Objective size and Mirage reduction

[Fred Bohl]

Rather than hi-jack one of the current threads on these topics, I'll post the following as IMHO based on my own understanding and experience:

The item provided with the March scopes is called a Modifier Disk. It is a thin disk with a 35 mm hole and is equipt with a threaded mounting ring that fits the objective end of the scope. When in place, it will:

Reduce light through the scope 50-55%. -- This may be usefull under bright conditions.

Increase depth of focus about 50%.

Reduce resolution by 33%. -- In bad seeing conditions (including particulate air pollution and mirage particularly with changing wind conditions), the normal high resolution of the scope may show too much detail for the user to interpret usefully. Reducing the resolution reduces the visibility of detail without having to reduce scope power.

Using the Modifier Disk can be very helpful in those afternoon relays when the mirage and wind would otherwise make target sighting and analysis very difficult.

The Modifier Disk works by reducing the clear objective diameter (aperture) which in turn reduces the visibility of small details (reduces resolution).

Before the enthusiasm for using the March Modifier Disk or any other aperture reduction gets out of hand please remember that with the potential benefits there are also some performance sacrifices. The following opinion on use of the Modifier Disk is based on my experience over the last 10 seasons. At or over 300 yards I only use the Modifier Disk under extreme mirage conditions do to the loss of resolution needed for aiming and analysis. For 100 or 200 yards there is sufficient resolution with the Modifier Disk on for use in moderate or worse mirage. However, I have also found that the more I use the scopes without the Modifier Disk and thus train my visual system to deal with the more data rich images, the less I feel the need to use them and the better I am able to aim precisely and analyze my bullet hole patterns.

There have been many postings of late commenting the comparative "quality" of various scope brands. On that topic I offer the following:

There are many tests on scope performance that we all can run with a little knowledge. Most of the more important ones do require some very specific tools and equipment to do with precision. Some can be done with home-made gear that will produce useful but not very repeatable (differences due to who does them and with what) results.

One of the big problems for typical users testing scopes is that they are not objective unbiased observers. We all have a tendency to be biased toward supporting decisions we made without adequate research. Say I read the posts on the new Brand Q scope and it sounds like the answer to my wishes so I go buy one. Then at the range my shooting buddy has a new Brand Z that he is convinced is the best. If we do a side by side Q versus Z comparison are the two of us objective or unbiased observers that will produce a result you would depend on to make your buying decision?

Then we add the problem that you and I look through the same scope with different visual systems. We have different visual acuity, different color perception and most importantly different visual experience. What we see is the sum of our visual training/experience with our individual system and our personal prejudice as what a quality image should be and/or what is important about that image.

 

[JRS]

Rather than hi-jack one of the current threads on these topics, I'll post the following as IMHO based on my own understanding and experience:

The item provided with the March scopes is called a Modifier Disk. It is a thin disk with a 35 mm hole and is equipt with a threaded mounting ring that fits the objective end of the scope. When in place, it will:

Reduce light through the scope 50-55%. -- This may be usefull under bright conditions.

Increase depth of focus about 50%.

Reduce resolution by 33%. -- In bad seeing conditions (including particulate air pollution and mirage particularly with changing wind conditions), the normal high resolution of the scope may show too much detail for the user to interpret usefully. Reducing the resolution reduces the visibility of detail without having to reduce scope power.

Using the Modifier Disk can be very helpful in those afternoon relays when the mirage and wind would otherwise make target sighting and analysis very difficult.

The Modifier Disk works by reducing the clear objective diameter (aperture) which in turn reduces the visibility of small details (reduces resolution).

Before the enthusiasm for using the March Modifier Disk or any other aperture reduction gets out of hand please remember that with the potential benefits there are also some performance sacrifices. The following opinion on use of the Modifier Disk is based on my experience over the last 10 seasons. At or over 300 yards I only use the Modifier Disk under extreme mirage conditions do to the loss of resolution needed for aiming and analysis. For 100 or 200 yards there is sufficient resolution with the Modifier Disk on for use in moderate or worse mirage. However, I have also found that the more I use the scopes without the Modifier Disk and thus train my visual system to deal with the more data rich images, the less I feel the need to use them and the better I am able to aim precisely and analyze my bullet hole patterns.

There have been many postings of late commenting the comparative "quality" of various scope brands. On that topic I offer the following:

There are many tests on scope performance that we all can run with a little knowledge. Most of the more important ones do require some very specific tools and equipment to do with precision. Some can be done with home-made gear that will produce useful but not very repeatable (differences due to who does them and with what) results.

