Prepper's Armory: Scopes, Part 1

When it comes to optics, especially magnified optics, there's a lot of confusion and misinformation out there. One aspect of this type of optic that's not very well understood is the First and Second Focal Plane Reticle.

In variable magnification scopes with a reticle in the First Focal Plane, the reticle will adjust along with the image. This means that any graduations, such as mil-dots or bullet drop compensator markings, will still be useable no matter the zoom level. The downside to this is that at higher magnifications, the reticle itself will appear noticeably thicker and is more likely to obscure part of the sight picture and at the lowest magnification, the reticle may be thinner than preferred. First Focal Plane scopes are also generally more expensive than Second Focal Plane scopes because of the additional manufacturing elements required.

Magnified scopes with Second Focal Plane reticles change the image size, but not the reticle, which means the crosshairs and other aiming marks are always the same size while the view gets bigger or smaller. This can be beneficial for aiming consistency, but if the scope has Mil-Dots, etc. they'll only be reliably usable in a small subset of the full magnification range. This type of scope is generally less expensive than those with First Focal Plane setup.

Scope Anatomy

Image courtesy of The NRA Blog

Starting from the front of the scope and on the right of the illustration is the objective lens, measured in millimeters. It affects both field of view (the width of viewable area at any given zoom level) and the amount of light entering the optic path. The larger the objective lens, the better for both these values.

Moving left and back, the erector tube assembly is next. This element contains the lenses needed for magnification, and may also include the reticle in a First Focal Plane setup. Another part of the erector tube is the focus lens which, as the name implies, helps keep the image crisp and clear. 

Behind the focus lens is the image reversal assembly. When light passes through the series of lenses in a modern scope, it can cause the image to appear upside down, and the image reversal assembly corrects this before it reaches the user's eye.

The magnification lenses move forward and back inside the scope body in a scope with adjustable zoom. When these lenses move forward (away from the ocular lens) magnification increases; when they move rearward (away from the objective lens), magnification decreases. For example, in a 3x9 variable magnification scope, at 9x magnification the erector tube will be closest to the objective lens, and at 3x it will be closest to the ocular lens.

Next is the Second Focal Plane aperture and, if appropriate, the reticle for this type of scope. Finally there is the ocular lens, which collects the light that passes through the scope and presents its image to the shooters eye. Many of these lenses will have coatings to reduce light reflection and improve clarity. 

All the different elements of the scope are sealed with rubber or silicone gaskets to make the scope waterproof, and before final assembly the scope is purged of air and filled with nitrogen (or a similar gas) to prevent fogging of the lenses.

Scope Operation

There are several external adjustment controls on a modern scope. The windage and elevation knobs, located on the top and side of the scope at the middle of the scope body or tube, allow the user to change the position of the reticle for zeroing a scope. These are adjusted in either half- or quarter-minute of angle increments.

If appropriate, there may also be a Bullet Drop Compensator knob. These days it is more likely that the BDC will be incorporated into the reticle.

Zoom adjustment is generally made by means of a collar in front of the ocular lens. Many scopes also have a focus ring at the rear of the eyepiece, used to adjust focus of the reticle for each user.

If the scope has a parallax adjustment, it may have another knob opposite the windage adjustment, or else be changed by rotating the objective end of the scope. This adjusts the reticle focus in relation to the target. If the scope has fixed parallax, it will usually be set at 100 yards in centerfire rifle scopes and 50 yards in rimfire, shotgun, and handgun scopes. This means that at other distances, both the reticle and the target may not be in focus simultaneously.

Another setting is what's called eye relief, the distance between the shooter's eye and the eyepiece of the ocular lens where the entire field of view is visible. Shotgun and handgun scopes have considerably longer eye relief than rifle scopes. While this can usually be adjusted in small increments on the scope itself, it is more coarsely set by the position of the scope on the firearm.

Hopefully this article clarifies both components and terminology in the modern scope. Have fun, and safe shooting.