The “modifier disk” (MD) supplied with March riflescopes consists of a cover for the objective (front) lens that has a hole in the center that’s smaller than the diameter of the lens. It therefore reduces the amount of light entering the scope and increases the depth of field of the viewed image due to the principles that photographers are familiar with.

In at least one Internet discussion of the MD, shooters claim that in addition to reducing the light and increasing the depth of field, using the device also reduces resolution. They make that claim in reference to using the MD to make it easier to see and read so-called “mirage” effects. But the “lower resolution” claim strikes me as very odd because I don’t understand how that would occur with an aperture reduction ring. In fact, stopping down the aperture of a camera lens usually improves the optical quality (to a certain point).

On the other hand, the March scope owner’s manual states that using the MD can indeed enhance the user’s ability to read mirage, but that’s because the image brightness is reduced and the depth of field is increased; nothing about lowered resolution.

Comments?

Another question that occurred to me relates to the effect of increased depth of field on parallax effects: I.e., does the increased depth of field reduce the effect of improper parallax adjustments?

My understanding of what causes parallax shifts when using a rifle scope is a little vague, so I can’t answer the question for myself based on my knowledge of optical theory. Of course, the obvious way to find out is to experiment with both the MD and without it. I have tried that, and it almost seems as though there is less parallax shift with the MD in place than without, but part (all?) of that may be because using the MD does not allow moving one’s eye as far off the optical center of the scope.

Again, any comments? Thanks for all replies.

I can't provide any insight that'll really help you, but I am intrigued by the concept, and I intend to rig something up, in order to try it out myself. I have heard tell of the same concept being used with night vision, as a quick focus adjustment. You'll have your monocular adjusted to view clearly for most tasks, but use a Butler Creek cover with a smaller aperture drilled, to flip down when you need to see up close.

The optical principle behind this endeavor seems to bear a remarkably close resemblance to that behind the 'pinhole' camera's or camera obscura, which gadget provides an 'infinite' or at least immense 'depth of field'.

This message has been edited. Last edited by: Orthogonal,

Excellent questions, let me see if I can explain.

The Modifier Disk is as you described but there are specifications for it. For you riflescope, the objective lens is 52mm. The modifier disk has a hole in the middle with a diameter of 38.8mm, call it 37mm. As we all know, the area of a circle is Pi time R-squared. For the 52mm lens, that works out to 2123.7 square-mm. Half of that is 1061.8 square-mm, which yields a diameter of 36.8mm.

In optics (by way of photography for me), halving the size of the aperture (in this case the objective lens) represents one full F-stop. As we all know by now, adding one F-stop to a lens cuts out half the light and as a by product, increases the depth of field of that lens at that setting. If you go check out my stickied thread here, I talk about depth of field, and another measure called the Circle of Confusion, CoC. This is what is used to determine the depth of field of a lens.

Photographers will use very large apertures to take pictures of subjects using a telephote lens and the shallow depth of field from the large aperture will force them to focus precisely on the subject and cause the background to be blurry, something called bokeh. This makes for very artistic pictures. If they want a deeper depth of field, they will use a smaller aperture and the bokeh slowly fades away.

Camera lenses are well understood and there is an aperture setting at which the resolution will start to be adversely affected. This will show up when you enlarge the pictures. That is usually several stops down from maximum, usually around F/8 or so, but that varies wildly.

For us poor riflescope users, we already start with a rather small aperture and we are already enlarging the picture using our erector zoom lenses. Now I don't know that my aging eyes would be able to detect any reduction in resolution coming from one F-stop, that's for others to see, but I do know that the DOF increases dramatically, where I can see it, when I use an MD on my scope. I have used it on my 4.5-28X52 for my PRS outing and it worked out great for me, but I rarely to almost never use it with my March-X 10-60X56HM in F-class shooting. I do know of other f-classers who do use it.

Increasing the DOF in the riflescope will catch more mirage, and that may help you read it better. Some of my friends with March scopes report that. I use a Kowa spotting scope at 27X to monitor the mirage, and I have a large wheel on my March-X 10-60X56HM to pop back and forth from 600ish to 1000 yards in an instant or less.

As for parallax: The side focus knob on the March (and other scopes) is used to focus the objective image more properly onto the first focal plane. That focusing action will cause the apparent parallax deviation to disappear as the image of the reticle and the objective merge in the same focal plane. A larger DOF only complicates this focusing because the objective appears to be in focus over a longer range of adjustment. That's the DOF stuff. If you want to be more certain of the focus, increase the magnification and go from there. If you are lined up pretty decently behind the riflescope, parallax is not an issue regardless of the setting. As you are probably aware by now, as you increase the magnification on a riflescope, you are forced to be placed properly behind it or you simply see nothing. In other words, parallax deviation will only bite you at lower magnifications where your eye can be off center and you are still able to see the objective.