One of the big problems for typical users testing scopes is that they are not objective unbiased observers. We all have a tendency to be biased toward supporting decisions we made without adequate research. Say I read the posts on the new Brand Q scope and it sounds like the answer to my wishes so I go buy one. Then at the range my shooting buddy has a new Brand Z that he is convinced is the best. If we do a side by side Q versus Z comparison are the two of us objective or unbiased observers that will produce a result you would depend on to make your buying decision?

Then we add the problem that you and I look through the same scope with different visual systems. We have different visual acuity, different color perception and most importantly different visual experience. What we see is the sum of our visual training/experience with our individual system and our personal prejudice as what a quality image should be and/or what is important about that image.

 

[centershot]

At one time I was an avid photographer. This is very good food for thought. Never heard of this before and I shoot informal bench rest. Now I have to figure out how to try this.

 

[RGRobinett]

Another overlooked factor: all else equal (variable magnification scopes are a perfect example), doubling the power halves resolution, and vice-evrsa. So, there is always the question - regarding resolution vs. magnification - where is your [individual] point of diminishing returns?
Most people do not realize that higher magnification, without enlarging the aperture, results in diminished resolution. RG

 

[JRS]

Increasing magnification also results in a decrease in image brightness by spreading available light over a greater area, decreases field of view; making objects harder to find and keep centered, introduces more image degrading vibrations, and accentuates atmospheric disturbances. Why use more magnification than you actually need to get the job done?

 

[6MMsteve]

At one time I was an avid photographer. This is very good food for thought. Never heard of this before and I shoot informal bench rest. Now I have to figure out how to try this.

Burris scopes been doin it for a long time

 

[Fred Bohl]

Another overlooked factor: all else equal (variable magnification scopes are a perfect example), doubling the power halves resolution, and vice-evrsa. So, there is always the question - regarding resolution vs. magnification - where is your [individual] point of diminishing returns?
Most people do not realize that higher magnification, without enlarging the aperture, results in diminished resolution. RG

I have once again created a problem by failing to define my terminology. We have stumbled on a difference in our definitions of resolution. As I use the word resolution when applied to target scopes I mean the ability to determine that two bullet holes that overlap by 50% (half a calibre) are in fact two separate bullet holes. Please reply with what you mean by resolution so that I might understand your posting.

 

[Fred Bohl]

Increasing magnification also results in a decrease in image brightness by spreading available light over a greater area...

I do not understand the above portion of your post could you please elaborate? I do follow and tend to generally agree with the rest of your post.

 

[Turbulent Turtle]

I think and please notice the underlined word "think", that what JRS is trying to say is that increasing magnification, in effect, involves a smaller section of the image projected by the objective lens to the first focal plane in the scope, with the corresponding "section" or "portion" of light forming that smaller section and this fills the eyepiece with that reduced amount of light. In fact, that's not reality, the reduced light does not really fill anything, as the increasing magnification reduces the size of the exit pupil while maintaining whatever level of light transmission is coming through the lenses.

Of course, if my interpretation is incorrect, then just ignore it.

 

[RGRobinett]

I have once again created a problem by failing to define my terminology. We have stumbled on a difference in our definitions of resolution. As I use the word resolution when applied to target scopes I mean the ability to determine that two bullet holes that overlap by 50% (half a calibre) are in fact two separate bullet holes. Please reply with what you mean by resolution so that I might understand your posting.

Fred, by resolution, I mean, the degree of detail visible in the image: in an optic, best measured using a resolution chart designed to measure the smallest measurable interval, such as line pairs per millimeter.
Being able to identify two overlapping holes, as opposed to one bullet-hole, would fall under resolution.

Dialing up the power reduces resolution - this is an optical law: via a variable X scope, at 50X, you are seeing a magnified image, exhibiting 1/2 the resolution of the same object viewed at 25X. The question remains, for each individual, where is the trade-off point between resolution and magnification? Oh, I left out the, "cure" . . . to maintain equal resolution, double the objective diameter! RG

 

[Turbulent Turtle]

Which is why I could never understand why NF decreased the objective size of their Comp scope by 14% from the NXS/BR model while simultaneously increasing the magnification 30%.

 

[Fred Bohl]

Fred, by resolution, I mean, the degree of detail visible in the image: in an optic, best measured using a resolution chart designed to measure the smallest measurable interval, such as line pairs per millimeter.
Being able to identify two overlapping holes, as opposed to one bullet-hole, would fall under resolution.

