Riflescope primer (Added new info and drawing 6/16/2020)

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June 30, 2020, 05:54 PM
Riflescope primer (Added new info and drawing 6/16/2020)
Thanks for the explanation of the eyepieces. I believe I am actually starting to understand some of this stuff. Smile

(Because the topic is pinned, I often don't notice new posts immediately.)

“The fundamental cause of trouble in the world today is that the stupid are cocksure while the intelligent are full of doubt.”
— Bertrand Russell
August 27, 2020, 11:59 AM
As I have explained many times earlier, I am a huge fan of March scopes. I believe them to be among the best is terms of optics (glass, zoom, eyepiece, etc) and incredibly robust in construction. The attention to detail is astounding.

That said, I have been informed that March is introducing an LPVO (Low Power Variable Optic) with some leading-edge features:

It is a 1-10X24 (10X zoom) and it features a Dual Reticle. This is not something new, but to have such a reticle from March means that they are confident in their design and manufacturing.

At least confident enough to produce a few for testing. Let's talk about reticles.

We are all familiar with the two main types: FFP and SFP. The FFP (First or Front Focal plane) reticle is located at the front of the erector tube and this is where the image from the objective lens group focuses the image. By placing the reticle in the focal plane in front of the erector zoom lenses, it means that the formed image including the reticle, will increase in apparent size as you zoom in. This has the benefit of keeping the reticle in constant relationship with the target image. The subtensions on the reticle remain the same regardless of the zoom setting. The downside is that your reticle will grow in size and will cover more and more of the image as you zoom. When you're dealing with the mundane 3X or 4X zoom ratios, that's not really an issue, but when you have 8X or 10X ratios, the reticle designers have to get more innovative to produce reticles that are useful throughout the zoom range.

The SFP reticle is located at the back end of the erector tube assembly and that's the image that the eyepiece presents to your Mark 1 eyeball. The image of the target can grow and shrink as you zoom in and out, but the reticle stays the same size at all times. This means that the subtensions on the SFP reticle are only easily meaningful at certain zoom settings. For instance, in my March-X 10-60X56, the MTR-5 reticle represents exact distances when it's set at 40X. When I boost to 50X, the subtensions are different by 25%.

In high magnification target scopes, the SFP reticle is more useful as you keep the same size reticle and you usually dial to make adjustments. The SFP reticle can be a lot finer and much more precise for that kind of application. For instance, no one uses an FFP scope in F-class where the minimum magnification is usually north of 30X and the distances and target sizes are well known.

In PRS-type competitions where surgical precision is not required but where targets are at varying distances and can be of different sizes and shape, an FFP design is eminently more useful, almost de rigueur. I doubt anyone uses the FFP reticle for ranging, but knowing that your subtensions are the same regardless of magnification allows you to correctly hold much more quickly than if you had to deal with an SFP reticle.

Now we come to the Dual Reticle. The concept is to have the FFP reticle set up as a scale with the subtensions that are useful for aiming, something like a line at every 0.1MIL and a thicker one at say 1.0MIL and/or 0.5MIL. Something along those lines. The SFP reticle can be just a simple crosshair with maybe a center dot that can be illuminated. This crosshair will stay the same size regardless of the zoom setting, but the scaled FFP will grow or shrink and will always have the same subtension values at whatever zoom setting. It will appear to grow and shrink in the eyepiece and the crosshair will remain the same size.

This gives you the best of both worlds, especially in a 10X zoom.

The following thoughts are mine and mine only. They do not represent March or anyone else's thoughts, concepts, designs or challenges. If I am wrong, and that's highly likely, it's my own fault.

Let's talk about the manufacturing challenges as I would expect them. In order to make this dual reticle work, the erector tube assembly will have to be near perfect. (I was going to say perfect, but I don't think there's such a thing.) The front and rear reticles will have to be perfectly (there's that concept again) aligned. But wait, there's more. Even if they are "perfectly aligned" the erector tube has moving parts; the zoom lenses. It is well-know that zoom scopes can display POA shifts when the zoom ratio changes. This is due to having zoom lenses that are not (oh no, not that word again) perfect. For example, I had a Nightforce NXS 12-42X56, a superb scope, that exhibited a 1.5 inch displacement when I zoomed from 42 down to 12, shooting at 50 yards with a .22LR. It was uncanny; it did it all the time and exactly the same amount and direction, every time I shifted the zoom setting from 12 to 42 and back.

