Reducer Ring

I'm thinking that some of what Bryan is describing has to do with eliminating "non-image-forming light" from the scope. Lots of extra light bouncing around in the optical path will lower contrast, blocking that light will maintain contrast. The result of eliminating non-image-forming light will be a greater ability of the viewer to see fine detail at the target.

I have noticed that very few BR shooters around here use a shade on their scope. (I use one all the time.) No photographer I know of would expect the best performance from their camera lenses used without proper shading. This even with the very best coatings on the glass and the very best internal anti-reflection measures.

The effect of "stopping down" or placing a smaller aperture in the light path will increase the depth of field (more of the object area in apparent focus). I wonder if some of the effect observed of "clearing up the view" might be caused by the covering of slight mis-focus of the target. On the other hand, and generally speaking, a brighter view would seem to help most folks see better through their scope.
 
Guys,

Sorry for the delay in my promised response. It is proving to be more difficult than I anticipated to avoid being too technical and even more difficult to be brief. I will finish and post tomorrow afternoon.
 
Thanks Fred,

Technical is OK, we're up to the task.

However, if we don't get what you are saying, we'll make false assumptions, assume that you are feedings us mumbo jumbo and end up posting our own, well proven, time tested, unscientifically acquired knowledge. It won't be technical, and it won't be pretty, but it will be the truth, at least as we see it through a scope.

Mustafa
 
Promised Response

To avoid a lot of typo errors, I did the response off line and attached it here as a PDF file.

Mustafa,

For the last few months I've been trying to put together an imaging setup to use digital images of the scope image rather than human eyes to assess scope performance. Even now that I'm using an imager with small enough pixels and a very good beam expander to get good data, it still takes human eyes to assess the images. Even though there is less disagreement that way it is not unanimous on that elusive quality judgment.

henrya,

I'm on your side re sun shades. Any time that conditions are bright I try to use one at least 2 objective diameters long. I will note though that is far less of that problem with the March scopes versus the Weaver T-36s. That would indicate better attention to coatings, blackened edges and internal glare stops. Of course that should be expected for 5x price boost.

All of you,

During my experimenting and testing last year I was trying to learn to read mirage more effectively (big plus for higher power). Bumped into a tidbit I'll share. On real calm days (near zero wind) found a serious thermal problem mirage pattern cause by the hot barrel gasses from the muzzle out to about 5 yards that lingered up to 3 minutes with no wind to clear it. Had to resort to a folded target used to fan it away.
 

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Thank you Fred.

So my next questions are:

1) Are any currently available scopes for BR shooting, by actual test, "diffraction limited" in resolving capability?

2) If the scopes operate at less resolution than the diffraction limit just what is the resolving ability of available scopes?

3) Would/could one measure resolving ability by using a line pair type of target pretty much as used for photographic testing? I'm thinking of a digital camera with appropriate lens system to image at the exit pupil. I'm pretty sure I can rig that up if its an appropriate test. Seems to me it would be.
 
Scope Testing

henrya,

1. We have tested the resolution of several popular target scopes using a resolution test target both by visual observation and using a digital imager. The scopes tested were: Weaver T-36, Leupold Competition 40x, Leupold Competition 45x, March 40x, March 50x, March 60x, and Nightforce BR 12-42x. Three observers found all to be able to show resolutions at or slightly better than the calculated Dawes Limit for their objective sizes for black on white, blue on white and yellow on white patterns. Two of the observers found slightly worse than Dawes limit resolution (visual only) by all but the March scopes for red on white patterns. One observer found slightly worse than Dawes limit resolution for the Weaver T-36 in both visual and digital image for red on white patterns.

2. Because of the relatively small objective sizes and high magnification of target scopes, we would expect and did find all tested to be in fact diffraction limited. We did do some brief visual only testing on an old Lyman 20x target scope. Because of its lower magnification being below the match magnification of the visual acuity of the three observers, the resolution was as expected limited by the visual acuity of each observer. Note that using the Nightforce variable we were able to demonstrate that from the match magnification for the three observers (26x, 28x, 34x) visual acuity, the resolution limit above those match magnifications was effectively constant at the calculated Dawes diffraction limit.

