posted February 01, 2011 09:15 PM                   

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Light transmission through the scope is what we're concerned with. The more light you let in (larger objective lens) the brighter the image. But, the higher the magnification, the darker the image. Divide the objective diameter in MM by the magnification (say 10x50mm) and you get an Exit Pupil (the little circle of light you see in the eyepiece when you hold a scope about 18 inches from your eye.) This EP corresponds to our own pupils which dilate from about 2mm in bright sun to 7mm in darkness. If the scope's EP is smaller than our own, we don't get as bright a view as we could. Ideally, a scope's EP should be 7mm wide to match our pupils in dimmest light. BUT, with age, our dilation decreases, perhaps as much as 2mm or 3mm. Highly variable. In general, I've found that a 4mm EP is adequate for putting a black crosshair onto a medium-toned animal (deer, coyote) 45 min. after sunset, perhaps an hour after (depending on other brightness factors I'll cover below.) So, a 42mm scope divided by 10X power gives you a 4.2mm objective, which I find more than adequate for 99 percent of my shooting. If I were specializing in super long range shooting after sunset and didn't need to carry the scope all day, I'd get a 50mm or even 56mm objective in order to keep the EP above 4mm at higher powers. A 15x50mm would yield a 3.5mm EP. A huge 56mm objective would only increase that to 3.7mm. A 10x55mm would give you 5.5mm. To get a maximum brightness from a 7mm EP, you'd need a 10x70mm scope. Probably ain't gonna find one.

Now, here's what really makes a huge difference in brightness: Anti-reflection coatings. They don't weigh anything, don't enlarge the physical size of the scope but more than double the light transmission. Here's how: raw glass reflects about 4 percent of light striking it and another 4 percent leaving it, for a loss of 8 percent per lens. Scopes have as many as 8 lenses total. Do the math, folks. One layer of magnesium sulfide anti-reflection coating cuts that reflection loss in half. Additional layers can knock it as low as 0.02 percent per air-to-glass surface. The end result is that using maximum layers of anti-reflection coatings produces scopes that transmit 95 percent of more of the light that enters. Combine that with a 4mm or larger EP and you're doing the best you can.

This means that a perfectly, multi-coated 10x40mm scope can easily transmit a brighter image than a poorly coated 10x56mm. Cheap 50mm and bigger objective scopes often add a "field stop" inside the main tube just behind the objective. This looks like a common washer and largely is. It's job is to eliminate poor-quality edge light from the big lens, which makes for a sharper image. This is what apertures (f-stops) in a camera do. It's a gimmick enabling companies to make/sell cheap scopes with huge objectives. But there is little or no brightness advantage.

The other thing anti-reflection coatings do is increase contrast by controlling flare and glare. Reflected light bounces around in a scope to create that fuzzy, foggy-looking haze we most commonly see when we look toward the low sun with a cheap scope. The entire view becomes muted, usually tinged orange. This is from all that uncontrolled, reflected and re-reflected light bouncing around inside. It's plenty bright, but not useably bright. Stop the reflections and you end up with a saturated view with crisp images that stand out. You can check all this stuff by aiming your scopes just off the low sun at sunrise and set. Do it with expensive and cheap models and you'll soon see the differences. Be sure to clean dust from the objective lens because all those dust specks reflect light to create haze and a dim image.

Increased contrast is useful because it helps us see the difference between subtle color hues like gray sagebrush and a coyote pelt, deer antlers and branches. Contrast increases resolution in low light situations.

By the way, if your scope has side parallax adjustment, it has an extra lens inside. Parallax adjustment is simply turning a lens to perfectly focus the image onto the reticle.

There's a lot going on in a scope, but since most of it is hidden in that dark tube, manufacturers can pull the wool over our eyes.

By Ron Spomer

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Night Force scopes appear to me to be slightly superior to Leupold's VX3 line in optical precision and overall fit/finish and durability. But I haven't compared NF against Leupold Mark Vs. This is an an important point. Many scope makers have several lines from cheap to high dollar, so you have to compare the proper classes. NF were/are designed for heavy duty, military sniper work, as are the Mark Vs. You pay a weight penalty for the big, tough sniper scopes, but they are durable and tough. Zeiss are super precise, of course, and their range finding scope is unbelievable. So's the price. My compromise for spot and stalk coyote work would be the Zeiss RF binocular (ranges to 1,700 yards in a half second, one push of the button -- best in the business) coupled with either a MOA dial scope or one with multiple reticles. Check out Vortex's newest sniper scopes. They are surprisingly excellent and precise. I'd opt for the one without the windage MOA turret and use the MOA harsh marks on the reticle for holding into the wind. Faster, just as effective. One dial to turn is enough for me.

Ron Spomer

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posted February 02, 2011 09:55 AM                  

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“Spend as much for glass as you do on the rifle.”

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posted February 02, 2011 11:20 AM                  

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Along with the Mexican wrestler sneakers

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posted February 02, 2011 01:07 PM                      

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I now have a very common burris scope on that rifle now

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