I get a bit irked when folks talk about the “good glass” in scopes. It’s not the glass…it’s the coatings! After touring several scope manufacturing plants and talking with several optical engineers, I’ve learned that glass is glass. They don’t buy “cheap,” “better” and “best” glass to put in these instruments. They WILL specify a certain type of glass with varying degrees of refractive index to deliver specific performance, but quality of Brand X’s glass isn’t any better than Brand Y’s or Z’s glass of the same type.
The coatings might be.
Coatings are not an afterthought. They make the difference between bright and dim. Once a scope’s objective lens has been ground and polished, maximum sharpness (resolution) has been achieved. The rest of the lenses in the scope can only degrade this. They are designed to reverse and erect the upside down image the objective lens produces. A parallax adjustment lens may be added for precise focus. Eyepiece lenses are used to magnify the initial image. That’s about it. Grind, polish and mount all these lenses properly and you maintain top-end resolution and sharpness.
As for brightness (more accurately, light transmission), the more lenses, the worse it gets. This is due to reflection. The reason you see a dim image of yourself in a window is because some of the light reflects off the window. In a scope, about 4 percent of the light striking each raw glass surface is lost to reflection. Another 4 percent is lost when the light leaves that lens. There can be eight- to 10 air/glass surfaces in a scope, costing you up to 80 percent of the light!
Anti-reflection coatings reduce this loss to as little as 0.02 percent per air/glass surface. It takes multiple layers of these to get to that level. This is a big difference between $150 and $1,500 scopes.
The next time you’re admiring a sharp, bright image through a scope, impress your friends (and probably the salesman) by exclaiming: “Wow, the glass in this scope is precisely ground and wears some killer anti-reflection coatings.”
Shop The Sportsman’s Guide for a fine selection of high-quality, value-priced Scopes and Optics.