Dialing up the power reduces resolution - this is an optical law: via a variable X scope, at 50X, you are seeing a magnified image, exhibiting 1/2 the resolution of the same object viewed at 25X. RG

RG we appear to be in general agreement on resolution. When I test scopes for resolution I use the USAF 1951 test target which yields line pairs per millimeter which I then convert to MOA for further uses. However, this brings me back to not understanding your assertion "Dialing up the power reduces resolution...". Please advise of and provide a link to what "optical law" you are basing your assertion on so that I might learn and understand.

 

[RGRobinett]

Fred,
Regrettably, due to a traumatic injury, I cannot direct you to a specific source - my defective memory won't accommodate that! It is only in the last 12-18 months that I have been able to read, comprehend, and write complete sentences again! Well, maybe.

I can recall only fragments - back in the mid 1990's to Y2K, I was involved, with [the late] George Meyer, in developing a HIGH resolution rifle scope, and studied several texts regarding optical design - the most comprehensible booklet, was one I obtained, which was a U.S. Naval training manual . . . I cannot recall that title, or, any other specific text - only snippets of attributes - this, resolution vs magnification, being one of them.

Still, I'll take a SWAG at it. By your method (aperture reduction) you are reducing available light. Increasing the magnification, by altering the focal length, is reducing the available light - light being the most important requirement of resolution. Perhaps a knowledgeable soul will rescue me here ().?

Since you are using the USAF resolution chart, you can prove this to your own satisfaction, as that method factors for both magnification AND distance: I recommend that you employ non-shooters (unbiased folks) in your, "testing". Sans prejudice, the USAF chart will weed out most cheating, and you may learn a great deal. For several years, I kept a copy of that chart in the print shop, and subjected everybody who would participate, "to the test". I apologize for my inability to direct your to specific literature - I don't recall where I stashed it all - including the collected DATA. RG

Update - bless Donna's (my best half) heart she asked me what I was working on for so long, and I complained about not being able to find my optical stuff - especially the U.S. Naval book . . . ten minutes later, she produces it! BASIC OPTICS AND OPTICAL INSTRUMENTS (revised edition) Prepared by the Naval Education and Training Program Development Center. Yep, I'm a fortunate guy. RG

 

[Turbulent Turtle]

Let me see if I can pull RG from under the faucet (bilingual joke, comprenne qui peut.)

The resolution does not diminish because of the increase in magnification per se. In a riflescope the objective lens allows light to come into the scope and it focuses the image of the objective into the first focal plane (this is not the focal plane where an FFP reticle is located.) The larger the objective, the larger the projected image at that focal plane. (This is also where an adjustable objective is used to focus the desired object into this plane. A side focus will have a lens between the objective and that first focal plane to focus the desired image on the that focal plane.)

The rest of the lenses in a riflescope only inspect and magnify a specific portion of that first focal plane image. The bigger the objective, the bigger the image in the first focal plane. This is why when you buy say a Kowa spotting scope with a 60mm objective, your eyepiece will provide a magnification of 25X, but when you get the 82mm objective, the 25X LER eyepiece now becomes a 27X. This is also why you hear about big lenses on telescopes, that take months and years to grind. Bigger is better, more light and more detail (resolution.)

Once the incoming light forms the image in the first focal plane, everything degrades from here on. The amount of detail (resolution) that can be discerned is fixed by that first focal plane image coming from the objective. In the inner tube of the riflescope, we find the erector cell; the optical device used to flip the image around so we see it properly instead of upside down and left to right. The erector cell will focus the portion of the image in the first focal plane at which the inner tube is looking into a second focal plane. If your riflescope is an FFP design, that's where the reticle is located. We already see that the image coming into the erector set is only a portion of the whole image from the first focal plane. The entire range of movement of the inner tube is all done against that first focal plane image, up and down and side to side.

I will use an analogy here in order to explain what happens from here. Compare the image coming in from the objective to form the first focal plane to the image captured by a CMOS sensor of a digital camera. What the other lenses in the riflescope do is similar to what you do when you blow up the digital image on your screen. If you take a 24MP image (6000X4000), and you want to view it as an image no bigger than say 300X200 (your eyepiece), you can see that as you zoom in, the original image of a forest starts resolving into clumps of trees, then into individual trees and then branches and you can start making out individual leaves and their detail. At the beginning, everything looks very sharp because there are a lot of "pixels" to view through a small window (your eyepiece.) As the magnification increases, there are still enough pixels to present a crisp image in your small window, but at some point, you can see that the pixel count is down, to the point where the crispness of the image, the resolution if you wish, makes it increasingly difficult to discern the details in the image.

