Scopes around 3-9x cover a majority of rifle sighting requirements: powerful enough for extended distances, but can be dialed down for moving targets close-in. Such a versatile optic is naturally partnered with the adaptable AR-15 modern sporting rifle, with the MK11 and MK12 rifles having done much to promote such a partnership. Over time, scopes in this application (usually termed tactical scopes) have evolved to feature exposed knobs and ranging reticles. This article evaluates a sample of such scopes.
Candidates
All candidates have exposed turrets and reticles marked in miliradians (mil), although some can also be had in minute of angle (moa). Scopes with mismatched turrets and reticles require the conversion of 1 mil = 3.44 moa when inputting observed corrections into the turret.
The exposed turrets are readily accessible for dialing-in corrections and are generally stiff enough to reject accidental bumps. They also feature a resettable zero so that the turrets can read “0” wherever the rifle is actually zeroed. Some scopes with multiple turret revolutions include a zero stop and revolution counter to prevent lining up at “0” in the wrong revolution, causing the user to mistakenly think it is back at zero. With a hard zero stop such as the clutch on Nightforce, one dials the turret down until it stops moving. With a soft zero stop such as shims on Vortex, one must visually reset back to “0” after the turret stop moving (usually less than 1mil below zero).
Terms and Definitions
From the optical perspective, eye relief is the ideal distance between the eye and the ocular lens. While the image may be usable at other distances, field-of-view and image quality are compromised. Eye relief that changes with magnification can be undesirable as it leads to inconsistent head position. Field-of-view (FOV) is self-explanatory and is not influenced by the size of the front objective. That size, denoted by the number after the “x” (e.g., 2.5-10x42mm has a 42mm diameter front objective), drives resolution and light gathering abilities, as well as exit pupil size. A larger front objective linearly increases resolution as described in Rayleigh’s criterion, and it nonlinearly increases light gathering power due to the circular area being pr2.
The exit pupil is the size of the image projected at the eye relief point, and is idealized as the front objective divide by magnification (e.g. 42mm objective at 10x has a theoretical 4.2mm exit pupil). A higher number is more forgiving of poor head position. A 24mm front objective can be challenging to use in low light at high magnifications given that an eye pupil is roughly 2-4mm in full sunlight and 3-8mm in low light. The tradeoff in front objective size is between resolution/light gathering/exit pupil and weight.
To avoid parallax error, a scope must bring the target and reticle on the same focal plane. As an example, driver and passenger will read a speedometer differently due to the needle and dial being on different planes and the slant view angle of the passenger causes parallax error. To remove the error, the reticle focus ring (a.k.a. fast focus eyepiece) is first used to compensate user vision deficiencies. Then the parallax adjustment knob brings the target image into focus on the same focal plane as the reticle. Note that reticle focus is user specific and should be a one-time setting, while target focus varies with distances. SWFA, Leupold MK4 and both Nightforce scopes feature locks on the reticle focus ring, while flip-open scope caps can also reasonably secure the ring.
Another way to avoid parallax is to look directly along the optical axis by centering the crosshairs in the ocular lens. The maximum parallax error, induced when the user’s pupil rests on the edge of the exit pupil, is given by the equation where D is the front objective diameter, T is the range to target, and P is the parallax-free distance. Take the SWFA 3-9x42mm with parallax-free distance fixed at 100yd (manufacturer information), it has a maximum error of 25mm (0.13mil) at 200m, 71mm (0.18mil) at 400m, 117mm (0.2mil) at 600m. Given the error magnitude and mitigation procedure, the author believes parallax adjustment is not necessary and that the chief benefit is a sharp target
at varying distances.
Chromatic Aberration (CA henceforth) manifests as color fringing between light and dark objects which causes the image to lose sharpness. Different colors have different wavelengths and refractive indices for the same lens; this is why a prism can split white light into the rainbow spectrum. As such, focus error is varying as a function of wavelength (e.g., zero error at only one wavelength). Keeping in mind that CA is eliminated if focus error is zero at all wavelengths, note that adding lenses together changes the error function into a second-order shape for achromatic (2-lenses) and third-order shape for apochromatic (3-lenses) for a flatter line which reduces focus error across the spectrum. However, more lenses mean more weight and cost.
