Scopes come in two main types fixed power and variable power. Fixed power scopes have less moving parts theoretically enhancing durability.
Variable power (adjustable magnification) have more moving parts but are tactically more efficient allowing more viewing options. By having the ability to power down you have the advantage of using lower power, enhancing the ability to scan a larger area, this also makes the scope more usable at closer ranges or when light conditions are less than favorable. This is called Field of View. This is the difference between viewing an object with the naked eye vs looking down a tube.
10 x 40 means the scope has ten times magnification and the objective lens is 40mm.
3.5×10x40 means the scope has the ability to magnify an object form 3.5 times to 10 times and the objective lens is 40mm
Optical Considerations and Terminology
Objective Lens: The objective lens is just like a funnel collecting available light. Usually the higher the magnification the larger the objective lens. This is also dependent on the quality of the lens itself not all lenses are created equal. The disadvantage of large objective lens is that the scope has to be raised high above the bore centerline.
The sniper scope objective usually has a minimum of 40mm and a maximum of 56mm. There are scopes as large as 72mm for .50 cal shooting.
Tube and Tube diameter: 1 inch, 30mm and 34mm scope tube diameters are common.
Advantages of larger tubes are:
Transmitted light does not need to bend as much when traveling down the tube allowing for less distortion around the edges of the image.
Larger tube allows larger internal lenses allowing more light to be transmitted to the shooters eye.
Ocular Lens: This is the lens you look through.
Eye Relief: The distance you can place the scope away from your eye and still get a full field of view. This usually is around 2.5 - 3.0 inches.
Exit Pupil: The diameter of the light column that is transmitted to the eye. This column of light should be roughly equal to your eyes pupil diameter in low light typically between 4mm to 7mm. This can be calculated by dividing the size of the objective lens by the power for example: 40 divided by 10 = 4 or 40 divided by 3.5 = 11.4 as you can see the lower power will transmit more light.
Lens Coatings: Coated lenses will enhance the passage of light and reduce reflection. Some manufacturers have multi coated lenses further enhancing the optical performance of the rifle scope. Scopes can have 8 or more internal lenses.
Objective Focus: Some scopes have an adjustable objective lens focus ring. This focuses the objective lens to the target. Most modern scopes do not have this feature anymore.
Many modern scopes have a Parallax adjustment knob on the side of the scope opposite the windage adjustment dial. With reticle you only have a sight in one position unlike iron sights. This is like looking at a bathroom scale. If you look straight down, the indicator needle reads one thing and if someone standing to the side view the scale they will get a different reading. Adjust your Parallax till the target looks perfect.
Magnification (Power) Ring: This is used to adjust the magnification of the scope.
Ocular Focus Ring: This adjustment is used to focus the eye to the Reticle ONLY. This should be setup when the scope is mounted and should not have to be re-adjusted.
Reticles: Used for Long range rifle applications. Post, German Post, Plain and Duplex. Not that good for ranging. We prefer Mil-Dot and the Leupold TMR reticle, but there are to many reticles to mention. The Mil-Dot and TMR are very good reticles for target ranging. Some scopes have illuminated reticles these work well in low light and when a target has a black background.
The Mil-Dot reticle is graduated in Mils 1. Mil = 3.6 Minutes of angle we usually round this off to 3.5 Mils. Apart from ranging these reticles are useful in many other ways and are used by most branches of the military and law enforcement.
Adjustments Mechanical: There are many different types of turrets. We are most familiar with the minute system. 1 minute of angle (MOA) = 1/60 of 1 degree. 1 MOA is 1.047 inches at 100 yards in most cases we round this off to 1inch. 1.047 x 1000 = 10.47 inches at 1000 yards. We round this off to 10 inches. Common scopes have ¼ min, ½ min and 1min resolution.
¼ Minute scopes are good for target, varmint hunting and Police sniper engagements. We prefer ½ minute and 1 minute graduations for tactical applications. Bullet drop compensator dials (BDC) are also available for various calibers. These turrets are graduated in distance in yards or meters for a specific caliber. Basically you set the elevation turret to 300 yards and take the shot. Most of the time these turrets are close but I would not bet your life or mine on the calculated graduations. Some manufacturers offer BDC turrets based off your dope.
There is no industry standard as to the quality of a rifle scope. The formula that is usually the most accurate is the amount of wallet damage on purchase.