What Do the Numbers on a Scope Mean? (3-9x40 Decoded)

In a 3-9x40 scope, the 3-9 is the magnification range, from 3x to 9x, and the x means times. The 40 is the objective lens diameter in millimeters. A single number before the x, as in 4x32, means a fixed power. Tube diameter, such as 30 mm, is usually listed separately and is not in the name.

What do the numbers on a scope mean? This page reads the box in order, starting with magnification, then the objective lens, the tube size that is not in the name, the other spec-sheet numbers, and the letters. It then walks through two real Ozark Armament scopes to show how the pieces line up.

The number customers misread most is the last one. That 40 is the lens size in millimeters, not a distance and not a power. Once you know that, the rest of the box starts making sense.

I founded Ozark Armament in 2016 and run it from Tigard, Oregon, to this day. Along the way I have also personally zeroed hundreds of rifles.

So What Do the Numbers Mean on a Rifle Scope?

The table below breaks three common designations into their parts. It shows the magnification, objective and exit pupil for 4x32, 1-6x24 and 3-9x40.

What Does the First Number on a Scope Mean?

The numbers before the x are the magnification, how many times larger the scope makes an object appear. At 1x an object looks about the same size as with the naked eye, and at 10x it looks about 10 times larger, Leupold's glossary explains. Fixed and variable power both show up in this spot. The NRA Blog gives a fixed 3X40 as its example, a scope that magnifies three times and cannot be changed, and a 3-9X40 as variable power, adjustable from three to nine times. On a variable scope, the low number sets how wide a view you get at the lowest power. As magnification goes up, the object appears closer and the field of view gets smaller, Swarovski Optik notes. How much magnification you need is a separate question with its own page. On the Razorback 1-6x24SFP, Ozark Armament's product page lists 129.2 ft at 100 yards at 1x and 21.5 ft at 6x.

One verified Rhino 4x buyer wrote, "old eyes appreciate the 4x esp out to 100-200 yards". A fixed power gives you one magnification, so there is no power ring to set.

What Does the Last Number on a Scope Mean?

The last number is the objective lens diameter in millimeters, measured across the front lens. Bass Pro Shops' rifle scope buyer's guide writes the pattern as "power-power x objective dia." and notes that a larger objective lets in more light and gives a bit larger field of view, at the cost of a bulkier scope. Exit pupil is the size of the beam of light that leaves the scope, as the guide explains. Divide the objective diameter by the power to get it. A 4x32 gives an 8 mm exit pupil. For the human pupil, the guide gives about 2 mm at noon to 4 mm later in the day, and about 5 mm to 9 mm once the eye adapts to the dark. In daylight your own pupil is smaller than the exit pupil of a 4x32.

Is a bigger objective always better? No, not by itself. It does not change the magnification, and the exit pupil still shrinks as you turn the power up. Run the division on two examples. A 3-9x40 gives about 13.3 mm at 3x and about 4.4 mm at 9x. A 1-6x24 gives 24 mm at 1x and 4 mm at 6x. Glass quality also sits outside these numbers. Leupold's glossary defines twilight factor as the square root of magnification times objective size in millimeters, and warns that it does not take into account the quality or type of glass and coatings. Read the objective number next to the power range, not on its own.

Objective lens and tube sizes compared in front view, with every dimension drawn to scale.

Where Do You Find the Tube Diameter?

Tube diameter is not in a name like 3-9x40. It is listed separately on the spec sheet. Leupold's glossary defines maintube diameter as the diameter of the main tube where the rings attach, and lists common sizes as 1 inch, 30 mm, 34 mm and 35 mm. In the U.S., rifle scope tubes are built in 1-inch or 30-millimeter models, the NRA Blog notes, and the rings must be sized correctly for the tube. One inch is 25.4 mm, so a 30 mm ring and a 1 inch tube do not match. A larger tube does not let more light reach your eye, Bass Pro Shops explains, since the exit pupil controls that, and instead adds strength, rigidity and more windage and elevation adjustment range. The Rhino 4x has a built-in Picatinny mount and the Razorback 1-6x24SFP ships with its own optic mount, so neither needs separate rings, and neither product page lists a tube diameter.

I always read the tube diameter off the spec sheet before buying rings, because it never appears in the scope name.

What Other Numbers Will You Find on the Spec Sheet?

The name is only part of the story, and the spec sheet prints more numbers you will want to read. Field of view can be printed two ways, and Leupold's glossary notes it can be measured as a length at a set distance or as an angle. The Rhino 4x product page prints both, listing 7.0 degrees and 36.8 ft at 100 yards. A variable scope can print two field of view numbers, one for each end of the power range, as the Razorback does with 129.2 and 21.5 ft at 100 yards. Eye relief is the distance from the eyepiece where you see the full image, and the Rhino 4x lists 3.5 inches. Weight shows up on the spec sheet too, and the Razorback lists 1.1 lb. That number is not part of the name either.

What Do the Letters on a Scope Box Mean?

After the numbers, a box or spec sheet adds letters like FFP, SFP, IR and AO. Each one gets a short section below.

First and Second Focal Plane

What do FFP and SFP tell you? They mark where the reticle sits. Leupold's glossary explains that a first focal plane reticle sits in front of the magnification system, so it grows and shrinks with the target as you change power, while a second focal plane reticle sits behind it and stays the same size. The Razorback 1-6x24SFP is a second focal plane scope, and its product page notes the reticle keeps the same visual size as you change magnification. The full first versus second focal plane explanation lives in how rifle scopes work. One Razorback reviewer wrote, "That one is a FFP this one is a SFP. I can't decide which focal plane I like better."

