What do scope numbers mean?

Anthony Cote
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Objective Lens Sizes

The first number in a lens’s description is the total diameter of the front lens in millimeters. This is the main measurement that determines a lens’s light-gathering capability. A larger diameter lens gathers more light and is able to project a bright, large image.

The diameter of the larger lens is the first number on a pair of binoculars. For example, a 7 x 35 pair of binoculars would mean that the 35mm front lens has a total diameter of 7mm. Another way to determine the actual size of the objective lens is to multiply it by the magnification.

For example, if we were to look at 5 x 35 binoculars:

35 x 5 = 175mm

This means the objective lens diameter is 175mm.

Second Number: A lens’s magnification is the second number in the lens’s description. As the magnification goes up so does the field of view. A higher power generally means a smaller field of view. A pair of binoculars with a higher power will show less, but at a closer range, compared to binoculars with a lower power.

The other numbers…

The mechanical device that the scope operates on does not have the same magnitude of effect on your shooting ability in such a short time as the scope does, but you should take a look at the other numbers and how they may correlate to your rifle or how it is set up.

Operating pressure will affect what you pull the trigger on. Take a look at this abbreviated list of operating pressures for common cartridges and if you have a rifle with a lot of kick, try to find a load that does not exceed the picatinny operating pressures. (All of the operating pressures on this list are the max allowable pressures for elk, which is a fairly violent recoiling animal.)

308 180,000 is the maximum pressure

5x55mm Swedish 139,000

307 Winchester 180,000

270 Winchester 165,000

7mm-08, 7mm Remington Mag, 7mm Dakota 140,000.

300 Winchester Magnum 180,000

Field of view (ft or degrees @ yardage)

The field of view is how big of an area you can see through the scope at a given distance. A good field of view enables you to see game birds without pivoting the rifle, or to make a correct shot when shooting around obstructions.

The scope is measured by the number of inches wide at the objective of the scope. The higher the number, the larger the field of view.

For example, a 7x36mm scope has a wider field of view than a 7x40mm scope.

Eye Relief (inches or mm – range)

The distance from your eye to the ocular lens of the scope. If you take your eye off the ocular lens, the image is not visible anymore.

Effective Yards (Yds) …

Indicates the maximum distance the scope can be zeroed for its maximum point blank range. This is usually at 200 yards and should be less than 100 yards.

The Number of Inches (or Millimeters) of Travel per Click at 100 Yards

The distance (in or mm) between clicks or per revolution of the turrets.

Net Clear Optics (Moa) …

The distance between the center of each click in inches (or millimeters). This is usually 3 MOA.

Tube Diameter (in) …

The diameter of the main tube of the scope. Generally, a larger tube diameter will give you a larger objective lens and a longer eye relief. This is not necessarily the case due to sizing of objectives and eye placement of the shooter.

Objective Focal Length (mm) …

The diameter of the front lens of the scope. Typically, more length will give a clearer image for a given tube diameter.

Exit Pupil (mm)

The exit pupil is the amount of the lens the view through when looking through it.

For e.g. I have a 50mm lens, the 50mm is the exit pupil

Exit pupil is the diameter of what the lens displays.

So In 50mm lens, only 50mm of that lens is used to view through while in a 10mm lens the view through is 10mm.

Exit pupil is the diameter of the view through of a lens.

Newtonian reflectors are said to have a broader view than refractors and the exit pupil is easy to match to your eyes. If you wear glasses the exit pupil can give you a close idea of how much of your eyepiece field of view you will lose.

The information may mean nothing to you, but if you are into astronomy and optics you might find this information useful.

For example:
2" objective – 17.5mm exit pupil

4" objective – 6.4mm exit pupil

6" objective – 4.3mm exit pupil

8" objective – 2.8mm exit pupil

12" objective – 1.74mm exit pupil

18" objective – 1.22mm exit pupil

24" objective – 1.02mm exit pupil

36" objective – 0.89mm exit pupil

Tube Diameter (mm)

This number is pretty obvious. It is the size of the eyepiece tube. This is the distance that the rear lens sits behind the forward lens. With most binoculars, the larger the scope, the larger the number.

The purpose of the scope should match the size of the tube. The scope should also be much larger than the glasses. It is okay to have a smaller scope than glasses since the optics inside is the most important part of a binoculars.

If the tubes are the same then the glasses have better lenses. This is because there is a limit on how much you can make the scope.

Length (inches or mm)

The length specification given with a scope number is often the erector tube length. This is the distance between the backs of the front and rear erector lenses.

This is important for magnification calculations and is also used in the formula for finding the approximate elevation change in MOA a given scope will cause at a given distance.

Weight (kg or oz)

With a number of manufacturers marketing different models with the same or similar product name, one way of differentiating them often is to be more specific with the weight. The difference in weights between models can be very small, where even a few hundred grams can be a big deal for some people.

The weight, listed on the packaging, of a scope is usually for the scope body only. With scopes, that may be without the eyepiece, any lens covers, or any additions such as a sunshade.