Magnification vs. Light

And the Vortex 20-60x85mm Razor HD Spotting Scope.

Two of the attributes most commonly advertised for marketing purposes by manufacturers are exit pupil and twilight factor. I was remembering some work I had done previously along these lines as I looked through a Vortex spotting scope on a hillside in Montana recently.

Scott Parks, Vortex’s R&D guy, was kind enough to send their best spotting scope and three lenses for a high country elk hunt. As I sat on a hillside in late afternoon and looked into dark timber, I had occasion to think about what I had said in previous articles. Namely, there is a point at which power, exit pupil and twilight factor hit an optimum viewing combination.

The Razor HD is the top of the Vortex line. He sent both the angled spotting version and the straight. The first thing I liked was the combination of coarse and fine focus wheels. This causes a lot less vibration than the helical variety (ring around the body) when focusing, and is easier to fine focus, although it may not be as fast.

An 85mm objective lens gives ample light transmission at most powers, although the exit pupil diameter drops to 1.42mm at 60X. But then so does every other industry scope with the same objective lens size and power. Why does it matter? Because anything below about 2.5mm exit pupil diameter does not transmit enough light to produce fine resolution. Just a fact of life. But visual acuity is not entirely based on exit pupil. It also has to do with the quality of the glass and its coatings. And in that regard, the Razor HD spotting scope is right up there with the best of them.

Vortex uses High Density (HD) premium, extra-low dispersion glass in the Razor, which enhances resolution and color fidelity. It also features a triple apochromatic lens system that reduces fringing, dilution of color fidelity and degradation of resolution.

The lenses are fully multi-coated, Armor Tek protects exterior lenses, and Argon gas is used to reduce fogging and promote waterproof performance. The body is a magnesium alloy.

The 20-60X eyepiece is included in the price, but Scott also sent a 30X HD lens with a mil dot (hashmark) reticle for calling shots at long range as well as Vortex’s new Long Eye Relief lens.

While Vortex optics incorporates the latest in features, optics are just that: It is all about the quality of the glass. So I proceeded to test the scope on my charts, during low light, under poor conditions and mirage.

Clearly the HD extra-low dispersion glass is better, but is also higher priced. While the majority of Vortex glass is good, the spotting scope really tops their list for quality glass and image. I will put it up against any on the market.


The straight version of the Vortex 20-60mmx85mm spotting
scope sits above Ashbury Precision Ordnance’s .338 Lapua Magnum.

Let me turn to the real intent of this article. Binoculars and spotting and riflescopes are marked with a formula such as “8×30” or “10×50.” The first number in the formula is the power, or how many times the image is enlarged. The second number in the formula is the diameter of the objective lens in millimeters. The bigger the objective, the more light can enter and the greater the potential resolution of the image.

Low-light performance is largely dependent on the exit pupil. Exit pupils are the small, bright circles you see in the eyepieces when you hold the instrument away from your eyes and up to the light. The exit pupil diameter is calculated by dividing the diameter of the objective lens by the power. A 7×35 binocular has an exit pupil diameter of 5 millimeters (35÷7 = 5).

In bright sunlight the pupils of your eyes contract from 2 to 4mm, and at night they may open to 7mm. If the beam of light exiting the eyepiece is wider than the pupil of the eye, the excess doesn’t get in: The eye can’t see it. The extra light is wasted, so to speak.

During daylight hours things look just as bright through binoculars and scopes with 4mm exit pupils as through those with 7mm exit pupils. In fact, if they have better coatings, binoculars and scopes with 4mm exit pupils may be brighter. Thus, the larger exit pupil is at greater advantage in low light. The larger exit pupil is also useful on ships where the constant motion makes it difficult to always keep the eye centered on the exit pupil.

As we age, our eyes lose some of their ability to adapt to darkness. While a 20-year-old person’s pupils might open to 7mm, at age 50, the pupils may open only to 5mm. Therefore, binoculars and scopes with large exit pupils may not help the older shooter. The best binoculars and scopes are bright as a result of their advanced multi-coatings and top-quality optics, which provides brightness you can see all the time, even in daylight.


This is an image of the Vortex Razor HD’s mil hashmark reticle lens for the 20-60x85mm
spotting scope. This allows the spotter to call the shooter’s hits accurately at long
range and guide him into the target. It can also be used to range targets of known dimensions.