It's the same thing in the riflescope; the ability to resolve details is limited by diffraction and now let me introduce the concept of the Airy pattern as the measure of the resolution capability of a lens and at the center of that pattern we find the Airy disk. (Think of that as the pixel element in the CMOS sensor.) The angular diameter of the Airy disk is a function of the wavelength and the diameter of the lens aperture. Two adjacent points in the image are separated by their diffraction pattern. If the distance between the two points is quite a bit smaller than the size of the Airy disk, you can't discern that the two points are separate and they appear as one; they are not resolvable by your optical device. The smaller the Airy disk the greater the resolution. I can provide the formula for calculating the Airy disk diameter but this post is hairy enough already.

What RG was alluding to is that at some point, your optical device will run into that diffraction limit, where the points smaller than the Airy disk actually become dominant in the eyepiece and you can resolve anything anymore. By that time however, the image will be so dark you won't even be able to tell your Airy disks from your bullet holes.

Bottom line is that the size of the objective dictates everything in a riflescope (or any scope.) The bigger the objective, the better everything is. But big glass is heavy, bulky and most of all, costly.

And for those who were following, the SFP reticle for variable scopes is found after the zoom lenses and before the eyepiece, the third focal plane, if you've been keeping score.

 

[Fred Bohl]

RG and Donna,

Thank you both for your efforts to locate BASIC OPTICS AND OPTICAL INSTRUMENTS (revised edition). I found and bought the kindle edition (Amazon) for under $10. While most is redundant to other references in my library, it is an excellent handy basic reference that should be in the library of all serious shooters - thanks again for remembering it and finding it.

By way of background, there is group of us old BR shooters (originally 5 but sadly only 3 surviving) that have over the last 20 years done many test projects including several involving target scopes. All of us have professional training and work experience in optics in addition to our interest in telescopic sights, spotting scopes and astronomical telescopes. Since your Monday 5/1 post #4 above the three of us have been discussing and researching your comment (including my reading the training guide you noted above - relevant sections).

We have been unable to date to find any reference that supports your recollection of any "optical law" or design principal that increasing magnification reduces resolution of the telescope. However, there are many "rules of thumb" in astronomy that suggest limits on useful magnification. Although those are based on distortions caused by atmospheric effects and are not limitations of the astronomic telescopes design.

My little group are all past the age of 70 and are also subject to some memory lapses (like where we park the car). I doubt that all three of us would have forgotten such an important fact at once but... So if you recall any additional references please remind us.

 

[Fred Bohl]

Turbulent Turtle,

I'm still working on trying to understand your Tuesday #9 post.

However, I'm doing much better with your #14 post.

 

[Turbulent Turtle]

FredBohl, it's all related. It's all about light. Just remember that whatever light comes in through the objective is what you have to work with. There is no amplification of said light in a riflescope, it's only reduced from that point on.

 

[RGRobinett]

Thank you, Turbulent Turtle. Granted another life-time, I couldn't have recalled that! This is good exercise - every thought leading to another recollection or two. The Naval Training booklet reduced the technical info to the simplest terms - understandable to a layman such as me.

As an aside, when, during the Y2K IWA show (European version of the SHOT Show), George and I discussed our project with representatives of one of the best known German lens/scope makers, and showed them our glass specifications (for which we paid dearly), the response of their team leader was, "Such optical quality is reserved for the finest optical instruments!"
My response was something like, "Correct - will you do it?" To which the gentleman replied, "No. Such resolution is not required in a rifle-scope."

Eventually the glass (to Spec.) was provided by a Japanese firm - the resulting 6X [actual magnification] scope provides an image which, upon looking through, most people believe to be 9-10X! I ran out of money prior to the mechanical parts and assembly were complete, and signed my part-ownership over to George. Regrettably, George passed away not long after completing and delivering, perhaps 2 dozen scopes. RG

 

[mikecr]

Quick question; what is the price of higher magnification purely through use of eyepiece instead of bigger/longer scopes?

 

[Turbulent Turtle]

I'm not sure what you mean by "price" but in a riflescope, the vast majority of the magnification is performed by the eyepiece and its associated zoom lenses.

This is unlike a camera lens where all the magnification is tied to the focal length of the lens to provide the proper size image in the focal plane of the sensor.

If you want to see what a 16X camera lens looks like for a 35mm-size sensor, look at this one:
http://www.nikonusa.com/en/nikon-pr....html#tab-ProductDetail-ProductTabs-TechSpecs

18 inches long and it only weighs 10 pounds. The price is cute too: $16,000.

Riflescopes are a marvel of optical engineering.