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fas-retired
03-10-2009, 02:40 PM
What is the correct experimental design, to show by statistical validation, what a scope's POA shift really is? I am not thinking of any particular brand of scope. Has a valid study been carried out and duplicated by a subsequent independent investigation? It appears to me that a .020" shift, as some are worried about, would be extremely hard to verify and would entail quite laborious mechanical and optical apparati. Most everyone appears to have accepted that the problem of shift has been resolved. Good face validity of experimental design does not necessarily imply statistical validation of measurement.

Frank Sweterlitsch

alinwa
03-10-2009, 03:31 PM
What is the correct experimental design, to show by statistical validation, what a scope's POA shift really is? I am not thinking of any particular brand of scope. Has a valid study been carried out and duplicated by a subsequent independent investigation? It appears to me that a .020" shift, as some are worried about, would be extremely hard to verify and would entail quite laborious mechanical and optical apparati. Most everyone appears to have accepted that the problem of shift has been resolved. Good face validity of experimental design does not necessarily imply statistical validation of measurement.

Frank Sweterlitsch

Charlie Hood's Scope Checker kinda' skips all the theoretical steps :)

al

jackie schmidt
03-10-2009, 03:42 PM
The Hood ScopeChecker, if used properly, (big if), will come about as close to ascertaining how much POA shift there is in a scope.

You have brought up the subject that has plagued Benchrest Shooters since the first shooter sat down at a bench. It is NOT the scope that suddenly shifts 1/2 inch that gives Benchrest Shooters nightmares. It is the scope that might be adding .010 to .030 to each group that makes life miserable. To move .030 on the target, the erector tube only has to shift such a small amount that there is really no way to check it.

If you look at how scopes are designed, you can see where the problems arise. The simple fact is, in order for a scope to be adjustable, it has to have moving parts. Keeping the various moving parts 100 percent stationary shot after shot is not an easy task.

Through the years, many shooters have simply changed scopes to see if the ills in a Rifle is optically related. If you have a Rifle that is locked into a .250 agg capability, and refuses to do any better regardless of what you feed it, and you change scopes, and it suddenly starts shooting at a sub .200 level, then you can be pretty sure you have a scope that is infected with the ailment I discribed.

As best I can tell, the manufacturers have no way of telling if something inside a scope ismoving the extremely small amount that it takes to cause .030 shift on the target. They build the things to the best of their ability and then, hope for the best.

As you might know, we Benchrest Shooters have gone to draconian extremes in order to insure that our scopes are up to the task. External Adjustable mounts with with a frozen scope is a real solution. Many balk at this, because they are more worried about keeping a warranty intact than they are winning matches. But, enough of us have done quite well with the frozen scope system over the past three years to prove its worth.

And then there is the March. As best I can tell, they are the only scope manufacturer who made POA shift the "number one" priority in there scopes. So far, it seems to be holding true.

So, what is "draconian" about the March. $2200 a pop would be my guess. But, many of us have put our money, and our trust, into their hands to insure that scope problems are a thing of the past.

I have had Weavers, Leupolds, Burris' B&L's and Sightrons apart. I know what can go wrong.

It sometimes tickles me when a shooter will say, "I have been using (name brand), of scope for years, with no problems". Ask him about his last Yardage, Grand Agg, or Two Gun win before you buy into that.......jackie

cwood3
03-10-2009, 04:56 PM
Jackie,

Didn't the old Lyman's and Unertl's just about have
that same setup ??? Seems to me that if you just had a tube that provided your desired magnification, and an external, for lack of a better term, hold-in-place-once-ya-got-it-where-ya-want-it "attachment, you'd have a lot of the problem licked. Heck I dunno, these readin' glasses give me hell all the time.

curtis

jackie schmidt
03-10-2009, 05:55 PM
The problem with the old Unertles, (besides not much magnification), was very little eye relief. In the Unertal Mounts, the scope actually sat still for an instant as the Rifle recoiled, the big spring returned it. Hence, the shooter didn't get a bloody nose.

That, and they were heavy as heck. Keep in mind, with one of my frozen 36x Leupolds, and the Jewell Mounts, the whole assy just weighed 18.5 ounces. The Buckys Mount, (TSI) is about the same........JACKIE

LARRY FEUSSE
03-11-2009, 09:42 AM
On the March 10-60 variable, do you think the variable apparatus in the scope would have any potential for adding to point of impact change? I know these are fine scopes with everything possible done to them to eliminate POI change, but is the variable mechanism even part of the POI equation, and if so is it made any better in the March than some others?