So erector tube assembly: tube, reticles, zoom lenses: will have to be (once more, with feeling this time) "perfect" or as perfect as possible. If any scope manufacturer can design and produce such a scope, 10X zoom dual reticle, I can think of no other outfit than March. I am looking forward to find out how it comes out; it's supposed to go to early testing shortly and if it works out, it will be available by year's end. I wish them well.

You can read more about the new designs they have going to early testing here:

The astute reader will see the mention of the 4.5-28X52 with the FML-PDK and FML-LDK reticles. Yep, those are the ones that offgrid and I put together for them.
August 27, 2020, 03:16 PM
Thanks for the heads-up in the other thread; I probably wouldn’t have noticed this for a bit.

I am confused by the “dual” reticle. Are both reticles visible through the scope at the same time? That seems unlikely, but I’m not picturing how they work.

And to quibble about one statement of your typically excellent discussion in reference to a first focal plane reticle:
“The downside is that your reticle will grow in size and will cover more and more of the image as you zoom.”

Because the image also grows in size as we zoom up in magnification, the amount of the image covered by the reticle does not change. If I can see the small central diamond of the targets I like to use behind the reticle at one magnification setting, I can see it at all settings because it changes apparent size along with the reticle. (If I misinterpreted your statement, my apology, but that’s how I understood it.)

I would say that the problem with the FFP reticle is that because it shrinks in apparent size as the magnification is lowered, it must be thick enough to see at minimum magnification. And therefore at high power it’s a lot thicker than it probably would be in a SFP scope. FWIW this hard-to-see-at-low-power and thick at high issue is why one law enforcement sniping authority recommends SFP scopes for that purpose. Not everyone agrees with him, but his point is easy to understand.

“The fundamental cause of trouble in the world today is that the stupid are cocksure while the intelligent are full of doubt.”
— Bertrand Russell
August 27, 2020, 03:36 PM
The dual reticle is exactly as the name says, you see both reticles in the scope. One superimposed over the other. So the SFP shows the crosshair at the exact same size throughout the zoom range. And while you are zooming, you actually see the FFP reticle grow in size on the crosshairs.

What you see is an actual SFP reticle that seems to grow lines as you zoom in.

You're right about my choice of words. What I meant to say is that what you see as a small reticle on a target will grow in size in relation to the size of the whole image, not the target itself. And for those of us who are used to fine reticles staying the same size as the target grown in the scope, we can be more surgical in placement, something that becomes difficult as the FFP reticle grows also.

While working on the FML-PDK, I was struck by how much the reticle grows in an 6.2X zoom. To the point where the reticle really takes over the entire image. (That's why we worked so hard to make it "airy.") I can only imagine how big a 10X zoom would grow the reticle.

Now, as an F-class shooter, I have to be able to place my reticle, for example, on the left lower quadrant of the X-ring, which measures 0.5MOA. My non-expanding SFP reticle allows me to place the 0.125 MOA dot at 40X exactly where I want it in the X-ring. If I want it finer, I can increase to 50X and that dot now subtends 0.1MOA. I would hate to see what a 0.125MOA dot would look like at 4X in an FFP design.

I hope this makes sense and thanks for drawing attention to it.
August 27, 2020, 03:50 PM
You know, sigfreund you ask good questions.

Following up on my answer to your question just above, it occurs to me (I know, I'm old and slow) that the DR-1 reticle will benefit from the increased precision of the SFP crosshair growing finer in relation to the target along with the consistent ranging and subtensions of the FFP scale as you go up the zoom range. That's probably a better way to look at it.
August 28, 2020, 05:55 AM
Here is a link that provides a lot more details about the dual reticles.