In this article, the individual parts of a riflescope’s anatomy are explained in detail along with their influence on the properties of the scope.
You will find Exit Pupils, Eye Relief, Main Tubes, Objective Lenses, Lens properties and coatings and finally a bit about what causes a scope to have good light transmission.
Riflescope Technical Terms
Exit Pupil and Eye Relief
The exit pupil is the point where the light exits after travelling through the scope. It is also the most likely part that will hit your eye due to recoil. It is linked to eye relief and as both are associated with this end of the riflescope it makes sense to group them together. Exit pupils are always smaller than the objective lens so it is usually very easy to tell which way round your scope is meant to go.
The lens on the exit pupil should always be clear and will usually have a coating on it. The purpose of the coating is to improve light transmission which we will go into in a separate section of this guide. The lens should also be airtight so if you can see any gaps between the lens and the surrounding body then you will have problems with fogging and durability of your scope.
You should send a scope back to the manufacturer if it has a damaged exit pupil on delivery or in fact any damaged part on delivery as it will be covered by the manufacturers guarantee and your warranty. The amount of eye relief given depends upon your own eyes and how much your pupils can dilate. The eyes of an older person may only dilate as much as 4 millimeters whereas younger eyes can dilate to 7 millimeters or more. The smaller pupil dilation you have, the more eye relief you will need. This is so you can keep your eye sufficiently far away from the exit pupil when focusing on a target so that the recoil from the weapon does not result in you getting scope eye – a semi circular gash above your eye.
To figure out the eye relief of a scope you can see the exit pupil in the eyepiece when you hold the scope at arm’s length. Switching between the smallest to the largest zoom on a variable scope will reduce the size of this exit pupil by quite a large amount. Trying to see through that small exit pupil to line your shot up in low light conditions can be tough.
With a larger exit pupil, the way your head is positioned in relation to where the scope is becomes less important. This is the reason why the eye relief on a low powered scope can be shorter than for a high powered scope where the eye relief is very specific. Most scopes offer eye relief of between three and three and a half inches but there are a few scopes where eye relief can be up to 5 inches.
When you mount a scope properly it should be done set at its highest magnification setting so as to allow it to be comfortable to sight through the whole range of magnifications.
Scope Main Tube
Common choices of materials used to make the main tube from are carbon steel, aluminum or titanium alloy. Carbon steel is relatively heavy compared to the other 2 materials. The strongest and lightest material is titanium alloy with aluminum being similar in weight so if you are looking for a high quality scope then these are the 2 best materials to choose from for main tube construction.
The other thing to look out for is whether the tube is 1 inch or 30mm. The 1 inch tube is the standard and is good for most situations but for high magnification ratios the larger 30mm tube is favored. A larger tube will also give a brighter and crisper image as well but as good scopes can have between 95-98% light transmission the increase in the scope diameter doesn’t have as large an impact on brightness as the market is lead to believe. In short, a larger diameter tube leads to a stronger scope.
Objective Lenses and Lens Information
Objective Lenses are located at the front forward part of the scope. The objective lens collects light and transmits it down through the scope and out of the exit pupil. A larger objective lens will give more brightness by being able to collect a larger amount of light, but the larger the lens means that larger rings are needed to mount the scope. Using larger rings means that the scope gets uncomfortable to use as you have to raise your head higher to look through it while keeping your chin resting on the stock. You want the comfortable position to be repeatedly easy to find as you use your scope and having high rings affects this. For medium power variable zoom scopes a 40-50mm lens is recommended as you just will not see a huge improvement on light transmission over the medium range around 10x.
The quality of a lens has a huge impact on light transmission and quality of image. Lenses used in top-quality riflescopes are often made from very pure sand brought all the way from Vietnam. This sand can be used to create an imperfection free glass. These lenses can gather more light due to reflection losses being lower. The highest quality lenses suffer much less from image distortion around the edges and you can expect up to 98% light transmission. The coating also has an impact on light transmission and the brightness and crispness of the image you see through the scope. Lens Coatings – Coating a lens reduces its reflection of light. If a scope reflects lots of light then no matter how much light it gathers it will always reflect a high percentage of it so you will not be able to see much through it.
The Different Types of Lens Coatings
Single Coating – Just one layer on one or two lens surfaces
Fully Coated – Single layer coating on all air-to-glass interfacing surfaces
Multicoated – Multiple layers of coating, possibly polarizing light on at least one lens surface
Fully Multicoated – The most complete form of coating and the best where all surfaces are covered with multiple layers of coating to reduce reflection and allow maximum light transmission.
Light Transmission
The light transmission of a lens is the percentage of light that the riflescope lets through. A lens always loses some light but the very best riflescopes can reach an incredible 98% light transmission. Above 95 % is considered to be great for all applications but most low cost scopes average around 90% or so. The larger the objective lens the more light that gets through to your eyepiece, the more the scope is zoomed in reduces the light reaching your eyepiece.