As this is written, I am firmly ensconced at the Raasay distillery on the Island of Raasay among the Inner Hebrides islands off the western coast of Scotland. I got lucky with the weather and this place is absolutely unbelievable. Some of the most beautiful views I have seen to date and incredible hospitality where I am staying.
What do you think is on my mind among all this beauty and hospitality? Well, mostly the fact that I miss my family. They are half way around the world. The original plan was to bring them here with me, but due to unforeseen circumstances it was not in the cards this year.
While that is what I am mostly thinking about, the issue of prismatic magnification is still lingering in the OCD dominated parts of my brain, so let's get it out.
I went and looked a little bit at FOVs and size and weight of what's out there. I also went and looked at the FOVs of different thermal clip-ons since that also makes a difference.
The FOVs of thermal clip-ons I have looked at in the last couple of years varied from about 6 degrees to 17 degrees and almost everything in between.
That is not hugely helpful, but if we assume you are willing to lose some thermal FOV when using a thermal monocular as a clip-on, we can narrow it down to the range of 6 degrees to 9 degrees. That gives you roughly QVGA pixel counts when in front of a scope with most configurations.
Let's look at how that matches to the FOVs of common modern prismatics that I have been discussing over the last however many months.
Compact 3x prismatics from PA, Burris and Vortex are in the 7.2 to 7.4 degree range.
SwampFox Trihawk is a large prism design so it has 3x FOV of nearly 10 degrees.
ACOG 4x32, crappy eye relief and all, has 7 degree FOV.
Elcan OS 4x that is arguably my primary prismatic to date, has 6 degree FOV and weighs right around 17 ounces with the mounts.
Large prism 5x Saber from SwampFox is almost 6 degrees as well and ~25 ounces with an integrated mount.
The new 5x30 prismatic from Element should be a bit less than that, but likely close to 5.5-6 degrees and about the same weight with the mount. I think the Element might be the best one optically this side of an Elcan, but I will withhold judgement until I get a full production model here. It does have the best reticle of the bunch, of course (not that I would be biased about that, of course).
With small prism 5x scopes like Burris RT-5, PA SLx 5x and Vortex Spitfire Gen2, the FOV drops down to ~4 to 4.3 degree range, so performance with a thermal clip-on begins to suffer.
With 6x prismatics, we do not have a whole lot of reasonable datapoints, but PA has introduced a fairly compact 6x magnifier as did Vortex. A magnifier is essentially a prismatic scope without a reticle. Both of those have FOVs of just under 4 degrees. That's pretty usable, but not great with a clip-on.
I hope you are still with me after this entire stream of numerical unconsciousness.
There are a few things we can draw from this. One is that if you can bear the weight and plan to use a thermal clip-on, a large prism design might be worthwhile.
If you want magnification, a larger prism design with magnification of more than 5x will get a little unwieldy, unless you go with an ultrashort eyerelief scope. Those do have their place, but mostly on airguns. Element has a few coming up and I might look at them. Until then, large prism 5x designs are about as heavy as I'd want to go.
Even when at the same weight as an LPVO, prismatics do handle better simply because the weight is a bit further back. This handling advantage is further extended when working with a clip-on. It ends up being a lot closer to you. With a clip-on like Burris BTC-50 that has a rear-biased mount, I can fit both the prismatic and the clip-on on the upper receiver without touching the handguard (picture attached).
Where does that leave us? If compatibility with thermals is important stick with 3x or 4x prismatics, preferably with large prisms.
That gets you a lot of capability in a comparatively compact package.
Looking at FOVs of high quality LPVOs, like SAI6 1-6x24, it turns out that small prism scopes like the micro 3x and 5x prisms give you similar FOV to LPVOs, but for less money and weight.
Large prism designs, get you a wider and more immersive FOV than an LPVOs at the same magnification but weight becomes comparable (that will all change if I convince someone to make me an LPVO for ARs only with consistent 2.2" FOV).
This is where you have to be clear with what you are looking to do.
If for you this is a choice between an LPVO and a prismatic and you are looking to save some money, get a large prism 4x or 5x design. You will have an excellent experience with the image, wide FOV and reasonable compatibility with thermals. You might even be able to get away with the new 5x designs. I certainly plan to give them a shot. Once the 5x scopes from Element get here, you'll understand why.
If LPVO is not in the cards because of weight concerns, but you need some magnification, 3x scopes are looking good. Again, you retain good compatibility with thermals and add significant ability to positively ID and engage targets at distance at very low weight.
Small prism 5x designs are not a lot bigger and if you do not care about clip-ons they have their place. They are less expensive and lighter than LPVOs. What gives me a little bit of a pause is that I am clearly more comfortable shooting offhand on 3x than on 5x. Interestingly, larger prism 5x scopes mitigate that to a good degree with larger eyebox and massive FOV.
Is there such a thing as a medium prism design? It is not like there are only two prism sizes out there. Well, Elcan 4x I have is sorta that. If it had an adjustable eyepiece, it would eventually end up buried with me right next to the 3-15x50 Tangent Theta (there must be something in the water in Canada). The optics on that thing are glorious.
The truth is that there is only so much FOV I am willing to give up for magnification. With LPVOs, I can always turn the magnification down, but with a prismatic, I'd rather err on the side of keeping the FOV even if that means a little less magnification or a little more weight.
I wonder if I can get someone to make me a dual power 2.5x/6x prismatic....
