about whose technique of "sighting in" is best or even
adequate. Rather it's about -- given time and interest -- what techniques
might minimize systemic error assuming mediocre component quality. In this
forum, I'm more interested in the optical theory -- about which I know
little -- than the issue of whether such theory makes a significant
difference in a world full of human nerves, flinches, wind conditions and
the like. ;-)
Zane
(snip)
>Where to point the telescope is another problem. Because of parallax and
>gravitational drop of bullets, that is a trade-off. I will repeat that a
>modest knowledge of high school physics is sufficient to do well enough.
With all respect, it sounds as if you may have missed one of the key
points. This is the possible parallax error induced by the reticle, or
parts of it, not being exactly at the focal position of the objective lens
(or its relayed focal "plane"), and the eye not centered exactly with the
scope's optical axis. As opposed to parallax defined by the displacement
of the scope's optical axis relative to the barrel.
If the reticle is on a flat plane, in general not all parts of it can lie
exactly in the (generally) non-flat focal plane of the lens(es). There
will therefore be the opportunity for parallax errors for any points in the
field of view other than those at only one radial position. If the center
of the reticle (assumed flat) is adjusted laterally for windage or drop,
for example, it (the center) will most likely leave the center in-focus
position to one where it is out of focus to some degree, even though it may
not appear so to the eye.
Whether or not this is a significant error depends, of course, on the
degree of curvature of the focal plane for a particular scope and how
accurately one places his eye. I suppose if one is trying to shoot to
better than 1/2 mil, these kinds of errors could be a non-negligible
factor, especially for scopes with large diameter objective lenses.
Zane
Bob May
you're doing several that you find out who has the error and correct that
one.
As to the curvature of the focal plane, I'd probably want the reticle to the
left or right of the axis of the optics. The reason for this is that you're
going to be changing the elevation depending upon the range you're shooting
at and that will make the change in focus to be less as you change the
elevation.
Lou Boyd
>
> Drexel, take your pick as to which surface you change. I suggest that if
> you're doing several that you find out who has the error and correct that
> one.
> As to the curvature of the focal plane, I'd probably want the reticle to the
> left or right of the axis of the optics. The reason for this is that you're
> going to be changing the elevation depending upon the range you're shooting
> at and that will make the change in focus to be less as you change the
> elevation.
Optically that may be true (though I don't think so) but when you offset
the image on a rifle scope you reduce the adjustment range in the other
axis. Also if you use the windage adjustments for their designed
purpose you'll be making the parallax error a function of wind velocity
which will really be hard to keep track of.
--
Lou Boyd
Greatest Prime
zane...@sansnetcom.com wrote on 2/17/02 11:07 AM:
> With all respect, it sounds as if you may have missed one of the key
> points. This is the possible parallax error induced by the reticle, or
> parts of it, not being exactly at the focal position of the objective lens
> (or its relayed focal "plane"), and the eye not centered exactly with the
> scope's optical axis. As opposed to parallax defined by the displacement
> of the scope's optical axis relative to the barrel.
I did not miss that. I just did not comment on it. The reticle must be in
the focal plane or its image. To the extent that it is not or that it shifts
with target distance, that cannot be corrected without some other
adjustment. I think of the offset between the telescope line-of-sight and
the bore sight. To that must be added drop.
An optimum adjustment may be to have the line-of-sight intersect the almost
parabolic trajectory in two points in a way that gives decent performance at
some predetermined range of range. That is something a dedicated shooter can
figure out for him or herself with a *decent* high school education in
physics and math. Some external ballistic information needs to be searched
out from the extensive information out for shooters.
Bill