Glass Quality Explained

There's a lot of noise out there on glass quality and cost. Many people believe that glass quality goes up with price and that you really do get what you pay for. This may have been the case for many years, but due to today's global economy and worldwide manufacturing capabilities, that rule may no longer apply.

The untrained eye usually doesn't see much difference when comparing an inexpensive low quality optic such as a binocular or rifle scope to one of much higher quality. On the other hand, the trained eye is often one of discerning taste, always critical of an optic's shortcomings and constantly looking for the best glass available.

This isn't really a bad thing, because plenty of people are content using optics that fall into the area of budget prices, while others will always upgrade to the latest and greatest optics on the market. It really is a matter of preference. But often there are questions asked on how one can determine quality.

Glass Quality at First Glance

First, understand that when we mention the word "glass", we are referring to the lenses in any sport optic, whether it be spotting scope, binoculars, or rifle scope.  There are several factors to consider when determining if a piece of glass can be deemed high quality. Each one of these factors contributes to an overall quality.

Clarity

The first factor to consider is clarity, which is literally how clear the image looks when viewing through an optic. The opposite of clarity would be a blurry or foggy view. Clarity is achieved by using glass with fewer imperfections. Clarity also contributes to light transmission and sharpness. The old rule that "you get what you pay for" still applies to a degree, because clearer glass costs more to produce.

Light Transmission

The term light transmission is used to describe the amount of ambient light that makes its way from the object being viewed to the eye of the viewer. More light is better, and that usually means that the glass will perform better when less light is available, such as during dawn, dusk, or at night. A larger objective lens size usually results in the highest light transmission, but light transmission can be affected by a number of other glass characteristics as well.

Sharpness

Sharpness is often tied directly to clarity. Sharpness is used to describe how detailed the image is when viewed through an optic.  The opposite of sharpness would be blurriness. A blurry picture usually either means the focus is set incorrectly, or the glass used is of lesser quality.

Color

All glass, especially coated glass, will make slight changes to the light that passes through it.  The changes it makes are described in terms of color temperature, which describes the specific colors that make their way to the viewer's eye.  Warmer color is color that appears more red or orange, while cooler color appears blue or white.  Color isn't really an indication of glass quality, and whether it's good or bad is usually dependent on the viewers preference.  The color seen by an optic is determined by the types of lens coatings used.  

Lens Coatings

Lens coatings are designed to achieve two goals. The first is to protect the exterior surface of the glass from damage. The second is to ensure maximum light transmission through the glass. An uncoated lens will typically reflect up to 10% of available light, meaning a maximum of 90% of the available light actually reaches the eye, even less if there are more uncoated lenses for the light to travel through. Coated lenses can reduce the amount of light
reflected to about 1%.

There are many different types of lens coatings, and many manufacturers have proprietary names for the type of coatings they use. Without getting into too much detail, there are three main terms used to describe coatings used on sport optics:

• Coated - Glass is given a single layer of coating on the exterior surfaces for protection from abrasion.
• Multi-coated - Exterior surfaces of glass are covered in multiple coatings to prevent abrasion and assist in light transmission.
• Fully mulit-coated - All glass surfaces utilize several coatings to ensure durability and the best possible light transmission.  Higher quality glass will be fully multi-coated. There are no exceptions to this.
   

Chromatic Aberration

Chromatic aberration is the term used to describe the fringes of color that are seen around bright objects on a dark background or vice-versa. Chromatic aberration occurs because of the way lenses bend light, and all glass will suffer from it to some degree. However, higher quality glass is manufactured in a way that produced so little chromatic aberration that it is commonly not visible to the human eye. Higher quality glass will do a better job of mitigating chromatic aberration than lower quality glass.

Glass Grades

As mentioned above, many manufacturers will give their glass or coatings proprietary names in order to set them apart.  Generally, there are a few categories used to distinguish higher grade glass:

• ED Glass - Extra-low dispersion glass
• FL Glass - Flouride glass
• HD Glass - High definition glass 

 
Each of these glass grades may have a different name, but they will all produce better color, clarity, and sharpness than glass that is not considered high grade.

Glass Quality and Cost

The old rule that "you get what you pay for" still holds true in some respects.  A $1000 optic is going to perform better than a $100 optic is most real world scenarios.  However, the line drawn between "good" glass and "great" glass is rapidly blurring as international manufacturing brings down costs and allows for tighter tolerances.  What used to be considered out of reach to the common man is actually quite attainable from a number of manufacturers.