---
title: "<span id=\"hs_cos_wrapper_name\" class=\"hs_cos_wrapper hs_cos_wrapper_meta_field hs_cos_wrapper_type_text\" style=\"\" data-hs-cos-general-type=\"meta_field\" data-hs-cos-type=\"text\" >What Makes a Prism a Prism?</span> | Newt's Notes"
description: Discover how elevation prisms enhance long-range shooting by offering precise optical adjustments without altering your scope's settings. Learn the science behind their function.
image: https://app-eu1.hubspot.com/api/filemanager/api/v2/files/474376856801/signed-url-redirect?portalId=148831916
---

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## [Newt's Tips From the Bench to the Range](https://blog.xringprecision.com/newt_tips)

From the Bench with Newt

 Riflescope · Elevation Prism · From the Bench to the Range

# What Makes a Prism a Prism?

NEWT X-Ring Precision Oct 15, 2026, 7:56:08 AM

![TYPES OF ELEVATION PRISMS - LEARN WHAT MAKES THEM DIFFERENT](https://app-eu1.hubspot.com/api/filemanager/api/v2/files/474376856801/signed-url-redirect?portalId=148831916)

You have dialed to the top of the turret. The steel is still further out. At that point you need more elevation than the scope has, and a prism is one of the cleanest ways to get it. It is also one of the least understood pieces of glass in long range shooting. Let's break it down.

## What makes a prism a prism

Start with a window. Its two faces are parallel, so light leaves it heading the same direction it arrived. Now tilt one face by a degree or two. You have a wedge, and light leaves a wedge at a slightly different angle than it entered. That is all an elevation prism is: optical glass with two faces that are deliberately not parallel. No moving parts, no batteries, no software.

Put that wedge in front of your objective and everything the scope sees is shifted by one fixed angle. Your scope, your reticle and your 100-yard zero are untouched. With a 50 MOA wedge in place, the crosshair sits on the target while the rifle is actually pointed 50 MOA higher. That is elevation you did not have to find in the turret.

How much shift you get comes from two things working together: the angle between the faces, and how strongly that particular glass bends light (its refractive index). For a thin wedge the rule of thumb is simple: deviation is roughly the wedge angle multiplied by the index minus one. In ordinary crown glass, a wedge of about a degree and a half gets you 50 MOA, which is 14.5 mils. Double the wedge angle and you are at 100 MOA, or 29.1 mils.

## The catch: glass does not bend every color the same

Glass bends blue light a little more than red. Optics people call this dispersion, and it is the same effect that makes a rainbow out of a chandelier crystal. In a wedge sitting in front of a rifle scope, it means the blue image of your target and the red image of your target land in very slightly different places.

Here is the back-of-the-bench math for a single wedge of common crown glass. Set for 50 MOA, it spreads blue and red apart by roughly three-quarters of an MOA. Set for 100 MOA, roughly one and a half. Your eye does not see that as two targets. It sees a soft colored fringe along high-contrast edges, and it gets easier to see as you add magnification.

At 300 yards, three-quarters of an MOA of color is about two inches and most shooters will never notice. At 1,000 yards the same angle is about eight inches, and the edge of the plate is the thing you are trying to aim at. Dispersion is an angle, so distance is what turns it from a footnote into a problem.

![Diagram comparing a single-element wedge, which bends light and splits the colors, with a two-glass achromatic wedge, where a crown element bends the light and a flint element brings the colors back together.](https://blog.xringprecision.com/hs-fs/hubfs/undefined-1.png?width=760&height=366&name=undefined-1.png)

## Crown and flint: why the type of glass matters

Optical glass comes in two broad families. **Crown** glass has a lower refractive index and low dispersion: it bends light modestly and keeps the colors close together. **Flint** glass has a higher index and high dispersion: it bends light harder and spreads the colors much more.

A few things follow from that.

- **The glass does not set the MOA on its own.** MOA comes from wedge angle and index together. Pick a higher-index glass and you can cut a shallower, thinner wedge for the same 50 MOA. Pick a lower-index glass and the wedge gets steeper.
- **What the glass does set is the side effects.** Color spread, thickness and weight all change with the glass. A thin high-index wedge sounds attractive until you see what flint-type dispersion does to a target edge.
- **50 and 100 MOA units are the same recipe cut to different angles.** The value engraved on the housing is a geometry choice, not a different kind of glass.

The smart move is to use both families at once. In an **achromatic wedge**, a crown element does the bending. A thinner flint element is bonded to it facing the opposite way. Because flint spreads color about twice as strongly for every bit of bending it does, a small flint wedge can cancel the crown's color spread while only giving back part of the bend. What comes out the back is the elevation you wanted with the colors still travelling together.

