Understanding and choosing optics part 1: some simple things you can do when choosing your next optic

Guns Magazine, March, 2008 by Jacob Gottfredson

I have been evaluating and writing about glass for close to 20 years. As an engineer, the complexity of the physics and the beauty of the result fascinate me.

The question is: What are the factors upon which you make your choice? Certainly the foremost consideration is image quality. After all, if we could see an object adequately without the aid of optics, we might not spend the money. We also want the unit to be reliable, functional, and in some cases waterproof. Many shooters just want them to be cheap.

There is a lot to say about optics, enough to fill a book. I will try to hit some of the fundamentals of the subject and add more as required in future. Unfortunately, the space available in a magazine is a matter of limited real estate.

The optical engineer and manufacturer are both faced with myriad challenges in producing quality optics. Some of these include waterproofing, cladding with rubber or polyurethane for quietness, comfort and protection. Reliability and durability as well as rust prevention, fogproofing and other features can become important. But I would like to leave these issues to the subject of future articles. For now, let's just figure out how to evaluate and chose a binocular, for example.

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People often call me and want to know what binocular to buy. I seldom hear them say, "I am going to use it to ..." or, "I am willing to spend ..." But these are important considerations when choosing a binocular, riflescope or spotting scope, even a laser rangefinder. For example, will you carry them around your neck across a mountain or two for a week or will they primarily stay in the vehicle? Do you expect a considerable amount of moisture, cold, heat, mirage? You get the point.

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There are two predominant binocular designs: Porro prism or those shaped like a couple of Z's put side by side and roof prism binoculars or those that are simply a couple of straight barrels laid side by side. Some have central focusing and some have individual eyepiece focusing. Some claim to be permanent focus while others have mechanical or electronic stabilization.

Tough Tests

In general, it takes significantly more effort and money to produce roof prism binoculars with the same quality image than it does Porro prism binoculars. But roof prism binoculars have become the current fashion statement. Most roof prism binoculars are armor coated and are waterproof even in several feet of water. Some authors actually test them by submerging them for hours, then freezing them, heating them, throwing them around, etc. I admit having done those things a few times myself. I guess we just like to test the manufacturer's claim with some, "Ah ha, I thought so," kind of remark. On the other hand, if you're paying the better part of $2K, it is reasonable to expect them to live up to their claims. Fortunately for us, we are now seeing binoculars in the $250 to $400 range rivaling those above $1,000.

Let's concentrate on what is most important first. Image quality is comprised of many things and we will talk about each in following issues. But certainly resolution, contrast and light transmission are at the top of the list. In simple terms, resolution defines the ability to see detail. Contrast describes how well some detail stands out from other detail.

Light transmission has a lot to do with how well the binocular resolves an object. For a given power, as the objective lens (the one at the end of the binocular as opposed to the ocular lens nearest your eye) increases in diameter, the more light is transmitted to the eye. But this not the whole story.

Coatings affect the ability of the optic to transmit light. There are different types of coatings, different ways of applying them, as well as how many lenses are coated that significantly affect light transmission and quality. The more coatings, and the better the quality and application, the more expensive the optic becomes. The numbers given for a sporting optic all say the same thing, but the quality and effectiveness stop at the brochure.

Exit pupil affects the ability to see well, too. Some manufactures like to make big of Twilight Factor, to be explained later, but exit pupil is more important. An optic with an exit pupil below about 2.5mm will seldom produce a very satisfactory image, particularly at twilight. Exit pupil is calculated by simply dividing the objective lens diameter by the power. Thus, a 10x40mm binocular has an exit pupil of 40/10 = 4mm.

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The exit pupil of a spotting scope or riflescope is found the same way. Young eyes can dilate to about 7mm and older eyes have difficulty dilating to 5mm. A variable optic such as a 20-60x80mm spotting scope's exit pupil will vary from 4mm to 1.33mm. Thus, turning the power to 60 in twilight might not give you as much knowledge about the object viewed as it would at 30 power, i.e. at 2.67mm.

Thus, when you evaluate binoculars, riflescopes, spotting scopes and the like, you need to determine what the optic will be used for, the quality of the image, as well as the reliability and durability required.