3. Yes that is essentially what we did for the above tests. Our preferred target was the Edmund Optics NT83-001 as it provides black on white, blue on white, yellow on white, and red on white patterns to get a sense of chromatic aberration for both visual and digital images. I use an Image Source DFK 31AU03 camera that has a Sony ICX204AK HAD CCD color sensor [1024 (H) × 768 (V), Active area: 4.8mm (H) × 3.6mm (V), Unit cell size: 4.65μm (H) × 4.65μm (V) square pixels] and a Tele Vue Optics 2.5x Powermate (with C to T to Powermate adapters) as a beam expander lens so that the small exit pupil image is magnified 2.5x to use most of the available image sensor size.
 
Fred Bohl said:
henrya,

1. We have tested the resolution of several popular target scopes using a resolution test target both by visual observation and using a digital imager. The scopes tested were: Weaver T-36, Leupold Competition 40x, Leupold Competition 45x, March 40x, March 50x, March 60x, and Nightforce BR 12-42x. Three observers found all to be able to show resolutions at or slightly better than the calculated Dawes Limit for their objective sizes for black on white, blue on white and yellow on white patterns. Two of the observers found slightly worse than Dawes limit resolution (visual only) by all but the March scopes for red on white patterns. One observer found slightly worse than Dawes limit resolution for the Weaver T-36 in both visual and digital image for red on white patterns.

2. Because of the relatively small objective sizes and high magnification of target scopes, we would expect and did find all tested to be in fact diffraction limited. We did do some brief visual only testing on an old Lyman 20x target scope. Because of its lower magnification being below the match magnification of the visual acuity of the three observers, the resolution was as expected limited by the visual acuity of each observer. Note that using the Nightforce variable we were able to demonstrate that from the match magnification for the three observers (26x, 28x, 34x) visual acuity, the resolution limit above those match magnifications was effectively constant at the calculated Dawes diffraction limit.

3. Yes that is essentially what we did for the above tests. Our preferred target was the Edmund Optics NT83-001 as it provides black on white, blue on white, yellow on white, and red on white patterns to get a sense of chromatic aberration for both visual and digital images. I use an Image Source DFK 31AU03 camera that has a Sony ICX204AK HAD CCD color sensor [1024 (H) × 768 (V), Active area: 4.8mm (H) × 3.6mm (V), Unit cell size: 4.65μm (H) × 4.65μm (V) square pixels] and a Tele Vue Optics 2.5x Powermate (with C to T to Powermate adapters) as a beam expander lens so that the small exit pupil image is magnified 2.5x to use most of the available image sensor size.
Click to expand...

Wow!
Great test and clear, detailed explanation.
I didn't know anyone had taken the time to do this.

So now I wonder if the "reducer ring" helps or hurts. I can imagine the smaller aperture increasing diffraction and darkening the image. Neither seems helpful. But then we hear otherwise from some users.

Henry
 
And just wondering, did you find particular values for line pairs per inch (or mm) resolution?

Thanks again,
Henry
 
Resolution Test Data in LP/mm

henrya,

I’ll answer your last two queries in reverse order.

First, yes we did get the measured results for resolution in line pairs per mm (LP/mm) and a summary is shown below (before you have to ask – I inferred that would be your next question). The way the test is done using that type target, the observer actually records the group and element number of the last element that is discernable as three lines and two intervening gaps. The design reference specification sheet is then consulted to determine the actual equivalent LP/mm resolution.
 

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Hi Fred, in looking at your last post to henrya, how do you account for the differing magnifications/powers of the scopes you are testing? I'm probably missing something obvious in your table, but with only practical optics knowledge, I probably won't find it unless I ask.

Also, don't you find that the perceived resolution of the captured digital image in your testing with the camera is subject to its own set of blurring, sharpening, and other optical characteristics that are built into its image processing abilities (of both the hardware and software)?

Thanks, Mustafa
 
Back to the Reducer Ring Function

henrya,

We expend a lot of effort in researching, discussing, testing and comparing the relative quality/merits of the various telescopic sight designs and manufactures in our quest for the best performance possible. This is all well and good but it tends to ignore the proverbial “500 pound gorilla” in the room. The biggest impact on actual optical performance at the range is that column of atmosphere extending from the front of the scope to the front of the target. What is happening and present in that column of atmosphere while we look through it creates more optical distortion and errors than nearly all scope internal optical quality issues.

That column of atmosphere contains particulates, aerosols (fine particulates suspended in gases), and water vapor that are all stirred by thermal variation (we call it mirage) and the wind. If you think of that column as a thick lens that is over 99% of the optical path you can start to grasp the problem. If the combination of problems is bad enough there will be rapidly changing optical disturbances that constitute optical data overload disturbing your eye/brain concentration on the more important tasks of aiming and/or target analysis.