Thanks,
Larry

Fred Bohl
03-11-2009, 04:42 PM
Frank Sweterlitsch

It is my carefully considered opinion that the extremely small POA shifts that plague the short range Benchrest shooters are not measurable at the range and further may also not be measurable in a optics lab. Jackie is probably correct that the only way to tell if your scope has a potential problem is to try another scope. Then if groups/aggs improve you are probably justified in assuming that the first scope is not holding POA.

For serious problems of POA shift you may be able to get an estimate of magnitude using a collimator and bore rod (popularly called a boresighter and spud). Measurement resolution will be inversely proportional to magnification (approximately 1 MOA for 6x scope and 0.1 MOA for 60x scope).

Let us go back to those extremely small POA shifts and an associated problem. Remember that these POA shifts are actually angular errors that manifest themselves as linear offsets. The following data set may put this better numeric perspective. Starting with the linear offsets at 100 yard range referred to in the posts by you and Jackie:

0.010 inch = 0.0096 MOA = 0.573 ArcS Note: 1 arc second is 1/60th Minute of Angle

0.020 inch = 0.0191 MOA = 1.146 ArcS

0.030 inch = 0.0286 MOA = 1.719 ArcS

Let us add the data for the Dawes Limit of Resolution for a 52mm clear objective diameter (e.g. the March fixed scopes):

0.039 inch = 0.0372 MOA = 2.223 ArcS

Since we cannot resolve/see details smaller than the resolution limit through the scope, we obviously cannot hope to measure or even see POA errors of the magnitude in question by viewing through the scope.

Therefore when Jackie and others have replaced suspect scopes with frozen scopes in external mounts, the reduction in sizes of groups/aggs on the target is the best we can do to measure the effect of POA shifts. Similarly, if we replace a suspect scope with a March and get a similar reduction or replace a frozen scope with a March and get no reduction then we can assume no POA shift with the March. This is not formal proof or even very conclusive statistical evidence but it may be all we will have for quite some time.

Greg Walley
03-11-2009, 07:27 PM
Frank:

I’m not sure where some of these numbers come from (i.e. .020” POI shift, “dialing in” this or that part to .0001”) that are thrown around with impunity in this forum. Remember, General Custer got into big trouble with overgeneralization of numbers, as has our present federal administration with budget and debt numbers.

Both you and I know that even a basic simplex method of experimentation in this game is out of the question with the confounding variables involved. It would be next to impossible and very expensive to optimize or establish a valid control variable for any single parameter that is statistically significant. I can’t think of a single component of the rifle, ammunition, or rest that wouldn’t be of statistical significance. This is especially true for the realm of precision we are trying to achieve.

The follow-up answers from Jackie Schmidt and Fred Bohl should be required reading for anybody in this game that is quick to point the bony finger of indignation at the scope manufacturer (or barrel maker, or bullet maker, or gun builder, et al.) when a shot goes out of the group.

Also, the research design may be faulty when the testing methodology is suspect…not that there is even a hint of scientific study taking place on the benchrest range, but sometimes the right conclusions are drawn for the wrong reasons…or vise versa.

For example:

One problem with a comparison between multiple scopes and looking for minute gradations of inaccuracy via the use of a “scope-checker” setup was answered for me by a story relayed to me a few years ago. A fellow shooter and close friend of mine took a tour of the Leupold optics laboratory. They demonstrated to him how unreliable this method is; and they had tried this setup many years before it became popular in the benchrest community. I don’t remember the details on how they demonstrated the faults, but I think they used laser interferometry for the control. He said the test was very convincing. He also said a visit to the lab by the naysayers would put to rest any doubts on their method…and I believe him.

Another example: A few years ago, I had two Jay Young railguns sitting next to each other on the same concrete bench, with both guns aimed on the same spot at the same target at 100 yards. The guns had the same make of barreled actions and scopes installed in the barrel blocks. The conditions were partly sunny with light winds. The guns were not being fired, but were only used for observation of the target.

I looked through each scope every minute or two, and noted that the image shift due to mirage was significant, and NOT SIMILAR between the scopes. If fact the image shifting appeared to be random between the scopes. Another shooter on the line verified this effect. The only explanations that I could come up with: the scope mechanisms were expanding or contracting at different rates due to temperature differentials and/or the light refraction in front of the objectives was not equivalent due to air density differences.

Greg Walley
Kelbly's Inc.

