Tactical Scopes: Optical Performance Part 2

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In the previous post, I reviewed the overall optical performance scores, and the details of the optical clarity tests. Those tests were the Snellen eye chart test, and the high contrast and low contrast US Air Force line charts. It was a lot of information! So I broke out the details for the other optical performance tests into Part 2. In this post, I’ll review the results I found for:

• Measured Field of View
• Measured Max Magnification
• Zoom Ratio

If you’re interested in optical clarity (i.e. image quality), or would like to see the overall optical score, check out Part 1 to these optical results.

Measured Field of View

Good news … measuring the field of view was much more straight-forward than the optical clarity tests! I essentially made a big tape measure that I could read at 100 yards, and then looked through each scope and recorded the apparent field of view. I did this for the maximum and minimum magnification for each scope, and also did it for exactly 18x on each scope. (Note: I carefully found 18x on each scope, and didn’t just rely on the index on the scope’s magnification ring. I explain how I measured that in detail in my How To Measure the Apparent Magnification of a Scope post.)

Since the field of view obviously changes significantly based on magnification, looking at the field of view of all the scopes at 18x is the fairest way to compare them.

The Kahles K 6-24×56 rifle scope offered the widest field of view at 18x magnification with 7.2 feet at 100 yards, which is quite impressive. But the Valdada IOR RECON 4-28×50, Schmidt and Bender PMII 5-25×56, Nightforce BEAST 5-25×56, Nightforce ATACR 5-25×56, US Optics ER25 5-25×58, and Zeiss Victory Diavari 6–24×56 were all right there at the top as well.

I realize being able to see 5.2 feet or 7.2 feet at 100 yards may not seem like much of a difference. It’s just 2 feet, right? But at 1000 yards, that equates to either 52 feet or 72 feet … a difference of 20 feet! That can be the difference between seeing the target … or it being hidden just outside your field of view. If you need to quickly find a target through your scope (like in a timed competition or a hunting scenario), a larger field of view can be the difference between getting a shot off or not.

I also measured the field of view at the maximum and minimum magnification for each scope, and the results for that are provided below. Keep in mind the minimum and maximum zoom varied by scope, and the field of view is dependent on the magnification. For example, a scope with a max zoom of 18x should have a much larger field of view at its max magnification than a scope with 25x zoom. That isn’t a fair comparison. So this data isn’t intended for comparison between scopes (which is why I didn’t display it in a chart), but is just informational purposes.

Rifle Scope	Measured FOV @ MIN Zoom (ft at 100yds)	Measured FOV @ MAX Zoom (ft at 100yds)
Bushnell Elite Tactical DMR 3.5-21×50	25.4	5.2
Bushnell Elite Tactical XRS 4.5-30×50	23.5	3.6
Hensoldt ZF 3.5-26×56	29.8	4.3
Kahles K 6-24×56	20.6	5.2
Leupold Mark 6 3-18×44	34.3	5.8
Leupold Mark 8 3.5-25×56	31.7	4.2
March Tactical 3-24×42 FFP	33.3	4.2
Nightforce ATACR 5-25×56	17.7	4.8
Nightforce BEAST 5-25×56	18.7	4.9
Nightforce NXS 5.5-22×50	17.8	4.7
Schmidt and Bender PMII 3-27×56 High Power	38.7*	4.4
Schmidt and Bender PMII 5-25×56	16.2	4.8
Steiner Military 5-25×56	21.5	4.3
US Optics ER25 5-25×58	14.5	5.0
Valdada IOR 3.5-18×50	27.3	6.2
Valdada IOR RECON Tactical 4-28×50	28.7	4.3
Vortex Razor HD 5-20×50	20.3	5.2
Zeiss Victory Diavari 6–24×56	17.6	5.1

*The Schmidt & Bender PMII 3-27×56 may seem to have a giant field of view for its max magnification of 27x, but I measured its apparent magnification at the max zoom to actually be 22.4x … not 27x. I explain how I measured that in the How To Measure the Apparent Magnification of a Scope post. More on that below as well.

Measured Max Magnification

In the book Applied Ballistics for Long-Range Shooting, Bryan Litz has a quote that really sums up my approach to a lot of things:

The overall philosophy of dealing with sights is: DON’T TAKE ANYTHING FOR GRANTED! The bottom line is to deal in absolute fundamentals, measure them, and remove all the assumptions. Don’t ever assume that something is what it says without measuring it. – Bryan Litz

Since the upper end of the magnification range is so important to a long-range shooter, I didn’t want to assume that the apparent magnification was exactly what the manufacturer claimed that it was. In fact, we know there are at least a few scopes that don’t match up with the magnification indicated in the model name. Here is one example:

Now to defend Leupold a little here, this is something virtually every scope manufacturer does. They’re just unique because they’re actually upfront with this information. I can appreciate that type of integrity. I can also say that they don’t have this type of “optimistic rounding” on all of their scopes. For example, they say the max magnification for the Leupold Mark 8 3.5-25×56 is exactly 25.0, and after measuring it myself … it is exactly 25.0. This was the same case for the Leupold Mark 6 3-18×44 … exactly 18.0. So I’m not trying to throw them under the bus here, but just trying to illustrate that it is a known practice even among the most reputable companies. While a few companies might own up to this type of thing in their published specs, it is worth measuring.