I focus my scopes as required and at higher mags, I have to get behind the scope properly or I can't see sh*t. Parallax is never an issue for me; it shouldn't be for you either,

Thanks for that discussion. I was familiar with a number of the principles involved, but the effect of increased depth of field on reading mirage because more of the disturbance was in focus wasn’t something that occurred to me. It obviously makes sense.

I had also known for a long time that stopping down a lens too much could affect resolution, but I didn’t know/anticipate that it could begin as early as f-8.

One thing about parallax adjustment that I discovered just yesterday was a little disconcerting. I have long enjoyed 20/20 distant vision in my shooting eye, but that has noticeably deteriorated over the past year or so. It’s not enough to require glasses normally, but I sometimes wear them now while driving. What I noticed when doing some informal resolution testing of several scopes was that when I adjusted them for maximum image sharpness, that wasn’t the best spot for minimum parallax shift. I’ll have to experiment further to ensure it’s a true phenomenon, but otherwise I guess I’ll have to think about corrective lenses for shooting. Or at least be extra careful about shooting position behind the scope. (Or not worry too much about it because the resolution target was only 50+ yards away.)

Thanks again for your comments and explanations.

This message has been edited. Last edited by: sigfreund,

Please be aware that resolution degradation as you step down is dependent on a lot of factors. I mentioned that it can start as high as F/8, but I am not aware that this is a rule or even a guideline. I used to read lots of reviews of lenses when I was much more into photography compared to now. That was a item that was tested and recorded in some of the reviews I read, however this was many years ago and memory fades.

As for your parallax problem, a riflescope is designed to provide parallax free viewing when the objective is crisply focused, meaning that the focal plane of the focused objective is the same as the focal plane of the reticle. If you are CERTAIN that the objective is perfectly focused and the objective is also perfectly focused, and yet there is still parallax, the riflescope may have an issue.

Hmm ….
I may have to check things again, but because I noticed the same thing in four different scopes, including two that I’d consider top tier besides the March, I don’t believe the issue lies there.

I have recently readjusted the reticle focus in the older scopes due to the eyesight issue I mentioned. Could that have had an effect if I didn’t do that right?

Trying to diagnose optical issues on a forum is, well, not optimum.

That said, the March-FX 4.5-28X52 has the fast focus eyepiece, which I find incredibly useful and easy to set. The diopter values are meaningful on there. Remember that you use your near vision to look at the reticle, not your far vision. The scope presents the merged focal plane to your eye, just a few inches in front of view.

March has a great FAQ for focusing the eyepiece properly.
https://marchscopes.com/wp-con...ust-the-Eyepiece.pdf

I will check the March site to see if its guidance is different from all other manufacturers' that I've followed for other scopes. I experimented a little this afternoon, and at longer distances than 50+ yards I can eliminate the parallax shift with a scope sharply focused.

Not really expecting a diagnosis, but sometimes we're surprised: "Oh, yes; that can be a problem because ...."

Thanks again.

Do you need cheaters to read? That should guide you for the diopter value to use,

quote:

Originally posted by NikonUser:
Remember that you use your near vision to look at the reticle, not your far vision.

Really? You obviously know much more about the subject than I, but I’m surprised to learn that. Based on 50+ years experience with rifle scopes, it always seemed to me that their optical systems somehow put the reticle image and the target image at the same far distance. That assumption was based on the usual guidance by scope manufacturers to adjust the reticle focus while looking at a featureless background like the sky, plus my own vision.

I have had “monovision” my entire adult life, being very near sighted in my left (nondominant) eye and having 20/20 or slightly better in my right. As I’ve aged, my ability to focus closely with my right eye has decreased, but until the last year or so, at infinite distance it was still 20/20. During those decades of having perfect far vision in my right eye, I’ve never had to make any significant adjustment to the factory focus setting for any of the scope reticles I’ve used. That includes the reticle in my Leupold spotting scope. That has always been true even in recent years as the close vision in my right eye has deteriorated.

In other words, even though I haven’t had good close vision in my dominant eye for many years now, I never had to change the reticle focus setting for any of my many scopes. Recently, though, my distant vision has deteriorated (shortened) a bit, and only then have I had to adjust the reticle focus of the older scopes I’ve owned since that deterioration started. As an example, I have a very old Beeman air rifle scope whose reticle focus cannot be changed from the factory setting, and now it’s no longer in focus the way it was literally for decades, and even when I didn’t have good close focus vision with my dominant eye.

It always seemed to me, therefore, that even though I’ve had poor close vision with my right eye for a very long time (and certainly couldn’t focus within a few inches for probably 40 years), the optical system of my scopes moved the image of the reticle far enough away to where I could focus on it.

Puzzling and something to examine further.

This message has been edited. Last edited by: sigfreund,

The eyepiece of a riflescope (or any other similar optical device) is an afocal optical device. The eyepiece does not focus the image it transmits, it just transmits the image to your eye eye's pupil which will then focus the image onto your retina.