Don
03-11-2009, 07:53 PM
Frank Sweterlitsch

It is my carefully considered opinion that the extremely small POA shifts that plague the short range Benchrest shooters are not measurable at the range and further may also not be measurable in a optics lab. Jackie is probably correct that the only way to tell if your scope has a potential problem is to try another scope. Then if groups/aggs improve you are probably justified in assuming that the first scope is not holding POA..


That is not what Jackie stated.

As Jackie correctly stated, if used properly, a "Hood Scope Checker" can resolve comparative position relationships, at least to the .020" resolution level at 100 yards, or about the dimensional projected width of a fine 36x Leupold crosshair projected onto a 100 yard BR target...............near as my 20/20 vision can ascertain as measured at the range.

I would imagine a 60x March to be capable of even greater resolution and therefore even better at range positional discrimation when used in a "Hood Scope Checker" system.




Let us go back to those extremely small POA shifts and an associated problem. Remember that these POA shifts are actually angular errors that manifest themselves as linear offsets. The following data set may put this better numeric perspective. Starting with the linear offsets at 100 yard range referred to in the posts by you and Jackie:

0.010 inch = 0.0096 MOA = 0.573 ArcS Note: 1 arc second is 1/60th Minute of Angle

0.020 inch = 0.0191 MOA = 1.146 ArcS

0.030 inch = 0.0286 MOA = 1.719 ArcS

Let us add the data for the Dawes Limit of Resolution for a 52mm clear objective diameter (e.g. the March fixed scopes):

0.039 inch = 0.0372 MOA = 2.223 ArcS

Since we cannot resolve/see details smaller than the resolution limit through the scope, we obviously cannot hope to measure or even see POA errors of the magnitude in question by viewing through the scope.

Therefore when Jackie and others have replaced suspect scopes with frozen scopes in external mounts, the reduction in sizes of groups/aggs on the target is the best we can do to measure the effect of POA shifts. Similarly, if we replace a suspect scope with a March and get a similar reduction or replace a frozen scope with a March and get no reduction then we can assume no POA shift with the March. This is not formal proof or even very conclusive statistical evidence but it may be all we will have for quite some time.


I think your "Dawes limit of resolution" does not match up to real world viewable resolution, at least as seen by my average 20/20 eyeballs, looking thru a 36x Leupold BR scope, at a 100 yard target................ .020" width lines on a 100 yard target, very viewable, show to be about the same projected width of the scopes fine x hairs...................Don

Don
03-11-2009, 08:23 PM
Frank:

I’m not sure where some of these numbers come from (i.e. .020” POI shift, “dialing in” this or that part to .0001”) that are thrown around with impunity in this forum. Remember, General Custer got into big trouble with overgeneralization of numbers, as has our present federal administration with budget and debt numbers.

Both you and I know that even a basic simplex method of experimentation in this game is out of the question with the confounding variables involved. It would be next to impossible and very expensive to optimize or establish a valid control variable for any single parameter that is statistically significant. I can’t think of a single component of the rifle, ammunition, or rest that wouldn’t be of statistical significance. This is especially true for the realm of precision we are trying to achieve.

The follow-up answers from Jackie Schmidt and Fred Bohl should be required reading for anybody in this game that is quick to point the bony finger of indignation at the scope manufacturer (or barrel maker, or bullet maker, or gun builder, et al.) when a shot goes out of the group.

Also, the research design may be faulty when the testing methodology is suspect…not that there is even a hint of scientific study taking place on the benchrest range, but sometimes the right conclusions are drawn for the wrong reasons…or vise versa.

For example:

One problem with a comparison between multiple scopes and looking for minute gradations of inaccuracy via the use of a “scope-checker” setup was answered for me by a story relayed to me a few years ago. A fellow shooter and close friend of mine took a tour of the Leupold optics laboratory. They demonstrated to him how unreliable this method is; and they had tried this setup many years before it became popular in the benchrest community. I don’t remember the details on how they demonstrated the faults, but I think they used laser interferometry for the control. He said the test was very convincing. He also said a visit to the lab by the naysayers would put to rest any doubts on their method…and I believe him.

Another example: A few years ago, I had two Jay Young railguns sitting next to each other on the same concrete bench, with both guns aimed on the same spot at the same target at 100 yards. The guns had the same make of barreled actions and scopes installed in the barrel blocks. The conditions were partly sunny with light winds. The guns were not being fired, but were only used for observation of the target.