On mechanically related matters, First-Focal-Plane (FFP) or Second-Focal-Plane (SFP) indicates where the reticle is placed. Reticle in SFP stays the same size independent of magnification, so the mil hash mark is only true at one magnification (usually the maximum). Reticle in FFP changes size, so the 1mil hash mark holds true across magnification. An illuminated reticle can be useful at night if dim enough to not overpower the target, and one can cover the front objective with a cap for use as an occluded gun sight during daytime at close-range. Lastly, some of the candidate scopes can be equipped with a Bullet-Drop-Compensator (BDC) turret which is marked in range. While easy to use, a BDC is only pinpoint accurate for a particular cartridge and atmospheric conditions.
Optical Performance
The author found the brightness and clarity of all candidates more than adequate for sighting purposes with none exhibiting bad lens flare. In fact, the SWFA and Nightforce x32 front objectives are set about 25cm inside the body and act as an integral sun shade. This section addresses optical artifacts that may cause eye fatigue or hinder speed: chromatic aberration, color cast, distortions, and eye box. Readers are reminded that the following evaluation is subjective to the author. Readers should also note that optical performance is degraded when the turrets are near their travel limits.
The SWFA has visible CA at 9x for far targets, but is well controlled at the 100yd parallax-free distance. The Leupold VXR has similar performance. The Vortex, Burris, and both of the Nightforce all have less CA that is still noticeable if carefully examined at maximum magnification. Leupold MK4 requires the eye to be positioned directly on the optical axis to obtain a nearly CA free image, but the image quality rapidly degrades when off axis.
SWFA and Leupold VXR are color neutral and fairly vivid at 3x, only falling off towards 9x with some purple tinge. Vortex and both Nightforce renders a rich image; but while Vortex has a visible purple tinge at 10x, both Nightforce only have a slight outline at 10x. Burris is color neutral but has noticeable purple tinge at 10x. In comparison, Leupold MK4 renders colors somewhat flatly with a cool cast across the magnification range.
Distortion is another optical artifact that can affect scope use. Barrel distortion is an artifact where a column of straight lines are rendered in a bulging manner akin to the stakes of a wooden barrel. Petzval field curvature is where a flat object (e.g. wall) is rendered on a curved image plane, such that only the central part is sharp. Tunneling is a phenomenon where, as magnification decreases, the image itself no longer fills the ocular lens and the black band between the image and ocular bell increases in size.
SWFA has noticeable tunneling from 3x to 4x accompanied by slight barrel distortions, and is not as sharp for distant targets due to the fixed parallax. Leupold VXR also suffers a lesser degree of tunneling from 3x to 3.5x. Vortex is sharp across the image with some fuzziness near the very edge. The Burris is distortion free at 2x, but has Petzval field curvature at 10x which causes smearing and noticeable CA at the edges. Leupold MK4 has slight tunneling from 3.5x to 4x, and slight distortions at the very edges throughout the magnification range. Lastly, the Nightforce x32 has noticeable barrel distortion at 2.5x which are mostly gone by 5x, causing the author some disorientation when panning for a moving target. The x42 scope has some barrel distortion at the outer fringes at 2.5x. Both are distortion free
and sharp at 10x.
Eye box refers to a combination of eye relief and exit pupil. As expected, the author found all scopes with 40/42mm front objectives much easier to mount than those with 32mm front objectives. This is especially true for low-light conditions when the pupil has dilated. Vortex has a noticeable black edge between the image and eyepiece, creating a sucked-in feel. In contrast, both Leupold candidates and the SWFA leave very little black space and feel less claustrophobic. Both Nightforce ocular lenses are not inset much from the eyepiece, but there is a noticeable black edge which can bleed into the image under the right lighting conditions. In addition, both Nightforce scopes seem less forgiving in eye relief as there is very little distance between having the ocular ring obscures the full image, to having the image not fill the ocular lens. As to the Leupold MK4 that varies eye relief with magnification, the author found it bothersome but workable especially since 10x is used typically in prone, which positions the head closer to the eyepiece anyway.
