A Primer For Airgunners On Prismatic Scopes – Part Two
In the second part of this story, Matt Coulter shares some detailed results from his testing of conventional and prismatic scopes.
Prismatic Scopes: Simplicity of a Fixed Focus With The Benefits of a Zoom
This is where we’re going to see the main benefit of giving up some of your eye relief. A prismatic scope provides the shooter with a reasonable magnification while experiencing the field of view of a lower magnification scope.
How much lower? This shooter’s backyard testing revealed that the Immersive Optics 10×24 scope gave a measured field of view of 15 Feet from left to right at 50 Yards.
To replicate this field of view on a traditional scope, a Sightmark Citadel 3-18×50, had to be zoomed back to just over 3x to achieve this same field of view. For reference, the Citadel is listed as having a field of view of 33 Feet at 100 Yards on its lowest magnification.
The following photos comparing scopes see details in photo section below…
1. Immersive Optics 10x prismatic scope: Full view.
2. Sightmark 3-18×50: Full view.
3. Sightmark zoomed out to 3.3x to copy 15-Foot field of view of 10x prismatic scope at 50 Yards.
In the interest of “science” (insert a sly wink and a nod here!) the target that was set up at 50 Yards to measure field of view and zoom levels couldn’t just be left there!
A pair of Kalibrgun Cricket IIs were ready and waiting to demonstrate what shooting at a matched field of view (not magnification) would do to the size of the resulting groups.
Here’s the results. In both cases the pellets used were 18 Grain JSBs. Both Crickets were in .22 caliber and tuned to produce about 36 Ft/Lbs of Muzzle Energy (950 FPS). The orange stickers got shot off each target!
No reader will be surprised to learn that the group on the right was significantly tighter at 10x with the prismatic scope. The group on the right was than with the traditional scope set at about 3.3x.
What this experience also demonstrated to me is how challenging it can be to use a First Focal Plane (FFP) scope at its lowest magnifications!
The FFP reticle at 3.3x was exceedingly difficult to read and this shooter had to turn on the scope’s illumination to accurately position the point of aim at the target. Aiming became more of an exercise in centering the crosshairs than identifying a specific point of aim with a specific part of the reticle.
The ½ inch sticker was easily seen and almost covered by the 10x prismatic scope reticle at 50 Yards. However, the sticker was not visible at all with 3.3x power on the traditional scope.
Let’s Look Closer
The Immersive Optics 10×24 prismatic scope has such a different feel from a traditional scope that I needed to understand if my eyes and mind were playing tricks on me. In using this scope my impression is that it’s not quite 10 power (10x) magnification.
To test this, I used Adobe Photoshop to view the photos at 400% their native size and measure the height of the letter (8.5×11 Inch) paper my target was printed on.
Through the Sightmark Citadel set at 10x, this paper measured approximately 190 pixels high. Through the Immersive Optics 10×24, the target’s height measured approximately 180 pixels. Uncropped, the vertical dimension of these images are 2268 pixels.
Here’s what I found. First the prismatic scope:
Now the conventional zoom scope:
Notice in the measurement photos above, that the Sightmark has substantial chromatic aberration (lots of purple fringing along the sides of straight lines where the Immersive Optics has little to none.
The Samsung S20 phone I used for these photographs has wide, normal and telephoto lenses. I was unable to capture the full field of view through my phone camera’s “normal” lens. The rectangular image size of my phone’s photographs crops the top and bottom of the field of view from the prismatic scope. My phone’s “wide” lens can capture it all but does so at a lower sharpness 🙁
This means that any full-circular images captured from the prismatic scope probably should not be directly compared to images taken through the Sightmark with my phone’s normal lens.
My takeaway from this is that the Immersive Optics 10×24 provides slightly less magnification, but 10 pixels is a small amount – in fact I was expecting this difference to be greater.
It’s also worth noting that photos taken through the Immersive Optics scope seem to lack detail when compared to the Sightmark images. I did not notice this when shooting. When shooting I am able to resolve equally small details between the scopes.
Final thought: Take a long look at these short prismatic scopes!
A medium power prismatic scope makes a great general-purpose scope for airguns! Will it replace a benchrest scope or other special use scopes? No, probably not.
Like the old car expression “There is no substitute for cubic inches” (regarding automotive engine displacement), there isn’t any substitute for higher magnification when it comes to long-range accuracy.
There is no doubt that the lack of eye relief of prismatic scopes will be challenge for some shooters. However, the versatility and simplicity of a medium-power, fixed-focus scope for general purpose airgunning is something worth considering!