I looked through each scope every minute or two, and noted that the image shift due to mirage was significant, and NOT SIMILAR between the scopes. If fact the image shifting appeared to be random between the scopes. Another shooter on the line verified this effect. The only explanations that I could come up with: the scope mechanisms were expanding or contracting at different rates due to temperature differentials and/or the light refraction in front of the oculars was not equivalent due to air density differences.

Greg Walley
Kelbly's Inc.

Greg, I think you are making this way more complicated than it needs to be.

I have spent countless hours positioning B2 Stealth Bomber global postioning aentenae and super sensitive wave guide systems, using theodolites and interferometers, and none of this high tech gadgetry would ever convince me that our present "scope checkers" cannot do an adequate job in discerning POA descrepencies to a .020"-.030" level on a 100 yard reference point.

There is a far cry difference between 2 railgun scopes, sitting independantly, 8 feet apart from one another, and a scope checker system with 2 scopes mounted monolithicaly 3 inches apart, sighted at the exact same reference point....................you will never see a change in POA of the "scope checker" optics unless somebody puts a hammer or heat gun to one of the test scopes or unless one of the scopes is bad and changes POA due to live firing....................Don

Greg Walley
03-11-2009, 11:01 PM
Don,

Incidentally, the two railgun test had both railguns on the same bench, with the scopes a foot apart or so.

I’m not sure how complicated the setup for testing would need to be to detect .020-.030” movement at 100 yards, but I still find it hard to believe it can be done reliably with a “scope-checker.”

I only posted the anecdote about the Leupold facility as food for thought, and I would assume their testing to be more valid than what we experience under most field conditions.

Allen Tucker from Leupold frequents benchrest matches, and someone could ask him for details and he would probably be happy to explain.

With the width of the center ring line at .050” on the 100 yard target, and being able to discriminate POI shift finer than this, when it is sometimes harder to aim the rifle this well in field conditions (even with the best of optics), makes the test suspect IMO.

However, I don’t doubt that one can detect LARGE differences with the “scope-checker”, say half a ring or more, under ideal circumstances.

Maybe it’s only with my setup, but on many occasions when I fire my railgun for multiple shots, I can see the image shift one ring or more at 100 yards in a random pattern, yet the gun will still be holding all of the shots in the zeros. If I move the railgun windage and elevation adjustments to correct, the shots will go almost exactly with the corrected hold and no POA shift.

alinwa
03-12-2009, 12:21 AM
Don,


Maybe it’s only with my setup, but on many occasions when I fire my railgun for multiple shots, I can see the image shift one ring or more at 100 yards in a random pattern, yet the gun will still be holding all of the shots in the zeros. If I move the railgun windage and elevation adjustments to correct, the shots will go almost exactly with the corrected hold and no POA shift.

Greg,

Aren't we talking about two different things? POA shift from mirage, "slow mirage," cloud/sun etc OPTICAL effects is completely unrelated to shifting in the actual scope mechanism, tube or mounting system isn't it? Wouldn't you agree that the items you mentioned are optical effects occurring between the scope and the target?

BTW, I can assure you that the Scope Checker will resolve crosshair shifts down to a lot less than 1/2 ring...... on a mirage-free day of course.

al

stiller
03-12-2009, 07:56 AM
I have been packaging optics for the military for about 25 years. We have some pretty sophisticated equipment we use at times when necessary, but it isnt always necessary. The hood type of checker should be pretty a pretty solid device to be used for what it is intended. As long as the "true" scope is good, the others shifts should be able to be determined within the resolution of the scope. 0.020 moa shift on the target is resolvable under correct optical conditions. That doesnt mean in Midland on a 100 degree day, but is doesnt mean in a lab either. The real ticket for a test would be a good simulator for a gun and a long hallway with a target. We have a machine gun simulator for our military optics at Raytheon. I put a few scopes on it and run them without destroying them, it was a little scary though. It puts anywhere from 1 to 500 rounds at a time on the scope per hit of a button. A device like this would quickly separate the good, bad and the ugly. By the way, most guns put about 3000 to 5000 g's of acceleration at about 3000 to 5000 hz into the scope. Even more interesting is that the up and down and left and right levels at these frequencies are almost the same as the front to back. The big hit that shoves your shoulder really isnt that much of an issue to the scope internals versus the other high frequency effects. The only way we could get real point of impact shifts to go away was to mount all moving parts on elastomers or flex pivots. Any metal to metal joints like sphericals etc just didnt pack it. I with I had more time, I have a computer design for a new pivot section for the Leu 30mm scopes that should fix the issue, just never have time to prototype and build it.