Mechanical Evaluation
A positive and relatively stiff click is necessary on the turret to prevent accidental changes and to provide users feedback. SWFA turrets are tactile with the right amount of resistance, but are not very audible. While the clicks are not mushy, they do not “clunk” into place either. The turrets also have a slightly sharp edge on the cutouts. Leupold VXR turrets have audible and tactile clicks, but are too easily moved. Curiously, both windage and elevation knobs move easier in one direction than the other. In addition, they lack revolution counters and zero stop, so one can easily get lost in the dials. Vortex turrets are comfortable in hand with tactile but not too audible adjustments. They also feature fiber optic to indicate the zero mark. Burris elevation turret is very stiff and positive that became easier to move after break-in, while the windage turret is just as stiff but less positive. It is also the only scope amongst those evaluated to have 10mils per revolution which can take a 77gr 5.56 cartridge to around 800m when using a 200m zero, obviating the need to track revolutions for most scenarios. Leupold MK4 M5 turrets have sharp edges that are not comfortable to adjust with bare hands. Initially, the elevation turret is stiff and reasonably positive but windage is very mushy. There are significant improvements after cycling the turrets from one end to another multiple times: the elevation clicks are audible and crisp while the windage clicks are now audible if still mushy. However, the revolution counter is not user friendly because it is readable only at the “0” mark and the revolutions are unnumbered while lacking a zero stop. In contrast, Nightforce turrets feature very crisp, audible, and positive clicks that while stiff enough to lend confidence, are easy to dial and easy on the hands.
The magnification ring on every scope feels very smooth with the Leupold VXR with just a trace of grit. Both of the Nightforce feature a “Power Throw Lever” that helps in changing magnification rapidly without breaking the shooting position. Similarly, both of the Leupold scopes feature a small nub that is unobtrusive but achieves the same goal. For other scopes, there are aftermarket “cat tails” that clamp onto the magnification rings for the same effect. Of note is the SWAF magnification marking that proves impossible to read from a shooting position.
Handling Impressions
The author was surprised by the change in handling of a lightweight AR-15 due to the center-of-gravity and weight of various scopes. Both Leupold MK4 and the Vortex have roughly the same weight, but the Vortex balances 25mm aft of the Leupold. The reduced rotational inertia makes a Vortex equipped rifle much easier to swing. The Burris weights only about 4oz more than the Leupold MK4, but it makes the rifle feel top-heavy.
Aside from physical attributes, reticle design also heavily influences the target acquisition speed. SWFA features a reversed German #1 reticle with the heavy post on top and extended mil marks on the bottom. The author found the heavy posts to be too distracting at max magnification but does an excellent job of drawing the eye to the center at low magnification. The Burris reticle strikes a better balance and the mil marks are large enough to read at about 4x against a light colored background. The 0.1mil marks of the Nightforce SFP MIL-R reticle are too fine to be usable, and the crosses on the vertical bar and the downward hashes on the horizontal bars make the reticle feel too busy. The mil marks on the Vortex is usable at lower magnifications even when the marks themselves are indistinguishable (starting about 7x) because of the numbering. In contrast, the marks on the Leupold TMR reticle are not visible below 6x on anything but than a white background.
To compare the utility of SFP and FFP reticle at intermediate magnifications, the author compared using holdovers with the FFP Leupold MK4 and SFP Nightforce x42 at 5x where the former acted like a duplex reticle and the latter still had visible hash marks. A healthy amount of changes were dialed into both elevation and windage to give a random impact point. The author fired into a dirt berm and proceeded to walk the splash onto target with holds. On average, it took the FFP one more round than the SFP to hit because the visible marks allow for a more systematic approach. The tradeoff is that if one must dial down the magnification (e.g. falling light) and use the reticle to range or holdover, then mental math must be performed. While it is difficult to determine whether FFP or SFP is a definite winner, there are scenarios where one would be superior.
Conclusion
Despite the author’s nit-picking, all evaluated scopes are fine pieces of kit. Optical performances as well as turret feel are mostly subjective, and the differences are subtle and only apparent when compared side-by-side or under challenging conditions. The biggest discriminator aside from price is the feature-set such as FFP or illumination. SWFA SS 3-9x40mm delivers a no frills scope that performs well for a very reasonable price. Vortex PST 2.5-10x32mm is a light-weight feature-packed scope that has good optics at a reasonable price. Burris trades some optical performance for 10mil per rev turrets and larger front objective. Leupold MK4 3.5-10x40mm gives up some of those features for better optics at nearly double the price. Lastly, the excellent optics and mechanics of Nightforce is reflected in their price.
