March Scope Question

[Boyd Allen]

Fred,
As I mentioned, the shooters that looked through our test scopes were all short range benchrest shooters, and they were given complete freedom to adjust the scopes any way that they wanted to. Also, as mentioned, all scopes were at least nominally 36X.

I had spent some time looking through the array of scope before we took it to the match, and was a bit surprised at some of the interest that was expressed in one of the scopes that I had not paid much attention to, probably because of brand bias. It was a Tasco, of which there were two, an earlier one that had a conventional AO and a later one that had an adjustment that was similar to the power ring on a variable.

No manufacturer has much incentive to set up a test like this, neither would any commercial publication that depends on advertising revenue, because the manufacturers that did not fare well would have little reason to support something that did not cast their product in a favorable light. Nevertheless, if this test could be redone, I think that the results would be of interest.

At the time that we built the device, a friend who had done some writing had a connection with Millet, and so we were able to get five sets of their Weaver style windage adjustable rings, and five bases that were built for model 336 Marlins which have flat topped receivers. I found a scrap of 3/8" aluminum plate, and a friend who has a mill helped me square it up and dimension it. After that we used a carpenters square to lay out parallel scribed lines, and then center punched, drilled and tapped the base mounting holes. After the bases were mounted, I built what looked like a short saw horse that the fixture was mounted to the top of, that positioned the scopes at eye level when sitting at a table that it rested on. Then I used an old scope that had had its reticule physically centered by inspection, using a V fixture, to adjust all of the rings so that they were all aligned to a single distant point. Then I centered the reticule of the first scope, mounted it, and then mounted and adjusted the other scopes so that their reticules were centered on the same distant target. I did this to ensure that we were looking through all the scopes very near to their optical centers. In any case, everything worked as it should, and it was very easy to sit comfortably at the table, with ones elbows resting on it, and shift from side to side, from scope to scope. This removed the whole memory of an image issue. Perhaps someone will take up this challenge and recreate our test with a modern selection of scopes.

Boyd

 

[Fred Bohl]

Boyd,

I would love to be able to use your concept for a comparison fixture and add provision for a digital imager in addition to human observers. It would be great to see such a test done with latest scopes, but it will have to b done by someone younger than me. I'm three months short of 71and don't even buy green bananas.

 

[Fred Bohl]

Diffraction Effects on Target Images

In defense of Jackie's "fuzzy" brain, I believe that all of us that use scopes with magnifications greater than the match magnification see the diffraction effects and may interpret those effects as "fuzzy" images. To clarify:

Diffraction Effects on Target Images

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:


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).

 

[Boyd Allen]

Fred,
I have done a couple of things that might be of interest to you. Some time back, I fashioned a very crude "stop" (hole smaller than the objective diameters of the scopes being tested, using a knife and a piece of cardboard from the back of a tablet. I went down the firing line, and politely asked if I could do a test with shooters scopes. As you undoubtedly know, camera lenses are not at their best for resolution when wide open, and do better when stopped down a couple of stops. My quest was to see if there was any apparent sharpening of the images seen through the various scopes when the aprature was placed in front of them. Obviously, the images were darkened. What I found was that in most cases the images seemed to be sharper, and of course were unacceptably darkened, but this was not universally the case. My B&L 4200 (36X) only seemed to get darker, but no sharper.

Another thing that I have done is to attach positive diopter lenses to the eyepieces of a couple of my scopes, shortening eyepieces' focal lengths, reducing scopes eye relief, and increasing magnification by what looks to be around 20% using a +2 diopter lens. The lenses are NOS Tiffen 37mm closeup lenses that I got for around $20 for a cased set off of Ebay. A couple of friends have done the same thing, and are very please with the result.

My main point for this discussion is that the whole field of view seems to be magnified, including field of view, which is apparently unchanged, the reticule, which stays the same size relative to the target image, and therefore appears to be larger, much as a first focal plane scope's but to a lesser degree, and finally the optical quality, or rather image shortcomings are similarly magnified. If you start out with marginal image quality, you will end up with an image that looks even worse. Although I have not had any issues with image quality with my scopes, I did notice a bit of chromatic aberration well off center when shifting my eye as I viewed a target at 200 yards. Fortunately this is not a problem for the use that I put these scopes to. Friends who have made this modification to their 36x Leupolds have given favorable reports and continue to use them that way. The reasod that I have limited myself to +2 is that with +3 with my vision and glasses, I run out of room to focus eyepieces properly.

