The numbers on a rifle, crossbow scope (our best picks), or viewing scope initially seem confusing, but they are really quite simple to understand. Let us take an example from a mid-sized spotting scope...
20 -- 60 x 80 mm
The first number 20 signifies the minimal magnification level the scope offers. Second amount 60 the maximum magnification level. Since the first 2 numbers are a range separated by a hyphen, we understand this is a zoom scope. The 80mm following the x denotes the objective lens diameter in mm.
The numbers before the x consistently denote magnification power or power range, and the numbers following the x represent the size of the objective lens in mm.
Here's another instance from a crossbow scope list that appears slightly different:
4 x 32
The first number means 4x magnification power or '4 power' for short, and the 32 signifies a 32mm objective lens. This scope is fixed. You can not change the magnification by zooming in or out. We know since the first number does not specify a range.
Notice this time the 'mm' component was missing. That does not matter that you still know it pertains to mm. That is the standard measurement for optical lens dimensions. What matters is that it comes after the 'x.'
If you ever see specifications or listings like this...
3x scope or 6x scope
The above is just denotation for magnification power. 6x implies 6 times magnification or '6 power’'. This could be a fixed magnification scope. 6x is a picture that's 6 times larger than what you see with the naked eye. Here's an illustration of the effects of different magnification levels within an object.
One final example from a high-end rifle scope list merely to be sure that you've altogether got it...
3 -- 20 x 50
A zoom scope with a 3x power minimal and a 20x power max magnification. No mm recorded, but we know that the 50 following the x describes a 50mm objective lens.
Objective Lens Sizes
A bigger objective lens will let in more light than a more compact lens. This makes for a brighter, clearer picture, particularly in low light conditions such as dusk.
Crossbow and rifle scopes contain smaller objective lenses than binoculars. Binoculars typically have smaller objective lenses than spotting scopes. Each is made for a different function.
A scope does not need as large an objective lens as binoculars as you are zoomed in on a single point, while the spotting scopes will need to allow as much light as possible to enable you to view clearly in their high 60-80x magnification capacities.
The other numbers...
You will see a few different numbers listed in range specifications, which we ought to also cover.
Field of View (feet or levels @ yardage)
A normal human field of vision is around 210 degrees arc. The nearer you magnify something that the smaller the field of view will get. Scopes usually list a field of view parameter at a certain distance so you can gauge the gap between them. This is a much wider range for spotting scopes compared to rifle or crossbow scopes. A larger FOV is better for acquiring targets or after moving targets. Something such as a spotting scope will have a much higher FOV compared to a rifle scope as they are made for spotting things before you zone in on them with the crossbow scope or rifle to Create a shot. A sample FOV from a spotting scope:
100-142 Feet @ 1000 yards or 1.9-2.7 degrees @ 1000 yards
While you can see at 1000 yards, you can just 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 a lot less than the spotting scope in a tenth of the distance.
Eye Relief (mm or inches -- range)
Eye relief shows you how close your eye needs to be on the surface of the scope's eyepiece, so you see the entire field of view. A lower power scope will usually have a bigger eye relief distance, whereas a higher power scope will ask you to get up close and personal to get the full view of the picture.
Usually, a crossbow scope or a low power rifle will record something like 4", meaning that you can be upto 4" away from the surface of the scope before you lose any of the images. Eye relief for high powered weapons and rifles is crucial since you do not need to receive your eyes too near something with a great deal of recoil!
A spotting scope may record something like 16.7-17mm for eye relief. This is a lot more precise and much nearer... nevertheless, a spotting scope will have an eyepiece enabling you to get your eye that close, and generally, you do not expect any recoil.
The reason for listing eye relief on this sort of scope is to understand how well you can work together with the scope if you wear glasses. If your eyeglasses push your eye farther away than the maximum eye relief, you may lose some of the field of view. Usually, something like 12-16mm works well with all.
Exit Pupil (mm)
The exit student of a scope is the circle of light's diameter, which leaves the extent and enters the eye. A little exit pupil will not fill the iris with light and provide a dim image, while one that's too big will waste light.
As a rough guide, the human iris is around 4-5 mm in low light, 2-3 mm in the day, and 6 mm in near dark conditions.
Not all Optics define this parameter.
Tube Diameter (mm)
Crossbow scopes and rifle may list their tube diameter. That is the diameter of the central part of the tube of the scope. Having a larger tube does not affect the optical quality, but it might mean you could adjust the scope higher from the rifle than with a thinner tube.
Length (inches or mm)
Simple enough, this is the maximum length of the scope from tip to tip.
Weight (kg or oz)
Again, pretty obvious that this is the total weight of this extent (without packaging). If You're concerned about how adding a particular scope will influence your rifle balance or add weight to your pack, this is sometimes considered.