April 11th, 2019

FOV, Exit Pupil, Twilight Factor — Optics Lingo Explained

Swarovski Optik exit pupil scope accurateshooter.com

Swarovski Optik exit pupil scope accurateshooter.comWhen shopping for a new riflescope or spotting scope it’s easy to get confused by all the technical terminology. Do you wish you had a better way to compare scopes — beyond just size, weight, and price? Well Swarovski Optik can help. The Swarovski Hunting Blog offers a helpful guide to technical terms used when comparing scope specifications. Here are some important definitions, expressed in layman’s language:

Objective Lens Diameter
The objective lens diameter determines the size of the optical system’s entrance pupil. The bigger the objective lens diameter, the more light the system can capture. However, the size of the objective lens does not determine the size of the field of view.

Exit Pupil
The size of the Exit Pupil is determined by the objective lens diameter and the magnification. If you look at the eyepiece from a distance of around 30 cm (11.8 in), the Exit Pupil appears as a bright disc.

For calculating the Exit Pupil the formula is:

Exit Pupil = objective lens diameter ÷ magnification (expressed in power number).

The larger the Exit Pupil, the more light will reach the eye.

Field of View
The Field of View is the size of the circular section of the area which can be observed when you look through a long-range optical device. In the case of rifle scopes, it is specified at a distance of 100 meters or 100 yards. For example, 42.5 m at 100 m or 127.5″ at 100 yards. As an alternative, the Field of View can also be stated in degrees (e.g. 6.6°).

Swarovski Optik exit pupil scope accurateshooter.com

NOTE: The technically-feasible size for the Field of View is essentially determined by the magnification. The higher the magnification the smaller the Field of View.

Twilight Factor
The Twilight Factor defines the optical system’s performance in poor light. The statement “the greater the twilight factor, the better the suitability for twilight” only applies if the exit pupil is larger than or at least as big as the eye’s pupil. The pupil in the human eye can only open to around 8 mm. As we get older, our eyes become less flexible, which limits our ability to see things in twilight or at night. Therefore [an optic’s] exit pupil cannot always be fully utilized.

For calculating the Twilight Factor the formula is:

Twilight Factor = root of ( magnification x objective lens diameter ).

NOTE: Spotting scopes have extremely high twilight factors because of their high magnification and large objective lens diameter. But [when used at high magnification] their small exit pupil can make them [somewhat difficult] to use in twilight.

CLICK HERE to Read Full Article (with more illustrations).

Photos copyright Swarovski Optik Blog, all rights reserved.

Story Tip by EdLongrange. We welcome reader submissions.
Permalink - Articles, Optics No Comments »
February 6th, 2017

Optics Terminology — Lessons from Swarovski Optik

Swarovski Optik exit pupil scope accurateshooter.com

Swarovski Optik exit pupil scope accurateshooter.comWhen shopping for a new riflescope or spotting scope it’s easy to get confused by all the technical terminology. Do you wish you had a better way to compare scopes — beyond just size, weight, and price? Well Swarovski Optik can help. The Swarovski Hunting Blog offers a helpful guide to technical terms used when comparing scope specifications. Here are some important definitions, expressed in layman’s language:

Objective Lens Diameter
The objective lens diameter determines the size of the optical system’s entrance pupil. The bigger the objective lens diameter, the more light the system can capture. However, the size of the objective lens does not determine the size of the field of view.

Exit Pupil
The size of the Exit Pupil is determined by the objective lens diameter and the magnification. If you look at the eyepiece from a distance of around 30 cm (11.8 in), the Exit Pupil appears as a bright disc.

For calculating the Exit Pupil the formula is:

Exit Pupil = objective lens diameter ÷ magnification (expressed in power number).

The larger the Exit Pupil, the more light will reach the eye.

Swarovski Optik exit pupil scope accurateshooter.com

Field of View
The Field of View is the size of the circular section of the area which can be observed when you look through a long-range optical device. In the case of rifle scopes, it is specified at a distance of 100 meters or 100 yards. For example, 42.5 m at 100 m or 127.5″ at 100 yards. As an alternative, the Field of View can also be stated in degrees (e.g. 6.6°).

NOTE: The technically-feasible size for the Field of View is essentially determined by the magnification. The higher the magnification the smaller the Field of View.

Twilight Factor
The Twilight Factor defines the optical system’s performance in poor light. The statement “the greater the twilight factor, the better the suitability for twilight” only applies if the exit pupil is larger than or at least as big as the eye’s pupil. The pupil in the human eye can only open to around 8 mm. As we get older, our eyes become less flexible, which limits our ability to see things in twilight or at night. Therefore [an optic’s] exit pupil cannot always be fully utilized.

For calculating the Twilight Factor the formula is:

Twilight Factor = root of ( magnification x objective lens diameter ).

NOTE: Spotting scopes have extremely high twilight factors because of their high magnification and large objective lens diameter. But [when used at high magnification] their small exit pupil can make them [somewhat difficult] to use in twilight.

