Prismatic Rifle Scopes: A Primer for Airgunners – Part One
Matt Coulter has been looking into prismatic rifle scopes. (Pun intended!) Because these scopes are so different from the regular riflescopes we’re all used to, he has put together this primer explaining his findings. Take it away Matt!
Background And Comparisons
For many shooting enthusiasts the phrase “Prismatic Scope” may not have great name recognition, but the term “ACOG” likely does.
Released in 1987, the 4×32 “Advanced Combat Optical Gunsight” made by Trijicon was chosen by the United States Special Operations Command to be the official scope for the M4 carbine. For many readers “ACOG” also conjures up memories of the cool rifle scope featured in countless video games.
Glyn Bindon, the designer of the ACOG rifle scope, based his initial design on a “half a pair of binoculars”. The binoculars that most of us are familiar with effectively fold the optical path using a combination of prisms and lenses. This enables prismatic optics to be more compact than traditional scopes where the light travels down the tube in a straight line.
Thanks to Sightron for the use of this diagram.
Prismatic Rifle Scopes – Eye Relief And Recoil Considerations
Most will agree that the breadth of available optic has never been as great as it is today. When it comes to selecting your next scope, when do prismatic scopes become a viable option to consider?
As mentioned in an earlier HAM article on the Immersive Optics line of prismatic scopes, eye relief can be very short and will limit the use of some of their prismatic scopes to rifles with no recoil (or very little recoil). Fortunately, with most PCPs (and all C02 airguns), recoil is minimal.
Airguns with more pronounced recoil (big bore PCPs) may still be good candidates to use with some prismatic scopes, but this is where close attention to manufacturer’s specifications are important.
Does the manufacturer list an “Impact Resistance” or “Shockproof Up To” value? If so, that is a clear indication that mechanically, the internals of the scope can handle the indicated recoil or shock force.
Likewise, what does the manufacturer list for eye relief? These values can vary greatly and may be inversely linked to the scope’s field of view.
A traditional 5x rifle scope such as the Sig Sauer Whiskey5 1-5×20 lists an eye relief of 3.9 – 4 Inches with a field of view of 20.2 Feet at 100 Yards.
Prismatic scopes designed for traditional firearms such as the Burris AR-536 5×36 prism scope lists an eye relief of 2.5 – 3.5 inches and field of view of 20 Feet at 100 Yards – so they are not too different from each other using those specifications.
Things get interesting when you’re shooting airguns and recoil is minimal and eye relief becomes less important!
The Immersive Optics 5×30 prismatic scope lists a shorter eye relief of 1.77 Inches but almost doubles the field of view of the other 5x scopes from Sig Sauer and Burris with the Immersive Optics offering a 37 Ft field of view at 100 Yards.
Here’s how these Fields of View compare.
Clearly there’s a big advantage for the “airgun use” prismatic scope. But, the view through any scope is two-dimensional. This means that the AREA we are able to view at any one time can be calculated by the equation Area = π x r squared.
Now we see there’s actually a much larger advantage for the short eye relief, airgun prismatic scope than might appear from the FoV comparison.
We know that when we choose option “A” we may need to give up option “B”. There’s an “opportunity cost” in nearly everything we do – including picking out optics for our airguns. The advent of affordable prismatic scopes (especially ones built with airguns in mind) introduces some new and interesting options.
So, are prismatic scopes fundamentally better than traditional scopes? No, not necessarily. But with optics manufacturers beginning to recognize airgunning as a specific market segment, this opens the door to reconsidering some scope design conventions with roots going back to firearm-specific needs such as generous eye relief.
Mounting Considerations For Prismatic Rifle Scopes
What is it like to mount and use a prismatic scope? How significant is losing much of the eye relief that we’re so accustomed to in traditional scopes?
No doubt, there will be an adjustment period for the shooter to become comfortable with less eye relief. The prismatic scope’s ocular (rear) lens will be close to the eye.
This writer’s experience is with a prismatic scope with exceptionally short eye relief of 0.55 In. (14 mm) and I wear eyeglasses. At this distance the shooter’s glasses may contact the scope’s ocular lens while shooting. This is how I position a prismatic scope on my Cricket II Tactical.
Mounting and sighting in a prismatic scope isn’t fundamentally any different than with traditional scopes. You may even be able to use one of your own scope rings for mounting if the prismatic scope has a standard tube size such as 30 mm. However, many prismatic scopes have tube diameters that are much larger and considerably less common than what we airgun shooters are familiar with.
Fortunately, prismatic scope makers recognize this and may include (or offer) dedicated mounting options. Some may include extensions (typically Picatinny rails) to shift the scope rearward on the rifle to make up for the reduced eye relief.