Greg Walley
03-13-2009, 10:36 PM
Greg,

Aren't we talking about two different things? POA shift from mirage, "slow mirage," cloud/sun etc OPTICAL effects is completely unrelated to shifting in the actual scope mechanism, tube or mounting system isn't it?

Wouldn't you agree that the items you mentioned are optical effects occurring between the scope and the target?

Yes. But there is no way to disentangle this extraneous variable from the test.




BTW, I can assure you that the Scope Checker will resolve crosshair shifts down to a lot less than 1/2 ring...... on a mirage-free day of course.
al

In my experience with several railguns, with scopes that range from Unertl, Lyman, B&L, new and old Leupold and March, there is no way to pinpoint this error…I’m now referring to a single railgun with a single scope. Even on “mirage-free” days, there are still atmospheric conditions that cause image shifting problems that can be greater than a ring at 100 yards.

With all due respect to the contributors to this thread that see otherwise, we will have to agree to disagree.

I contend that there is no meaningful way to reliably diagnose a scope for POA mechanical error of .020 to .030” of an inch at 100 yards solely using a “scope-checker” device.

A highly skilled BR shooter might be able to identify .020-.030” of error by shooting the rifle under controlled conditions (preferably in a tunnel), and would probably wear out a barrel in the process. I’ve been in this game since 1995, and I can assure you that I couldn’t do this (I’m not a top shooter ;) ).

As Jackie has stated in many threads over a period of years, we should simply try what is best by using the best equipment available to us over time (meaningful range time), and learning the subtleties through its practical use and observation.

There is simply no way to do a corollary test in the field to establish a standard with a riflescope and isolating its minute mechanical gradations of error.

I do believe one can practically use this test to identify POA errors when there are gross accuracy problems. That is...aggregates that are greater than what can be expected for a well tuned BR rifle.

Greg Walley
Kelbly's Inc.

Don
03-14-2009, 12:16 AM
Yes. But there is no way to disentangle this extraneous variable from the test.

In my experience with several railguns, with scopes that range from Unertl, Lyman, B&L, new and old Leupold and March, there is no way to pinpoint this error…I’m now referring to a single railgun with a single scope. Even on “mirage-free” days, there are still atmospheric conditions that cause image shifting problems that can be greater than a ring at 100 yards.

With all due respect to the contributors to this thread that see otherwise, we will have to agree to disagree.

I contend that there is no meaningful way to reliably diagnose a scope for POA mechanical error of .020 to .030” of an inch at 100 yards solely using a “scope-checker” device.

A highly skilled BR shooter might be able to identify .020-.030” of error by shooting the rifle under controlled conditions (preferably in a tunnel), and would probably wear out a barrel in the process. I’ve been in this game since 1995, and I can assure you that I couldn’t do this (I’m not a top shooter ;) ).

As Jackie has stated in many threads over a period of years, we should simply try what is best by using the best equipment available to us over time (meaningful range time), and learning the subtleties through its practical use and observation.

There is simply no way to do a corollary test in the field to establish a standard with a riflescope and isolating its minute mechanical gradations of error.

I do believe one can practically use this test to identify POA errors when there are gross accuracy problems. That is...aggregates that are greater than what can be expected for a well tuned BR rifle.

Greg Walley
Kelbly's Inc.


Fortunately with the proper use of a "scope checker", 2 scopes are mounted side-by-side, within inches of each other, and when properly adjusted for the exact same parallex and focal distance, will see the exact same mirage image and atmospheric conditions..............so when a "scope checker" is used properly, atmospheric conditions are illiminated as a factor.

Now, Greg has brought up an interesting secondary atmospheric question, what if the "scope checker" is used improperly, and the 2 different scopes used in the "scope checker" are set at 2 different parallex settings? Will the different parallex settings then show different images as mirage/atmospherics change, because of the different focal distances?

Perhaps this could explain the differing results that Greg might be getting.

I have never encountered this problem while using "scope checkers" because I have always meticulously adjusted both scopes to attain identical parrallex distances. But, I must admit that I have never used a "scope checker" to see what effect varying parrallex focus adjustments might have due to exterior mirage/atmospheric changes........................Don

Fred Bohl
03-14-2009, 05:22 PM
Gentlemen,

I'm a member of a small group of shooters that share both interest in telescopic sight and spotting scope testing. We also have all had training in optic design, professional experience with various uses of optics, and long experience in use of optics in shooting sports including past or current competition (we are all now retired and all also have more than 50 years experience). Several of us are now or have been amature astronomers. This background is provided to hopefully avoid distracting references to "keyboard experts" and "cabin fever" as the source of opinions offered or hard won knowldge gain from real range testing.

