5X Supremacy

and the case for the Primary Arms SLx 5X MicroPrism

There exists a few primary magnification nodes:

• 1X, the Mk1 eyeball’s strong suit

• 10X, a typical military high-end for DMRs

• 25X, the typical high-end of long-range scopes

Secondary nodes come from factors of 2 on these nodes, as the effect of magnification is geometric rather than linear, i.e. a 1X increase from 1X to 2X is huge, but 20X vs 21X is negligible. So, we end up with 1, 2, 2.5, 5, 10, 20, and 25X.

If we consider that most engagements do not require more than 25X, especially given that full daylight the human pupil is 2-3mm in diameter. If we calculate an objective lens based on this, we get a requirement for a 50-75mm objective, which is where larger rifle scopes and smaller spotting scopes land.

Now, because the effects are geometric, let’s look at the square and cubic roots of 25: 2.924 and 5, or basically 3 and 5.

Now, look at all of the combat optics used for the last 100 years. Excepting precision optics, we find 2.5X-4X being typical going back to clip-on WWII sniper scopes. With combat ranges typically falling within 300 meters, Russian doctrinal defensive spacing being 500 meters, and the typical effective range of a carbine-equipped rifleman with magnification being about 500 meters (excluding the well-trained), it all comes together: 500 meters is the minimum goal range for accurate semi-automatic fires. Considering also the transonic range of M855 and similar cartridges out of an M16, we land at a ~700 meter max effective range, past which performance rapidly falls.

Let’s also consider the role and positioning of the 1X gunsight.

1X has its primary use in dynamic shooting.

Magnification is best used supported, as it is most-applicable to distant and small targets.

If 1X redundancy can be waived, we can look at a low-end magnification value. Geometrically:

• the center is 5 (the square root)

• the middle third runs from 2.92 to 8.55 (cubic divisions)

• the middle half runs from 2.24 to 11.18 (quartic divisions)

So, practically, this covers:

• 2-3X prisms

• 2.5-10X scopes (and similar)

• 5X prisms

• 1-8X LPVOs (or 1-10X)

For the typical-but-large 56mm objective lens, using the eye diameter to limit exit pupil, we get the following magnification limits:

• Bright sun: 28X

• Normal sunny day: 20-24X

• Overcast: 12-15X

For the more common 50mm objective lens, using the eye diameter to limit exit pupil, we get the following magnification limits:

• Bright sun: 25X

• Normal sunny day: 17-20X

• Overcast: 10-12.5X

This is why there are so many 4-16x50mm scopes out there, and these numbers are only meaningful in a nice atmosphere. Mirage will rapidly cut into this venture.

Looking at the Primary Arms MicroPrism family:

• 3x25: 8.3 mm exit pupil

• 5x36mm: 7.2 mm exit pupil

Both of these are useable until NODs come down for transitional lighting, and even afterwards for mixed lighting and close-range magnified targeting, at a fantastically small size and weight.

So, the 5X sits right at the threshold of all-day function with a little wiggle room for eye placement, and the 3X is a generous, faster alternative.

It’s halfway between a Mk1 eyeball and a precision riflescope at maximum magnification, and at a quarter of the weight of the typical PRS optic.

Waiving the 1X redundancy and applying a 5X positional optic with a red dot high or offset is a flexible solution. Paired with an 800m BDC and good light, the lens assembly can facilitate hits at over 800 yards under favorable conditions. Magnification doesn’t solve the atmospheric issues.