If that sounds familiar, it should. It is the same trick as the achromatic doublet that John Dollond patented in 1758, and a version of it is sitting in the objective of every good riflescope you own.

The trade is honest and simple. Two elements means more glass, a steeper crown to make up for the bend the flint gives back, two parts to grind and polish, and a precise alignment and bonding step. That costs more than cutting one wedge. REACHR is built this way: two glasses, bonded, with the red-to-blue shift held to 12 arcseconds on the drawing. That is 0.2 MOA.

## A word from the man who designed it

I can explain the principle. Danny Schoening has built it for a living. Danny is the former CEO of Optex Systems and the original inventor of the REACHR before it came to X-Ring Precision. He offered to share his knowledge and gave us permission to post it with his name on it.

Prism technology · In his own words

*I've been asked what the optical difference is between the single-element wedge prisms on the market and two-element achromatic wedges like the XRing Reach-R. There are great products of both types, and ultimately, it depends on the intended use.*

*Before we dig into the optics, a little history on the use of wedges might give you the confidence to try them out. Optically, wedges have been used in many applications, but a good ballistic example is the “wedge” used in the M728 Combat Engineering Vehicle (1968). Its cannon was designed primarily to knock down walls and had terrible ballistics. Haven't heard of that one? Well, how about the M1A1 Abrams (1978)? Basically, both systems use an achromatic wedge with thousands of units in the field with very high reliability over the last ~50 years!*

*An achromatic wedge uses two different optical materials, typically cemented or otherwise combined, with different refractive-index and dispersion characteristics. The first piece of glass bends the light, while the second corrects the chromatic error introduced by the first. This minimizes color dispersion while still providing the desired angular deviation.*

*Dispersion becomes a more noticeable issue as distance increases. So, in my opinion, if you're primarily shooting at shorter ranges (less than 600 yards), a single-piece-of-glass wedge should work fine. And, as you might expect, a single piece of glass generally means less cost.*

*If you want the cleanest target edges with minimal color dispersion, then go with the two-piece achromatic wedge used in the XRing Reach-R.*

— Danny Schoening  
Former CEO, Optex Systems · Original inventor of the REACHR

The statement above is Mr. Schoening's personal opinion, shared with his permission.

## Not every sight-line bender is a wedge

Before we line everything up, two other optical approaches are worth understanding, because they show how wide the field really is.

### The periscope: mirrors instead of a wedge

A periscope uses two reflecting surfaces, either mirrors or reflecting prisms. The first catches the view and sends it to the second, which sends it into your scope. Set the two surfaces parallel and the sight line is simply moved up or sideways. That is a trench periscope, or the sight on an armored vehicle.

Tilt one of those surfaces and it gets interesting. A mirror turns the sight line by twice its own tilt, so one degree of mirror movement is 120 MOA of elevation. That is how periscope-style devices reach hundreds of MOA and can be made adjustable. And because reflection treats every color the same, mirrors do not produce color fringing at all.

The price is packaging. Two surfaces, a housing big enough to hold them, more weight, and an alignment that has to survive recoil, because the same doubling that gives you range also doubles any error.

### Fresnel prisms: a wedge collapsed into a sheet

Augustin-Jean Fresnel worked out in the 1820s that the bending in a lens or prism comes from the angle of its surface, not from its thickness. His lighthouse lenses proved it. A Fresnel prism applies the same idea to a wedge: slice the wedge into hundreds of tiny identical ridges and lay them side by side on a flat sheet. Each ridge has the same face angle, so the sheet bends light like the full wedge while staying thin, light and cheap. Most are molded plastic rather than glass. The ones you are most likely to have seen are the press-on prisms an optometrist sticks to a pair of eyeglasses. For scale, 50 MOA is about one and a half of the prism diopters used in that world.

So why not put one on a rifle? Because of what happens at the ridges. Every ridge edge scatters a little light, and a fine repeating pattern diffracts it. The optometry literature has measured this for decades: a Fresnel prism costs more sharpness and contrast than a solid prism of the same power. It is also still a single material, so it splits color like any single wedge, and molded plastic scratches and shifts with temperature in ways polished glass does not.

On eyeglasses at 1x, that is a fair trade for something thin and light. In front of a riflescope at 25x or 35x, where you are asking the system to resolve the edge of a plate a mile away, it is the wrong trade. That is why serious elevation wedges are ground and polished from solid optical glass.

