The numbers on a rifle, crossbow or spotting scope at first look confusing but they’re really very easy to understand. Let’s take an example from a mid-range spotting scope...
20 - 60 x 80 mm
The first number 20 means the minimum magnification level that the scope offers.
Second number 60 the maximum magnification level.
Because the first 2 numbers are a range separated by a hyphen we know this is a zoom scope.
The 80mm after the x refers to the objective lens diameter in mm.
The numbers before the x always denote magnification power or power range and the numbers after the x denote the size of the objective lens in mm.
Here’s another example from a crossbow scope listing that looks slightly different:
4 x 32
First number before the x means 4x magnification power or ‘4 power’ for short and the 32 means a 32mm objective lens. This scope is fixed, you can’t alter the magnification by zooming in or out. We know because the first number doesn’t specify a range.
Notice this time the ‘mm’ part was missing, that doesn’t matter you still know it refers to mm, that’s the standard measurement for optical lens sizes. What matters is that it comes after the ‘x’.
If you ever see listings or specifications like this…
The above is just shorthand for magnification power. 8x means 8 times magnification or '8 power'. This would be a fixed magnification scope. 8x is an image that’s 8 times bigger than what you see with the naked eye. Here's an example of the effects of different magnification levels on an object.
One final example from a high end rifle scope listing just to make sure you’ve completely got it…. 😉
3 - 20 x 50
A zoom scope with a 3x power minimum and a 20x power maximum magnification. No mm listed, but we know that’s what the 50 after the x refers to, a 50mm objective lens.
Objective Lens Sizes
A larger objective lens will let in more light than a smaller lens. That makes for a brighter clearer image especially in low light conditions like dusk.
Rifle and crossbow scopes have smaller objective lenses than binoculars. And binoculars generally have smaller lenses than spotting scopes. Each is designed for a different purpose.
A scope doesn’t need as big an objective lens as binoculars because you’re zoomed in on a single target, whereas spotting scopes need to allow as much light in as possible to allow you to see clearly at their high 60-80x magnification capabilities.
The other numbers...
There’s a few other numbers you’ll see listed in scope specifications that we should also cover.
Field of view (ft or degrees @ yardage)
A normal human field of vision is about a 210 degrees horizontal arc. The closer you magnify something the smaller the field of view will get. Scopes normally list a field of view parameter at a certain distance so that you can gauge the difference between them. This is a much longer range for spotting scopes than rifle or crossbow scopes.
A bigger FOV is better for acquiring targets or following moving targets. Something like a spotting scope will have a much higher FOV than a rifle scope as they are designed for spotting things before you zone in on them with the rifle or crossbow scope to make a shot.
A sample field of view from a spotting scope:
100-142 ft @ 1000 yards
1.9-2.7 degrees @ 1000 yards
As you can see at 1000 yards you can only see 100th of the field of view you see with your naked eye.
A sample field of view from a long range rifle scope:
7.6-19 ft @ 100 yards
See the difference? This is much less than the spotting scope at a tenth of the distance.
Eye Relief (inches or mm - range)
Eye relief tells you how close your eye has to be to the surface of the eyepiece of a scope so that you see the full field of view. A lower power scope will generally have a larger eye relief distance whereas a higher power scope will require you to get up close and personal in order to get the full view of the image.
Typically a low power rifle or crossbow scope will list something like 4”, meaning your can be upto 4” away from the surface of the scope before you lose any of the image. Eye relief for rifles and high powered weapons is crucial as you don’t want to get your eyes too close to something with a lot of recoil!
A spotting scope may list something like 16.7-17mm for eye relief. This is much more precise and much closer... however a spotting scope will come with an eyepiece allowing you to get your eye comfortably that close and generally you don’t expect any recoil.
The reason for listing eye relief on this type of scope is so that you can understand how well you can work with the scope if you wear eyeglasses. If your glasses push your eye further away than the maximum eye relief you will lose some of the field of view. Generally something like 12-16mm works fine with all but the thickest of spectacle wearers.
Exit Pupil (mm)
The exit pupil of a scope is the diameter of the circle of light that leaves the scope and enters the eye. A small exit pupil won’t will fill the iris with light and give a dim image whilst one that is too large will waste available light.
As a rough guide the human iris is approximately 2-3 mm in the day, 4-5 mm in low light and 6 mm in near dark conditions.
Not all optics specify this parameter.
Tube Diameter (mm)
Rifle and crossbow scopes may list their tube diameter. That’s the diameter of the central part of the tube of the scope. Having a bigger tube doesn’t affect the optical quality, but it can mean that you can adjust the scope higher from the rifle than with a slimmer tube.
Length (inches or mm)
Simple enough, this is the longest length of the scope from tip to tip.
Weight (kg or oz)
Again, pretty obvious this is the total weight of the scope (without packaging). If you’re worried about how adding a certain scope will affect your rifle balance or add weight to your pack, this is sometimes a consideration.