 

[Chism G]

Test Protocols

When conducting tests of scope optics,would it be advisable to give the testors an eye exam? Kinda like what we used to do in Chemistry class when testing chemical reactions. The professor called it pre-test protocol. Of course,eye exams are not necessary for tests in the Chemistry Lab. Been so long now,I cant remember all the details necessary for pre-test protocol in chemistry.

If eye exams are not necessary for scope optics tests, then explain why not. Just Curious.




Glenn

 

[btc624]

Uhhhhh changed my mind

Geeze the people at bullets.com are gonna think im nuts nah just ah newbie trying to get the best value for his money. I called and changed my order from the straight 40 to the 36x55 ep-zoom. After reading all the replies and getting a long detailed email from a guy down under staurt elliott? and the fact that i can get this at 15% off im gonna live dangerous and go for it. This thread really got informative and enjoy reading not just answers to my questions but others as well. Thank you guys for sharing your knowledge.

 

[Dusty Stevens]

Take weight into account on scopes. The variable moving parts are heavy and more parts to shift.

 

[Fred Bohl]

Glenn,

The type of testing that both Boyd and I have ben posting about is comparative rather than absolute. Each individual observer tries all of the scopes and reports on his opinion of their relative quality to him. Under that scheme, the differences between individuals eyesight (visual acuity) would have to be accounted for nor tested.

If the goal of the testing was to establish absolute relative optical performance then it would be necessary to screen the participants to ensure that they all had "normal" or at least "corrected to normal" (20/20 vision - 1MOA acuity). For absolute testing it would be better to a digital imager so hat all testing was entirely objective and consistent.

 

[btc624]

Thanks Dusty for sure weight is a factor, i think with my scoville and bat ds action im gonna be ok but if not i'll have my smith take ah smidge off my barrell.?

 

[Boyd Allen]

FYI since this is a discussion in which ones ability to see is relevant, and may be a determining factor in the weight that may be given to opinions, at my last eye exam the fellow was somewhat surprised that a fellow past 60 was able to read the the 20/15 line (with corrective lenses). Recently I found an eye chart on the internet, taped it to a shelf, and measured back to the required distance, and was still able to perform at that level quite comfortably. I was born in '48.

 

[Fred Bohl]

Boyd,

On those "couple of things that might be of interest" and they are.

Your aperture stop has two effects that are of interest:

First, the smaller aperture will tend to increase the apparent depth of field (distance from the nearest place of acceptable focus thru the true best focus and out to the furthest place acceptable focus). This would rarely be of interest in target observation unless the target was located outside the depth of focus (parallax/focus adjustment way off?).

Second, the smaller aperture reduces the available resolution which in turn reduces the fine details without changing the field of view. This is the intent of the March Modifier Disk which is used to reduce the visible mirage instead of reducing power. If we install the Modifier Disk (a 35mm aperture that screws into the 52 mm objective) on the March we reduce the available resolution by 33% (from 0.037 MOA to 0.055 MOA) and eliminate 33% of the small scale data and much of the annoying atmospheric disturbances that tends to overloaded our eye/brain system. We do this at the expense of that 33% increase in error circle size for aiming and also lose that much resolution is analyzing our target bullet hole pattern.

I also have use positive diopter lenses on eyepieces:

Back before my very successful cataract surgeries (removing the lens and replacing it with an intraocular lens implant) I used add on positive diopter lenses on a couple of scopes that had inadequate diopter correction built into the eyepiece. My results were very similar to yours. I also tried the "optical boosters" but with my eye issues, they did not work out for me.

