How to Evaluate Optics With a Flashlight
If you want to know whether your hunting optics are worth a spit, then look at them through the business...
If you want to know whether your hunting optics are worth a spit, then look at them through the business end of a flashlight. A simple penlight – the smaller and brighter the better – will reveal flaws in coatings, indicate whether your optic was made in a competent facility, and whether you are getting your money for the glass.
This visual inspection is a skill that sharpens with repetition, but here’s how to get started.
EVIDENCE OF COATINGS**
Flash your light over the objective lens (those are the bigger lenses) of a binocular, riflescope, or spotting scope. You are looking for the presence of optical coatings, the colored film that helps reduce reflection and corrects the colors coming through the optic. Ensure that this lens looks purple, or green, or magenta–the specific colors depend on the type of coatings that the manufacturer uses. I’d be shocked if objective lenses are not coated. If they are not–and you get only white light reflecting off the objective lens–then put the binocular down and walk away. It’s junk.
Next, shine your light down the objective lenses, into the barrel of the binocular. Now you are looking for the presence of coatings on internal lenses. This is probably the best indication of poor-quality optics. If you see white light reflect back at you, then there are probably one or more lens surfaces that were not coated, and the manufacturer’s claim of “fully multi-coated lenses” is bogus.
Ideally, you want to see a variety of colors reflecting back at you, evidence that each of the many lens surfaces in the guts of the optics are coated, and effectively canceling reflection and internal glare.
As you shine your light down the barrel, look for shiny reflection off internal screws, shims or bare metal. A good optic will be completely blackened internally, meaning that all surfaces are painted with a light-absorbing black finish. If you see reflection through the objective lens, then your eyes will see flaring and glare through the eyepieces.
Now look at the internal lens elements. You’ll be amazed at how much glass is inside even cheap binoculars and riflescopes. Each of those surfaces should be squeaky clean. If you see dust, debris or fingerprints (on several occasions I’ve seen greasy fingerprints left by the optic assembler) then you’re probably looking at an optic that was made at a facility with low quality-control standards, and the presence of debris is a good indication that you’ll be disappointed with the image the optic delivers to your eye.
Lastly, shine your light down the eyepiece of the optic. While you hold the optic a couple inches above a white sheet of paper, look at the size of the bright circle that comes from the objective lens. That light circle should be approximately the same size as the objective lens.
If it’s not–if the light circle is significantly smaller than the objective–then there’s a good chance the optic is “stopped down,” meaning that there’s a physical washer that’s keeping the objective lens from revealing all the light that shines through the instrument. This stoppage is a classic ploy used by optics companies that use cheap glass and poor lens-grinding techniques. They are keeping your eye from seeing the imperfect edge of the glass.
But they’re also cheating you. In some of the worse offenses, companies charge you for a 50mm objective, but deliver only 44mm of light.
That’s an offense you’d be hard-pressed to reveal without the aid of your handy penlight.