Illuminated Reticles and IR

The Rhino 4x lists red, green and blue illumination, with three settings for each color. IR is the short form many boxes use for an illuminated reticle, meaning a light inside it makes the reticle glow. Swarovski Optik notes that in its own model names a lowercase "i" stands for illuminated, one maker's habit rather than an industry rule. One verified Rhino 4x buyer wrote, "The only drawback to this optic is the weak illumination in daylight." A lit reticle is a low-light aid, and that is the standard to judge it against.

Reticle Names on the Box

BDC on one box, Mil Dot on the other. The Rhino 4x lists a BDC reticle marked for 100 to 600 yards, and the Razorback 1-6x24SFP lists a Mil Dot reticle. A reticle name tells you the aiming pattern you will see when you look through the glass. BDC stands for bullet drop compensator. The name describes the reticle and nothing else.

AO, SF and Click Values

AO stands for adjustable objective and SF stands for side focus. Leupold's glossary describes an adjustable objective ring on the objective bell and side focus adjustments, and both change the distance at which the scope is free of parallax. Neither Ozark scope on this page lists AO or SF. Spec sheets also print a click value. The Rhino 4x carries 1/4 MOA windage and elevation adjustments, and the Razorback 1-6x24SFP is listed at 1/2 MOA per click.

How Do You Read Two Real Scopes from Their Spec Sheets?

The Rhino 4x reads as a 4x32, which tells you fixed 4x power and a 32 mm objective. Divide 32 by 4 and you get its listed 8 mm exit pupil, the same number the Bass Pro Shops rule gives for a 4x32. Field of view is listed at 7.0 degrees, which works out to 36.8 ft at 100 yards. Eye relief is 3.5 inches. The reticle is a BDC marked for 100 to 600 yards, with 1/4 MOA windage and elevation adjustments. Illumination comes in red, green and blue, with 3 settings each. A Picatinny mount is built in, and the construction is all metal. It runs $69.99 on Ozark Armament's Rhino 4x32 scope product page, which lists no tube diameter. Those are the numbers, straight off the product page. One verified Rhino 4x buyer wrote, "Good eye relief, crisp view, zero complaints".

The Rhino 4x is a fixed 4x32 with a 32 mm objective lens.

What does the name 1-6x24SFP tell you? The 1-6 is the power range, 1x to 6x, and the 24 after the x is a 24 mm objective lens. SFP at the end means second focal plane. Work the exit pupil from that objective and you get 24 mm at 1x and 4 mm at 6x. Field of view runs from 129.2 ft at 100 yards at the low end to 21.5 ft at the high end. A Mil Dot reticle with red and green illumination sits inside, adjustments click in 1/2 MOA steps, and weight comes in at 1.1 lb. None of that lives in the name. The box includes the optic, an optic mount, a battery, flip up lens covers and mounting tools. Ozark Armament's product page lists the price at $119.99. No tube diameter is listed.

Before anyone picks between a fixed 4x32 and a variable 1-6x24, I ask what they will use it for. Everything else follows from that answer.

A simple order keeps a spec sheet from becoming a puzzle. Read the magnification first, then the objective, then go looking for the tube diameter and the letters on the spec sheet. The order is a habit, not a rule for every scope made. Ozark Armament is a Tigard, Oregon company, and it covers its products with a No B.S. Lifetime Warranty and a 30 day return policy.

For this page, Ozark Armament checked the Leupold glossary, the Bass Pro Shops buyer's guide, the NRA Blog, Swarovski Optik and both Ozark product pages on October 23, 2026.

Frequently Asked Questions

Q: What does a 4x32 scope do?

A: A 4x32 scope magnifies at fixed 4x power, so a target appears four times larger than it does to the naked eye, and there is no power ring to adjust. The 32 is the objective lens diameter in millimeters. Dividing 32 by four gives an 8 mm exit pupil. The Rhino 4x is one example of the format, and its field of view is 36.8 ft at 100 yards.

Q: How far does a 3-9x40 scope zoom?

A: A 3-9x40 does not zoom to a distance. Magnification is how much larger the target looks, and the NRA Blog gives the 3-9x40 as its example of variable power, adjustable from three to nine times. At 9x the field of view is smaller than at 3x, and the exit pupil drops to about 4.4 mm from the 40 mm objective.

Q: How to read a rifle scope reticle?

A: Start with the reticle name printed on the box, such as the BDC reticle on the Rhino 4x or the Mil Dot on the Razorback 1-6x24SFP. That name tells you which aiming pattern the glass carries. Leupold's glossary explains that on a second focal plane reticle, any off-center measurements only work with the scope set to a specific magnification, which on Leupold scopes is normally the highest power. On the Razorback, the product page notes its reticle subtensions are calibrated at the listed magnification.

Q: What is side focus on a Leupold scope?

A: Leupold's glossary describes the turret as the part of the scope body that holds the windage, elevation and side focus adjustments, so side focus is a knob on the turret in the middle of the scope. It changes the distance at which the scope is free of parallax. An adjustable objective does the same job with a ring around the objective bell at the front. Neither Ozark scope on this page lists side focus or AO.

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ARTICLE WRITTEN BY MATT RICE, OWNER OF OZARK ARMAMENT