I explain how I measured this in exhaustive detail in my How To Measure the Apparent Magnification of a Scope post, so I won’t repeat all that information here. I’d suggest you go check that out if you’re interested, or have doubts about the reliability of this data. Here is a quick image that shows the basic idea, but you should read the whole post if you’d like to know more.

One additional note is that I measured this at least two times, on two different occasions for each scope. If the scope was off from what the manufacturer claimed, then I measured it a third time. There was a complete teardown and setup in between each test. I never want to publish anything that might be considered negative without feeling confident I’d done the best I could to ensure the data is accurate.

Note: Since I did run this multiple times, I can estimate the accuracy of this information to be ±0.5x.

Rifle Scope	Claimed Max Zoom	Measured Max Zoom
Bushnell Elite Tactical DMR 3.5-21×50	21	19.8
Bushnell Elite Tactical XRS 4.5-30×50	30	29.7
Hensoldt ZF 3.5-26×56	26	26.0
Kahles K 6-24×56	24	24.5
Leupold Mark 6 3-18×44	18	18.0
Leupold Mark 8 3.5-25×56	25	25.0
March Tactical 3-24×42 FFP	24	23.6
Nightforce ATACR 5-25×56	25	25.0
Nightforce BEAST 5-25×56	25	24.6
Nightforce NXS 5.5-22×50	22	22.1
Schmidt and Bender PMII 3-27×56 High Power	27	22.4
Schmidt and Bender PMII 5-25×56	25	24.8
Steiner Military 5-25×56	25	25.6
US Optics ER25 5-25×58	25	24.6
Valdada IOR 3.5-18×50	18	18.1
Valdada IOR RECON Tactical 4-28×50	28	28.5
Vortex Razor HD 5-20×50	20	17.7
Zeiss Victory Diavari 6–24×56	24	24.1

The new Schmidt and Bender PMII 3-27×56 scope is the only one that is grossly off. It is advertised to have a max magnification of 27x, but I found the apparent magnification only went up to 22.4x. When I first calculated it, I thought I must have done something wrong … but after repeating the process several times, I’m confident this is reality (at least for the scope I had in hand). I may have just got a bad unit, but I tried to contact Schmidt and Bender a few times during this field test and didn’t get a response. This was one of the brand new scopes that the guys at EuroOptic.com let me borrow for the tests, and since it has a $7,000 price tag … I didn’t feel comfortable asking them for another one to double-check.

One optics expert did tell me during the peer review period of these tests that you can typically expect 3-5% variance in magnification due to variations in the eyepiece or objective focal systems. For an 18x scope, 5% is 0.9x, and for a 25x scope 5% is 1.25x. So some slight differences may be inherent to the complexity of these optical systems. However, a few of these companies nailed it, so I’m not sure if they have tighter tolerances or if I lucked into a good test unit.

I didn’t measure the min magnification, because of how time consuming all this was. Most long-range shooters are more focused on what the high-end is on the magnification range, which is evident from the screenshot shown previously of the Leupold website. They have a column to indicate the “Max Magnification,” but not the “Min Magnification.” While the low end of the magnification range isn’t unimportant, it is simply less important to most shooters.

Zoom Ratio

Roughly speaking, zoom ratio describes how large the magnification range is. It is simple to calculate:

For example, a 6-24x scope would be 24 ÷ 6, which equates to a zoom ratio of 4. Likewise, a 4-28x scope would be 28 ÷ 4, which is a zoom ratio of 7. Pretty simple, right?

High zoom ratios are obviously desired. I wish I had a 1-50x scope, just like everyone else! Rifle scopes with high zoom ratios are simply more flexible, because they give you good low end magnification for a wide field of view and quick target acquisition, but they also give you a lot of magnification for those long-range shots. You simply don’t have to compromise. But to say that cranking up the zoom ratio is difficult, would be an understatement. I don’t claim to be an expert in optics design, but I’ve talked to enough to know how complex these systems have become over the past few years. I won’t bore you with the details, but suffice to say … they’re all putting a lot of effort into continually raising the bar on this front, and we’re all benefiting from it.

Right now, most high-end rifle scope designs from the past decade have a zoom ratio of 4 or 5, with a few reaching 6. But in the past couple years, new scope designs have continued to press the limits. Here are a few of the scopes in this test that have huge zoom ratios of 7 or 8, and some are aspiring to even more. There are even a few new designs in the works that are pressing a zoom ratio of 10, like a 5-50x scope!