The image being transmitted is the focused image at the rear end of the erector tube, where the second focal plane reticle is located if this is a second focal plane riflescope. If it's a first focal plane, the merged image is focused there after having been erected and probably zoomed in (magnifified.)

The diopter adjustment allows the shooter to adjust this image's focus to get the crispest image. This gets complicated when you wear glasses and have bi or even trifocals. I really like the March fast focus eyepiece on the March-FX 4.5-28X52. It's much easier to adjust.

I get a security warning about the link but I would think that a device intended to help ensure proper head/eye positioning with respect to the scope would be better placed on the eyepiece rather than the objective lens. White Oak Armament offers rear lens covers with an insert that has a small hole in the middle for that purpose. I don’t believe I need anything like that, but I am a gear junkie, so ….

https://www.whiteoakarmament.c...ns-reducer-1707.html

Yes, you're correct, sigfreund, MD is on the objective, what I was reading in this article and the other that I can't recover (yet) is about the rear.

And from the aperture sight article I posted, this comment intrigued me, personally:

"However, as a practical matter, the amount of head
movement involved in moving the front sight within the field of view is
very small (less than 1 mm), and is not likely to be a significant factor.
With the front sight and target positioned near the extremes of the
field of view, the light from these objects enters the eye off-axis from
the center of the pupil. Depending on aberrations in the lens of the
shooters eye, this could cause image distortion compared to light
entering the pupil through its center. It is equally possible that some
people could experience an improvement in sight picture if the center
of their vision has aberrations."

Please remember that if you place the pinhole on the eyepiece instead of the objective lens, you will not get the benefits of the expanded depth of field due to the reduction of the aperture. In other words you won't see the additional mirage.

The product of the objective lens is the first image formed at the first focal plane, located at the front of the erector tube. This image dictates everything about what the user sees at the end. Without electronics, this image is at its best right there and will only degrade in the rest of the riflescope. The depth of field, the focus, the contrast, the colors, everything that makes up the image is what is focused on the first focal plane. The rest of the scope just looks at the image, flips it, erects it, merges it with reticle(s) and then sends out a magnified portion of it to the eye of the user.

quote:

Originally posted by sigfreund:

quote:

Originally posted by NikonUser:
Remember that you use your near vision to look at the reticle, not your far vision.

Really? You obviously know much more about the subject than I, but I’m surprised to learn that. Based on 50+ years experience with rifle scopes, it always seemed to me that their optical systems somehow put the reticle image and the target image at the same far distance. That assumption was based on the usual guidance by scope manufacturers to adjust the reticle focus while looking at a featureless background like the sky, plus my own vision.

I'm with Sigfreund on this one.

In optics, a "focal plane" is a plane (flat surface) for which the image being transmitted through the optical system is perfectly in focus.

In a rifle scope, there are three relevant focal planes. One is downrange at the exact distance at which the scope is focused. One is inside the scope between the objective and the erector (in rifle scope terms, this is the "First Focal Plane"). One is inside the scope between the erector and the eyepiece (in rifle scope terms, this is the "Second Focal Plane").

By putting the reticle on an element at either the First Focal Plane or Second Focal Plane inside the scope, you're putting the reticle on the same focal plane as anything you see through the rifle scope at the exact distance at which the rifle scope is focused.

Optical details aside, that conclusion should be obvious. One eyeball can't focus at two distances at the same time. If you're looking through a rifle scope, your eyeball is focused the same for both the target and the reticle. If they're both crisp, they're both at the same apparent distance.

quote:

Originally posted by NikonUser:
The eyepiece of a riflescope (or any other similar optical device) is an afocal optical device. The eyepiece does not focus the image it transmits, it just transmits the image to your eye eye's pupil which will then focus the image onto your retina.

If the eyepiece is truly afocal (not sure if it is or not), that would mean the light coming out of the eyepiece had an effectively infinite focal length and looking through the eyepiece (at something that was in focus) would require your eye to focus at infinity (which really just means "like you're looking at something far away"). This would apply to both the target image and the reticle, which are in the same focal plane.

The eyepiece on a riflescope and any other similar optical device is afocal. One quick way to verify that is that as you get closer or further away from the exact nice spot where you see the whole picture, the focus does not change. The issue you run into is something known as "vignetting," where the image develops a donut and gets smaller and then it's gone. It's always in focus. Scout scopes have their eyepieces designed to have the full image unvigneted, further away. I have one revolver which has a pistol scope on it and the image is complete at about 15-20 inches away. I do not have to focus that image, of course because it's afocal, but I have to line up very well behind it.

And before someone comes bask and says that you use the diopter setting on the scope to focus as needed, let me just explain that the diopter setting is equivalent to a pair of prescription glasses. Once your prescription is correct for your eyesight, you get your glasses made with that prescription and you don't run around focusing your glasses after that. You only need a new prescription if your eyesight changes. Same thing with the diopter setting on the riflescope. A positive number is for near vision. My near vision has been deteriorating and I set my 4.5-28X52 at +1.5, equivalent to cheater glasses.