One of the test rigs we used in our testing was based on a similar rail gun to this one (from the builders website - Magnum Metal):

http://www.benchrest.com/forums/attachment.php?attachmentid=7425&stc=1&d=1237063883

The group member had replace the original "scope wing" with one adapted to accept a Jackie Schmidt frozen Leupold target scope in Arnold Jewell external adjustable mount which served as our "reference/aiming scope". He had also made a replacement top half for the barell block with two sets of double dovetail scope bases and appropriate Burris Signature Rings/Inserts for mounting two "comparison scopes".

The rig was originally built as a platform for comparitive visual optical quality testing of multiple scopes under as near identical conditions as possible. However, we could not resist using it in the maner of a "properly used Hood Scope Checker" as discribed in this post. All of us owned one or more scopes that we suspected to suffer from POA shift so we thought this would provide a means to verify that. By the way, so as not to add frivolus wear to any "hummer barrels", two barrels that had been relegated to fire forming duty were use for these tests.

We found that all of the suspect scopes did in fact show some detectable change in POA compared to the frozen reference scope after shooting. While the shifts were detectable (some barely and some easily), it was not possible to "measure" the magnitude of the shifts.

I will do a follow up post to try to explain why a measurement of the POA shifts was not possible.

Fred Bohl
03-21-2009, 05:48 PM
First let me apologize for the delay in this promised follow-up posting. I was tied up in gun club business, wading through test data and preparing this post and its illustration.

The telescopic sight must do three primary tasks to provide a useful image to the eye/brain for it to perceive the needed target detail. First, it must preserve enough contrast (ratio of relative light levels). Second, it must provide sufficient resolution (clarity or sharpness). And third, it must provide sufficient magnification (apparent image size) so that the required details are detectable at the user’s visual acuity level.

Fortunately almost all modern target scopes have progressed to the point that all internal optical aberrations (deficiencies) have been reduced to a level less than that caused by diffraction of the incoming wave front by the limited size of the objective clear aperture.

The effect of diffraction on the perceived image is to blur the edges of details of a size at or above the resolution limit by widening the edge due to diverting part of the incoming energy into the blur tail. For details of a size below the resolution limit the contrast is lowered as the energy is spread even further until the even the peak value will have too little contrast to be perceived. This is shown in the following illustration:

http://www.benchrest.com/forums/attachment.php?attachmentid=7460&stc=1&d=1237725514

To put the sizes of the details and resolution limits being discussed into perspective, for the range of target scopes we tested the resolution limits were from 0.035 MOA (0.036 inch at 100 yards) for the Nightforce 12-42x 56mm to 0.048 MOA (0.051 inch at 100 yards) for the Weaver T36. It is also helpful to realize that these values are about 0.12% of the whole field of view.

From the illustration it should be obvious that as the width of the line printed on the target gets smaller, the edge blur becomes a larger portion of the apparent width as seen through the scope. Also note that apparent width is also dependent on the contrast threshold so that at the resolution limit the edge blur can make the line appear to be from 1.5 to 2 times actual width at the resolution limit. When the width of the line printed on the target is less than the resolution limit, the contrast ratio drops significantly so that even seeing the broad blur becomes difficult.

An additional limiting complication to using the scope for extremely fine measurement or even extremely precise aiming is the eyepiece. The eyepiece is itself a magnifier and usually an adjustable magnifier (the "diopter" corrections for adjusting focus for a particular user). Since the eyepiece is behind the reticle even in a fixed scope, adjustment of the eyepiece modifies the apparent size and clarity of the cross hair and/or dot. Also, there is a minimal blur of the edges of the cross hair and/or dot itself even at "best focus" but fortunately in high quality target scopes this blur is smaller than that on target details (about 0.25 to 0.5 that of the target blur on the test scopes).

Looking at the digital images and discussing the visual observations from the test of scopes for POA shift tests mentioned in my last post, I will concede to Don and alinwa that if very carefully done we could detect POA shifts of 1/2 the apparent with of a high contrast target line with a width at or wider than the resolution limit of the scope in question.

Edit 3/22/09 7:40 am CDT to restore deleted/missing image FB ???