• Hensoldt ZF 3.5-26×56
• Leupold Mark 8 3.5-25×56
• March Tactical 3-24×42 FFP
• Schmidt and Bender PMII 3-27×56 High Power
• Valdada IOR RECON 4-28×50

Here is a visual representation of the actual magnification range of each scope, with the max magnification reflecting what I measured it to be and the min reflects what the manufacturer advertises.

See how some lines are longer than others in the chart above? The length of the line represents the magnification range the scope covers. Zoom ratio is closely related to the length of the line, although they aren’t exactly the same thing.

Here is the calculated zoom ratio of each scope based on the magnification ranges shown above:

You can see the March Tactical 3-24×42 FFP scope is way out ahead, and they are one of a leading innovator in this area. In fact, they’re a company pressing that 1:10 zoom ratio in their newest designs. The Schmidt and Bender PMII 3-27×56 High Power scope should theoretically have a zoom ratio of 9, but I measured the max magnification to be 22.4 … not 27. So that meant the zoom ratio was closer to 7.5, but that is still outstanding. The scope Hensoldt released a few months ago in the Hensoldt ZF 3.5-26×56 is an impressive scope all around, including its 7.4 zoom ratio. The Leupold Mark 8 3.5-25×56 and Valdada IOR RECON 4-28×50 are also pressing the limits with zoom ratios above 7 as well. Valdada IOR is a perennial innovator. They actually released this design in 2007, which I believe made it the first to have a zoom ratio of 7 or more.

Other Optical Aspects

A few people have asked me how other optical aspects compared, like chromatic aberration (CA), tunneling effect, etc. There are actually several optical aspects like that which I didn’t test either because I couldn’t find an objective way to quantify them, or I considered them secondary to the other aspects I focused on. For example, the guys at Leupold spend a lot of time trying to get the entire field of view clear and sharp, so they were encouraging me to somehow rate edge-to-edge clarity. However, although I don’t think that is unimportant, I tend to side more towards ILya on this point (and chromatic aberration as well).

For a riflescope, it is important that whatever is in the center of the field of view is as sharp and clear as possible. The performance at the edges is comparatively less important. … Generally, a lot of variable magnification riflescopes have visible edge distortion at lower magnifications and, by and large, it is not very interesting. We aim with the center portion of the image, so whatever is at the edge is not all that important for as long as it is not strong enough to distract your attention from the point of aim. … My take on Chromatic Aberration in riflescopes, by and large, is the same as that on all aberrations and distortions: I want the center of the image (i.e. the aiming point) to be clear, sharp and as devoid of undesirable artefacts as possible. I am, however, far more forgiving toward edge effects as long as they are not strong enough to be distracting. – ILya Koshkin (from Riflescope Fundamentals)

I have a pragmatic view of this. It’s not that I’m okay with tunneling effect, or poor CA, edge clarity, or depth of field … they just aren’t reasons I missed a shot. I can’t say I’ve ever missed an animal or dropped a point in a competition because my scope lacked in those areas. I’ve missed lots of shots, for lots of reasons … just not those.

But, if you’re interested in seeing that information, ILya has published an outstanding article that looks at a few of these exact scopes and evaluates those finer points of optical performance. You can find that at http://opticsthoughts.com/?page_id=1254.

Other Post in this Series

This is just one of a whole series of posts related to this high-end tactical scope field test. Here are links to the others:

1. Field Test Overview & Rifle Scope Line-Up Overview of how I came up with the tests, what scopes were included, and where each scope came from.
2. Optical Performance Results
   • Summary & Part 1: Provides summary and overall score for optical performance. Explain optical clarity was measured (i.e. image quality), and provides detailed results for those tests.
   • Part 2: Covers detailed results for measured field of view, max magnification, and zoom ratio.
3. Ergonomics & Experience Behind the Scope
   • Part 1: Side-by-side comparisons on topics like weight, size, eye relief, and how easy turrets are to use and read
   • Part 2 & Part 3: Goes through each scope highlighting the unique features, provides a demo video from the shooter’s perspective, and includes a photo gallery with shots from every angle.
   • Summary: Provides overall scores related to ergonomics and explains what those are based on.
4. Advanced Features
   • Reticles: See every tactical reticle offered on each scope.
   • Misc Features: Covers features like illumination, focal plane, zero stop, locking turrets, MTC, mil-spec anodozing, one-piece tubes
   • Warranty & Where They’re Made: Shows where each scope is made, and covers the details of the warranty terms and where the work is performed.
   • Summary: Overall scores related to advanced features and how those were calculated.
5. Mechanical Performance
   • Part 1: Shows how precisely calibrated the clicks are on each scope.
   • Part 2: Reticle cant, measured elevation travel for each scope, and other mechanical tests
   • Summary: Overall scores related to mechanical performance.
6. Summary & Overall Scores: Provides summary and overall score for entire field test.