## Prisms next to the other ways of getting elevation

A prism is one answer to running out of clicks. It is not the only one, and the others are not wrong. Here is how they line up.

| Approach | How it adds elevation | What it does well | What you give up |
| --- | --- | --- | --- |
| Achromatic wedge (two glasses) | Crown and flint elements bonded; bends the sight line by a fixed value | Keeps your zero; on and off in seconds; clean target edges at distance | Fixed value; costs more to make than one wedge |
| Single-element wedge | One wedge of glass bends the sight line by a fixed value | Simple, light, lower cost; well suited to short and intermediate range | Color fringing grows with MOA value and distance |
| Periscope (mirror) device | Two reflecting surfaces redirect the sight line; tilt is often adjustable | Very large, adjustable range; reflection does not split color | Larger and heavier; alignment errors are doubled; higher cost |
| Fresnel prism sheet | Hundreds of tiny wedge ridges on a thin, usually plastic, sheet | Very thin, light and inexpensive; fine for eyeglasses at 1x | Ridges scatter and diffract light, costing sharpness and contrast; still splits color; not suited to high magnification |
| Rotating wedge pair (Risley) | Two wedges counter-rotate to vary the bend from zero to double | Continuously adjustable with glass alone; common in lasers and eye-exam equipment | Precision rotating mechanics; any rotation error becomes windage; color unless both wedges are achromatic |
| Canted base or rings | Tilts the whole scope down, usually 20 to 40 MOA | Rugged, inexpensive, nothing in front of the glass | Permanent; limited value; big cants can cost you a 100-yard zero |
| Adjustable tilting mount | A mechanical mount dials in extra cant on demand | Wide range in one unit; nothing in front of the glass | Moving parts, added height and weight; repeatability depends on the mechanism |
| Reticle holdover | Hold on a lower mark in the reticle | Free, instant, always with you | Runs out fast; in first focal plane you may need to drop magnification to see the marks |

Technology categories described generically. Color-spread figures in this post are first-order calculations for a thin wedge of common crown glass, not measurements of any specific product. Real-world results vary with glass choice, coatings and scope.

These also stack. Plenty of ELR rifles run a 20 or 30 MOA base for everyday use and add a wedge only when the distance calls for it.

## Prisms, honestly: the pros and the cons

### What you gain

- **Your zero stays put.** No new rail, no new rings, no change to cheek weld. Take it off and you are back on your 100-yard zero.
- **Your scope stays in the middle of its travel,** which is where an erector system tracks and resolves best.
- **The rifle stays general purpose.** You are not building a one-distance rig.
- **Nothing to power, nothing to wear out.** It is a piece of glass in a housing.
- **The principle has a long service record,** as Danny points out above.

### What you should know going in

- **It is a step, not a dial.** 50 MOA is 50 MOA. Enter the offset in your solver and true it.
- **Nominal is not actual.** Run a tall-target test with the unit on and record the value your rifle really gets. Use that number.
- **Level matters more than people expect.** A wedge rotated by one degree sends about 1.7 percent of its value sideways. On a 100 MOA unit that is 1.75 MOA of windage you did not ask for. The mount has to index the same way every time, and you should confirm it at the target.
- **It is glass in front of your glass.** Every surface costs a little light and collects dust. Keep it as clean as your objective.
- **Fit is about the bell, not the lens.** Bell-mounted units fit the outside diameter of the objective bell. A 56 mm scope does not have a 56 mm bell. Measure before you order.
- **Single-element units show color at distance.** Whether that matters depends on how far you shoot and how small the target is.

## So which one do you need?

Danny said it first: it depends on the intended use. If most of your shooting is inside 600 yards, or you want a fast offset on a carbine, a single-element wedge is a sensible, lower-cost tool. If you need hundreds of MOA and want to dial it, a mirror device is built for that. If the rifle never leaves long range duty, a canted base is cheap and tough.

If you shoot far, at high magnification, at targets where the edge is the aiming point, put your money into the glass. Two elements, bonded, colors kept together. That is the job REACHR was designed for.

> **Newt's Tip —** The glass is not lying to you. One piece of glass does what one piece of glass does. When the edge of the plate matters, use two.

Out of clicks? See how REACHR adds 50 or 100 MOA.

[See the REACHR →](https://blog.xringprecision.com/reach-r)

Until next time,  
**— Newt**  
Precision Specialist, X-Ring Precision

![Newt](https://blog.xringprecision.com/hubfs/Got%20it%20Newt.png)

NEWT

Precision Specialist · X-Ring Precision

Practical precision-shooting notes from the bench — optics, elevation, reloading, and gear that holds up when you measure it.

Go long · REACH-R systems

### Price the whole setup, not just the part.

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### More from Newt's Notes

[![TYPES OF ELEVATION PRISMS - LEARN WHAT MAKES THEM DIFFERENT](https://app-eu1.hubspot.com/api/filemanager/api/v2/files/474376856801/signed-url-redirect?portalId=148831916)](https://blog.xringprecision.com/newt_tips/what-makes-a-prism-a-prism)

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