However, for the purposes of this discussion let us assume that have correctly mounted and aligned to rifle and cartridge one of the best scopes made (March). We have the best quality glass lenses, mechanical systems that deliver solid stable adjustments and the best scope optical performance currently attainable.

In a match we are stuck with the weather, relay time and bench assignment due to the vagaries of scheduling, and the luck of the draw. What can we do by ourselves to improve our ability to overcome bad atmospherics (bad seeing conditions)?

If we install the Modifier Disk on that March (a.k.a. Reducer Ring in this thread) 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 have 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.

Whether the use of the Modifier Disk (Reducer Ring) helps or hurts depends on the severity of the atmospheric effects and the ability of the user to deal with the attendant problems. In my case the more I use and adapt to my high power scopes the less I have to use the Modifier Disk but make no mistake that there are bad enough conditions were I either use the Modifier Disk or don’t shoot.
 
Mustafa,

Remember that the testing was a trial to see if we could find meaningful ways to due comparative testing. In that the work was as much to establish procedures and methods as to collect data. Also note that we chose to examine only resolution limits and color impact on resolution. For the high power and relatively small objective sizes tested we knew that they would be diffraction limited at much lower than there design magnifications.

As expected the differences in magnifications presented no discernable problem for the visual tests at all as all of us were used to using such scopes. For the digital images I had initially tried to use the camera at prime focus with the eyepiece image on the face of the imager with no intermediate lenses. However, the very small exit pupil sizes (0.87 - 1.33 mm) did not provide a sufficient number of pixels of imager resolution to reliably assess scope resolution so I added the 2.5x Powermate which still is just adequate imager resolution (I may go up to a 5x Powermate in future tests).

Yes you are correct that the imager has limitations in both resolution (pixel density) and color response that do not match that of the human visual system. The differences must be allowed for in designing the test apparatus. Even more importantly, these differences must be accommodated in the protocol and preparation of the observers that will evaluate the comparative images. The one saving grace of the imager for collecting the data is that it is objective and consistent in its shortcomings.
 
Fred,

I like seeing that your calculated Dawes limit resolution and the measured resolution correlated so closely and I bet you did too. Thanks you very much for sharing this and the excellent explanations.

Henry
 
Fred,

I realize that we may have moved off topic, but your posts have been very enlightening, so to speak.

Your comment about using and adapting to one’s scope is very true, and applies to rest of the equipment we use in BR.

Personally, I found that using and adapting my eyeballs to the conditions is critical. By getting out of the dark loading area and arriving at the bench early enough, my eyes have time to get “seasoned” to the brighter conditions and distances.

I guess the same thing applies to our brain, we need to use it and exercise it regularly as well.

Mustafa
 
Resolution Definition Clarification

Based on an email question, I am apparently confusing some or at least not being sufficiently clear about the meaning of resolution as I've been using the term. Perhaps the illustration below will help.

LPresLSF1.jpg

At the top is a pair of bars (lines) as they would be on the target. The next pair is the intensity plot of the wave front as it would be distorted by diffraction caused by the objective diameter and then a representation of what the observer would visualize as the image through the scope. Each of next two pairs shows the same for successively smaller objective diameters (less resolution). The three pairs are shown as just resolved, at the resolution limit and just not resolved.

Part of the confusion may lie in resolution effects being a continuum with the Resolution Limit being a point along the continuum between seeing two medium width slightly blurred lines and one wide very blurred line. Progressing further upward (better resolution) from that point (resolution limit) leads toward the two narrow, sharp, clear lines. Whereas regressing downward (poorer resolution) from that point (resolution limit) leads toward a dim extremely blurred smudge.

It is probable that at least for those users that are not accustomed to the fringing effects of diffraction, they would conclude the blurry edges were caused by quality defects of the scope. With sufficient magnification and practice to train the eye/brain visual system to recognize the ridges in the edge fringe, they would then understand the high quality necessary to see those diffraction fringes.
 
henrya,

Re: "I like seeing that your calculated Dawes limit resolution and the measured resolution correlated so closely and I bet you did too."

We were not at all supprised by the correlation between the calculated Dawes Resolution Limit and the measured resolution limit. My associates and I have been confirming that correlation for both telescopic sights and spotting scopes for 3 years of testing now.

The most pleasing result for us is that all three of use were in very close agreement for both direct visual observations and when evaluating the digital images. That indicates to us that we are finally getting to two test protocols that correlate with each other and can be used with multiple observers to provide dependable and objective comparative tests.
 
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