 

[Boyd Allen]

Fred,
Before I had any experience with rifle scopes, I was into photography. I even did a little professional work. In any case, you are probably familiar with how camera lenses are tested, and resolution reported in terms of lines per millimeter. My point is that it is pretty common knowledge that for lenses of this type, even of the very highest quality, that their highest resolution is not at their maximum aperture, but is usually found a couple of stops down from maximum.

For those that are not familiar with what F stops represent in terms of the actual size of the aperture, they represent the ratio of the focal length to the aperture, so if a lens is stopped down one stop from say F2 to F4 (two stops) the diameter of the aperture is halved and the area quartered. It was based on this knowledge that I conducted my crude test.

Camera lenses and most likely rifle scopes are not so much limited in their resolution by a theoretical limit as they are by the quality of their builds, which would include lens quality, optical design, accuracy of assembly, and other factors like baffling and coatings.

When a user looks through a scope, and compares what he sees through another scope, assuming they are both properly adjusted, his perception of any difference in overall sharpness is by definition correct, someone else may see something else, but if we are selecting a scope for our own use, why would that matter?

One difference between a camera and our eyes is that the image that is projected on the film (old school here) is going to be magnified a significant number of times for viewing, as contrasted with telescopes including rifle scopes, where the image is projected directly onto our retinas. This may reduce the resolution requirement to the theoretical limit of the eye itself, rather than the lens system, but if the lens system is the limiting factor, this will become apparent if what we see sharpens up when it is stopped down. Obviously, stopping down to this extent would not be a practical approach to improving the sharpness of rifle scopes because of the darkening of the image that goes along with it. We cannot increase the exposure time of our eyes to compensate as we can with a camera.

What I found was that for most of the scopes stopping them down (putting the piece of cardboard with the hole close in front of the objective) sharpened the image, while in the case of my own scope it did not. To me this seemed to say that its quality was such that my eye rather than the quality of its optics was very likely the limiting factor with regard to resolution, and that in the case of those that saw a sharper image stopped down that they could would be able to see an improvement in resolution if their scopes were of better quality, that their eyes were better than their scopes.

 

[Andy Cross]

[QUOTE=When a user looks through a scope, and compares what he sees through another scope, assuming they are both properly adjusted, his perception of any difference in overall sharpness is by definition correct, someone else may see something else.

Based on what we , that is the technical staff in the lab in corporation with two ophthalomologists found was that even when the same operator set up the focusing of an optical system each day for their own use they set it up a little differently each time. The people selected for the tests had according to the Dr's 20/20 or 6/6 visual acuity. The test subjects also commented that the image quality varied from excellent to good to acceptable throughout the week.
So what are the chances that BR shooters who are for the most part over 50 years of age and don't have 20/20 vision any more are likely to find a scope that they say is clear, sharp or what ever. My guess is this is the reason you see shooters selling scopes of one brand that are a couple of years old and replacing them with another. I don't really believe there are many bad name brand scopes out there. Just a population of shooters with constantly changing visual acuity.
Andy.

 

[Butch Lambert]

Fred, Jackie is a Friend and he knows that I was ribbing him.

 

[Boyd Allen]

Andy,
I have no idea why others do what they do, or if what they see is the same as what you or I do, but I see obvious differences between scopes that are quite easy to spot, and which are consistent from viewing to viewing. I am pretty sure that there are multiple factors that determine scope users' satisfaction with their scopes, and several different reasons why they might want to make a change.
When I compare notes with others about the quality of the images of the various scopes that we have looked through, they are generally consistent and in agreement. The comparison of 36x scopes that I described was done close to 20 years ago. Before we let others look at the set of scopes, I spent quite a bit of time looking through them, with each of them adjusted to my vision. As far as I know, putting them side by side is the best way to do a subjective comparison. I recommend it to you if you have not tried it. It works.

 

[Chism G]

I am not an Optical Engineer or any kind of eye Doctor. Andy's Statement makes a lot of sense to me. In a sport where there are few absolutes.

The minimum uncorrected vision requirement in my previous employment was 20/200 in both eyes, correctable to 20/20 in one eye and 20/40 in the other. Radial keratotomy was disqualifying.

