Given good lenses of around F1.2, that’s pretty constant however even with this, the System Resolution will vary significantly between two devices. It tells you how much detail you can see and at what range. For many, this is perhaps the single most significant measurement yet very few people understand the relationship between tube resolution, objective lens focal length and system resolution.

First, System Resolution is measured in Cycles per Milliradian. A Radius is the distance from you to wherever if you take that distance and draw a circle and if you measure the circumference of that circle, you’ll find 2 x pi radians in it and a milliradian is a thousandth of a radian… So you could take 360 degrees and divide by 2 and then by pi and then by a thousand… Or you can take a practical example to understand it.

The Nato Specification is such a practical example:
CYCLES PER MILLIRADIAN (CY/MR) – Units used to measure resolution. A milliradian is the angle created by one yard at a distance of 1,000 yards. This means that a device that can detect two 1/2 yard objects separated by 1/2 yard at 1,000 yards has a resolution of 1.0 cy/mr.

OK… So understanding that, it means that a milliradian means that at 1000 inches, you can detect a half inch object separated from another half-inch object by half an inch… Or the same in feet, yards, meters, kilometres or anything else.

Except the word Cycle somewhat defines what the two objects have to be and also what has to be between them… Like a couple of black rubbish bins at 1000 yards, standing in front of a while wall.

Because if they were grey bins in front of a grey wall, you’re probably not going to notice them at 10 feet, let alone 1000 yards.

Now you understand that, let’s look at what it means. Your eyes, assuming you have 20/20 vision, have a natural visual acuity of 1.71 cycles per milliradian. That’s at their best. So that one-yard space at 1000 yards can be 1.71 times smaller before you can’t tell the object apart from the space and they blur into one. So simply put, if you have 20/20 vision and someone lines up a series of black bins against a white wall evenly spaced at 1000 yards, you’ll notice they are separate objects. At 10 yards more, they will just form a line of grey. You will no longer see the difference.

System resolution works like this. At what distance can you spot a man? How about a dog or a deer? Or a tank? With night vision, if you know the system resolution, you can predict this value.

So what affects System Resolution?

First, the tube resolution makes a difference. Most tubes have a resolution of around 64 line pairs per millimetre, and a line pair is about the same thing as a cycle (Line pairs cycle from dark to light ).

The second factor and probably the biggest is the lens focal length. The longer the lens, the smaller the angle for a given tube and hence the more cycles per milliradian.

After that, the remaining factor is the glass and lens quality. Losses through lens quality are fairly high and the losses increase significantly the further you get away from the center, so measurements of cy/mr are ALWAYS measured at the center of image. Finally, you have the eyepiece losses and losses through the tube itself ( though these are inherent in the lp/mm measurement ).

After that, there’s one other factor - Light. Looking through a few cy/mr measured values, I’ve noticed that the losses are around 20% for moonlight and 70% for starlight.

Based on this, even without knowing the lens quality, an approximation of system resolution and performance can be made, giving an insight into how these devices perform.



From the above, you can reverse out the calculations and you’ll see they hold fairly well with real-world examples. Of course, this doesn’t tell you how well these devices shoot, hold zero or if they come with lenses capable of achieving these goals.

A PVS-14 for example has a stated system resolution of 1.3 cy/mr and other systems such as the PVS-22 and PVS-27 are well known.

But as a rough working example, I included one final calculation - The width of a man ( 18 inches ) and at what range he "blurs” into the background noise - so that a group of men walking together and evenly spaced could not be detected. It’s rough by any measure and doesn’t really take into account many factors, but the relationship between the detection range calculated on raw lens size details holds an uncanny level of correlation to the real estimated figures.