Lens size is partially responsible for the amount of light transferred to the eye. You can compare the light transmitting ability of two binoculars by squaring their objective lens diameters. A 50mm objective lenses would yield: 50 squared = 2,500, while a 35mm objective lenses would be 35 squared = 1,225. Also, resolution (the ability of a binocular to show small details) is proportional to the size of the objective. That is, the bigger the lens, the smaller the detail you can see. But coatings and other factors enter into the ability of a binocular or scope to transmit light and resolve detail.

Another specification called “relative brightness” is calculated by squaring the exit pupil diameter. For example, all binoculars or scopes with a 4mm exit pupil diameter would yield: 4 squared = 16. Since relative brightness is a mathematical relationship, it doesn’t take into account aperture differences between optics. An 8x32mm and a 20x80mm binocular have identical a 4mm exit pupils, but they are hardly identical low-light performers due to the 635 percent larger light transmitting area of a 20×80. Relative brightness is only useful when comparing the low-light performance of binoculars of similar aperture, and then only when your eye’s pupil is as large as the exit pupils of the binocular or scope.

Twilight factor provides an indication of the relative performance of sizes of optics in low-light levels, and it is dependent on both the objective diameter (how much light enters the binocular or scope) and the exit pupil (how much light passes from the binocular or scope to the eye). It is calculated from the formula “the square root of the magnification times the objective diameter.” For example, an 8×56 has a twilight factor of 21.2, while an 8×30 has a twilight factor of 15.5 and an 8×20 a twilight factor of 12.6. The larger the twilight factor, the better the relative low-light performance of the binocular or scope.

Exit pupil, relative brightness and relative light efficiency comparisons are interesting and often useful, but they are not always the best indicators of how well binoculars and scopes perform in low light. For example, 8×32 and 20×80 both have 4mm exit pupils, a relative brightness of 16 and a relative light efficiency of 24, but the 80mm binoculars are much better in low light due to the 625 percent larger light transmitting capacity and higher twilight factor.


A big star for Vortex is their classy Razor HD spotting scope. Vortex didn’t mess around when
they designed this baby: High-density extra-low dispersion glass, fully multi-coated lenses
and argon gas purged. The HD has an 85mm objective lens with a triple apochromatic lens
system using coarse and fine focus knobs, and can be rotated in the tripod.

Twilight factor is a more useful judge of a binocular’s low-light performance than exit pupil, etc., as it takes into account both light transmission and magnification, both of which affect how much detail you can see. Seeing detail is what binoculars and scopes are all about.

The larger the image, the easier it is for you to see details in that image. By the same token, with a smaller image, the brighter it gets, the easier it is for you to see the same details clearly. So, within reason, if magnification goes up, brightness can go down without seriously affecting resolution, and vice versa. That is, small bright images can show you as much detail as large dim images.

Twilight factor allows you to compare various combinations of aperture vs. magnification to determine the one that best balances an increase in magnification against a decrease in brightness (or vice versa). The larger the twilight factor, the better a binocular is for low-light observing.
A twilight factor of 17 and above is best for twilight or early morning use.

But here again, the twilight factor is a mathematical relationship only. It does not take into account light transmission differences between binoculars and scopes, so small numerical differences in twilight factors may not be visible in the field.

Keep in mind that just because an inexpensive binocular and a premium model have identical twilight factors, exit pupil, relative brightness, etc., you can’t assume their optical performance will be the same. Distortions and optical flaws in the less expensive binocular or scope can severely compromise its sharpness and clarity.
By Jacob Gottfredson

Performance: Vortex Razor HD
20-60x85mm Spotting Scope

Resolution: 15 (Excellent)
Contrast: Excellent
Astigmatism: None
Curvature of Field: None
Rolling Distortion: None
Pin Cushion Distortion: None
Barrel Distortion: None
Color Fidelity: Excellent

Razor HD 20-60x85mm
Spotting Scope

Maker: Vortex Optics
2120 W Greenview Dr.
Middleton, WI 53562
(800) 426-0048

Magnification: 20X-60X
Objective: 85mm
Ranging: Via mil hashmarks
Exit Pupil Diameter: 4.25mm (20X) – 1.42mm (60X)
Field Of View: 117 feet at 1,000 yards (20X), 60 feet (60X)
Waterproof: Yes, Weight: 65.7 ounces
Length: 15.25 inches, Close Focus: 16.4 feet
Price: $2,000

Read More Optics Articles


Order Your Copy Of The GUNS Magazine June 2014 Issue Today!

Download A PDF Of The GUNS Magazine June 2014 Issue Now!

Leave a Reply

Your email address will not be published. Required fields are marked *

(Spamcheck Enabled)