Mandatory periodic eye exams were required to identify a disease or condition that might lead to a rapid deterioration of eyesight(Acuity). Examples; Glaucoma, cataract, chronic inflammation.

How many Benchrest shooters get regular eye exams?

If you’re happy with what you see when looking through your Rifle Scope. That’s all that matter.

I have two March 40X scopes,and a 40X Leupold. I can't tell the difference in the Clarity of these scopes. The Two March scopes absolutely hold POI.
This is one feature that really makes a difference in Benchrest Competition.


Glenn

 

[Fred Bohl]

Fred, Jackie is a Friend and he knows that I was ribbing him.

I know that but it gave me an excuse to put in my pitch about 'Diffraction Effects on Target Images'.

By the way, those Shadetree rest tops I bought last year are performing just great.

 

[Butch Lambert]

Fred,
The other forum touched on nitrogen filled or nitrogen purged scopes. My limited research says they actually purge the scopes with nitrogen or some with argon. Do they actually fill them and seal the nitrogen in forever?

 

[Fred Bohl]

Fred,
The other forum touched on nitrogen filled or nitrogen purged scopes. My limited research says they actually purge the scopes with nitrogen or some with argon. Do they actually fill them and seal the nitrogen in forever?

Butch,

The simple answer for high quality scopes is that the last step of assembly is to purge with "dry" or moisture free pure gas and seal the final port (typically a turret) in a dry gas environment to leave the scope filled with the dry pure gas. In very high quality lens subassemblies and scope final assembly the work will be done in glove box or with remote manipulators so all the parts stay in a dry gas atmosphere until the final assembly is sealed.

Nitrogen and Argon are the gasses of choice due to availability of high purity dry and plentiful supplies at modest cost since they are used as shield gasses in welding. Argon, while more expensive, is chemically inert and more thermally stable (less temperature sensitive) than Nitrogen and therefore more desirable.

I can't attest to your "forever" comment since the seals are usually synthetic rubber O-rings and while the internal side is exposed to dry pure gas the outside is not and may be exposed to corrosive compounds in the atmosphere. Also note that the thermal and pressure cycling that a scopes sees over years of use tends to cause the outgassing of the fill gas and in-flow of atmospheric contaminants and moisture (think of a burp).

I can't speak for you but at our age my experience suggest that the only things I think of as forever are death and taxes.

 

[Andy Cross]

Yes Argon and Nitrogen

Butch,

The simple answer for high quality scopes is that the last step of assembly is to purge with "dry" or moisture free pure gas and seal the final port (typically a turret) in a dry gas environment to leave the scope filled with the dry pure gas. In very high quality lens subassemblies and scope final assembly the work will be done in glove box or with remote manipulators so all the parts stay in a dry gas atmosphere until the final assembly is sealed.

Nitrogen and Argon are the gasses of choice due to availability of high purity dry and plentiful supplies at modest cost since they are used as shield gasses in welding. Argon, while more expensive, is chemically inert and more thermally stable (less temperature sensitive) than Nitrogen and therefore more desirable.

I can't attest to your "forever" comment since the seals are usually synthetic rubber O-rings and while the internal side is exposed to dry pure gas the outside is not and may be exposed to corrosive compounds in the atmosphere. Also note that the thermal and pressure cycling that a scopes sees over years of use tends to cause the outgassing of the fill gas and in-flow of atmospheric contaminants and moisture (think of a burp).

I can't speak for you but at our age my experience suggest that the only things I think of as forever are death and taxes.

Nitrogen is a relatively inert gas that is cheap and often used in creating a dry environment in optics which in simple terms doesn't alter the systems refractive powers as air is approximately 70% N anyway. High end optics often use the noble gas argon. It's molecular size and shape make it less prone to leaking away.
It's not uncommon to have the N leaked out of a system in 5 years unless special seals are used.
In systems I worked on pressurized argon was used. If the pressure inside is slightly higher than atmospheric pressure crap can't get in. As mentioned the seals used are the weak link.Gold plated copper crusher types are often used as primary seals with metalised silicon and nylon as secondary. The equipment to do this type of isolated assembly is fairly expensive and time consuming to do.
Andy.