## Saturday, January 24, 2015

### All About Scopes - II

In our last post, we looked at some basics of rifle scopes. We will continue our discussion in this post.

As we saw in our last post, there are mainly two types of scopes: the fixed power scope and the variable power scope. The big difference between these two is that the variable scope has adjustable magnification.

We will now look at how these scopes are specified. Fixed power scopes are usually specified as two numbers separated by x. For instance: 4x32, 12x40 etc. So what do these two numbers mean? The first number is the magnification factor of the scope. Therefore, in a scope marked as "4x32", this means it magnifies the image 4x times (i.e.) the object appears 4 times larger when viewed through the scope, than if it was viewed using just the eye. So what is the second number mean? The second number is the diameter of the objective lens in millimeters. Therefore, in a scope marked as "4x32", this means the objective lens is 32 mm. in diameter. In many cases, the unit of measurement is specified, so instead of "4x32", it may be more clearly specified as "4x32 mm."

A Bushnell 10x40 Fixed Power Scope. Click on the image to enlarge.

In the above image, we have a fixed power 10x40 scope made by Bushnell. What this means is that it has a 10x magnification and the objective lens is 40 mm. in diameter.

Variable power scopes also have similar designations, except that they have three numbers. The first two numbers are separated by a hyphen (-) and the third number is separated by x. For instance: 4-16x42, 6-24x50 etc. The first two numbers indicate the range of magnification power of the scope. Therefore, in a scope marked as "4-16x42", this means that the magnification factor of this scope can be varied between 4x and 16x. The third number indicates the size of the objective lens in millimeters. Therefore, in a scope marked as "4-16x42", the objective lens is 42 mm. in diameter. As with the fixed scopes, sometimes the specification includes the unit of measurement as well, so instead of "4-16x42", it may be more clearly specified as "4-16x42 mm."

A variable power 4-16x42 variable power scope made by Nikon. Click on the image to enlarge.

In the above image, we have a Nikon model M-223 scope, which is a 4-16x42 mm. scope. This is the model we studied in our last post, when we were studying the different parts of a scope.

So, a 10x magnification is better than a 4x magnification, right? Not quite. It is true that the object appears a lot larger on a higher magnification scope, but you see less of the surrounding area through the scope. For instance, if you're looking at a herd of deer through a powerful scope, you can probably see the fur very clearly, but you will be unable to tell which particular deer you're looking at, because you can only see a part of a deer's body through the scope. Also, it is very easy to lose sight of a particular deer if it moves off a bit, because the powerful scope only shows a small area at a time. Bear in mind that with a 10x scope, the field of view of an object at 100 yards (90 meters) is about 2 feet (0.66 meters) diameter. With a lower powered scope, you may be able to see both the head and the body of the deer and can tell which one it is in the herd.

Higher magnification also reduces the brightness of the image. For instance, if you have two scopes, a 4x40 and a 10x40. They both have the same size objective lens (40 mm.), but they have different magnification power 4x and 10x. The image seen through the 4x40 will be brighter than that seen through the 10x40. This has to do with the exit pupil, which we studied about in the last post. The 4x40 scope has an exit pupil of size 10 mm., whereas the 10x40 has an exit pupil of 4 mm.

A scope with higher magnification is useful against targets at a long distance, but not as useful against targets close by.

Therefore, for general purpose hunting, a scope with magnification in the range of 3x to 10x works fine for many hunters. Some use variable power scopes that work in this range (such as a 3-7x or a 3.5-10x scope), others are perfectly happy with a 4x or 6x fixed power scope, some even go for lower power, such as 1.5x or 3x, because they don't hunt at longer distances. For long distance shooting, scopes with magnification of 9x to 18x or so are used and anything more than that can only be used for shooting at targets that don't move.

For most soldiers, the US military have generally equipped them with fixed power scopes, because soldiers work in stressful environments and a fixed power scope saves them worrying about which magnification factor the scope is currently set at. Most military scopes have relatively low magnification, so that they are useful at ranges where combat usually occurs. The US Army, Air Force and Marines use the Trijicon TA31RCO ACOG sight, which uses a 4x32 fixed power scope. The scope has advanced features, such as dual illumination technology provided by fiber optics and tritium.

US Marine using his ACOG scope. Click on the image to enlarge. Public domain image.

Most other military forces also do the same thing for their soldiers. For example, Canada's soldiers are equipped with a C79 optical sight which is a 3.4x28 scope, British soldiers have a standard SUSAT L9A1 sight which has a 4x25.5 scope, Steyr AUG rifles (used by Austria and Australia) have a built-in 1.5x scope made by Swarovski (the same people that make luxury glass chandeliers and jewelry).

Canadian C79 Elcan sight. Click on the image to enlarge. Public domain image.

Snipers have also traditionally used fixed power scopes until recently. During World War II, German snipers used 4x fixed power scopes and US snipers used 8x scopes made by Unertl through World War II and the Korean war. By the Vietnam era, 10x fixed power Unertl scopes were in use by the US Marine snipers, although a variable power Redfield 3-9x scope was also tried out. The Unertl model MST-100 which is a 10x42 fixed power scope, remained in US Marines sniper service for quite a while (until about 2007 or so). The US Army snipers used the Leupold Ultra M3A 10x42 mm. scope or the Leupold Mk 4 LR/T M3 10x40 mm. scope until recently as well. In the recent years, US snipers have been experimenting with variable power scopes. For instance, US Marine snipers have been working with the Schmidt & Bender 3-12x50 mm. scope and the US Army snipers have been working with the Leupold Mk 4 3.5-10x40 mm., Leupold Mk 4 M1LR/T 8.5–25×50 mm. and Leupold Mk 4 6.5–20×50 mm. ER/T M5 scopes. Sandia National Labs also recently demonstrated the RAZAR (Rapid Adaptive Zoom for Assault Rifles) technology based on a request from the US military to develop a compact zoom rifle scope.

In our next post, we will look further into some of the technologies inside a scope.