CLICK HERE to Read Full Article (with more illustrations).

Photos copyright Swarovski Optik Blog, all rights reserved.

Story Tip by EdLongrange. We welcome reader submissions.
Permalink Optics No Comments »
April 13th, 2013

Gauge Low-Light Scope Performance with ScopeCalc.com

Zeiss DiavariHunters and tactical shooters need scopes with good low-light performance. For a scope to perform well at dawn and dusk, it needs good light transmission, plus a reasonably large exit pupil to make maximum use of your eye’s light processing abilty.* And generally speaking, the bigger the front objective, the better the low-light performance, other factors being equal. Given these basic principles, how can we quickly evaluate the low-light performance of different makes and models of scopes?

Here’s the answer: ScopeCalc.com offers a FREE web-based Low-Light Performance Calculator that lets you compare the light gain, perceived brightness, and overall low-light performance of various optics. Using this scope comparison tool is pretty easy — just input the magnification, objective diameter, exit pupil size, and light transmission ratio. If the scope’s manufacturer doesn’t publish an exit pupil size, then divide the objective diameter in millimeters by the magnification level. For example a 20-power scope with a 40mm objective should have a 2mm exit pupil. For most premium scopes, light transmission rates are typically 90% or better (averaged across the visible spectrum). However, not many manufacturers publish this data, so you may have to dig a little.

ScopeCalc.com

ScopeCalc.com’s calculator can be used for a single scope, a pair of scopes, or multiple scopes. Once you’ve typed in the needed data, click “Calculate” and the program will produce comparison charts showing Light Gain, Perceived Brightness, and Low-Light Performance. In the example below, we compared a “generic” 5-18×50 Tactical scope with a “generic” 8-32 Benchrest scope.

ScopeCalc.com

Though the program is easy to use, and quickly generates comparative data, assessing scope brightness, as perceived by the human eye, is not a simple matter. You’ll want to read the annotations that appear below the generated charts. For example, ScopeCalc’s creators explain that: “Perceived brightness is calculated as the cube root of the light gain, which is the basis for modern computer color space brightness scaling.” In addition, the way ScopeCalc measures Low-Light Performance is pretty sophisticated: “Low Light Performance [is calculated] as the average of light gain and resolution gain through magnification, as a measure of target image acuity gain in low light similar to Twilight Performance specified by scope manufacturers. Low Light Performance calculated here is much more useful than Twilight Performance, as Twilight performance is the average of the just the objective lens diameter times magnification, while Low Light Performance is the average of the actual Perceived Brightness times magnification, which also includes the exit pupil/eye pupil relation, light transmission, approximated diffraction, as well as the perception of relative light gain. Just as with Twilight Performance, this Low Light Performance calculation does not yet include lens resolution and contrast as factors. Therefore lower quality optics will yield relatively less gains at higher magnifications.” Got that?

Eye exit pupil*In low light, the human eye can typically dilate to 5mm – 7mm. The exact amount of dilation varies with the individual, and typically declines, with increasing age, from 7mm (at age 20) to a dark-adapted pupil of about 5.5mm by age 65. To take full advantage of a scope’s light-gathering capacity, the diameter of an eyepiece exit pupil should be no larger than the max diameter of your eye’s dark-adapted pupil, so that all of the light collected by the scope enters your eye, rather than falling on the iris. A large 8mm exit pupil may seem good, but it would be partly “wasted” on a shooter in his 60s.

Permalink Hunting/Varminting, Optics, Tech Tip No Comments »
October 3rd, 2011

NEW IOR/Valdada 12-52x56mm 100-MOA Scope — $3695.00

It’s big (40mm tube, 48 ounces), it’s powerful (52X max magnification), and it’s wickedly expensive ($3,695.00 MSRP). The new IOR 12-52x56mm “Terminator” is designed to “raise the bar” among rifle optics — to be the new Gold Standard. Is the new fat-tube 12-52X IOR really as “bad-ass” as its “Terminator” name implies? Well it does have some impressive features, starting with 100 MOA of vertical elevation travel (25 MOA per revolution). It also boasts ultra-high-grade Schott HD glass, digital illumination with auto shut-off, optional interchangeable BDC rings, and a Fast-Focus eyepiece.

IOR Valdada Terminator Scope 12-52x56mm

But the real innovation is the Terminator’s proprietary Mid-Focus Parallax adjustment. Handier than old-fashioned front-adjusting objectives, this new mid-focus system dispenses with sometimes-troublesome side-focus knobs. Look at the photo below. You’ll see a wide ring about 3″ ahead of the turret housing. Rotate that to set the focus for your target distance — anything from 21.5 feet to infinity.

IOR Valdada Terminator Scope 12-52x56mm

1/4″ Clicks and Tiny Exit Pupil at Full Magnification
With 52-power magnification on tap, you’d think IOR might offer 1/8th MOA clicks, at least as an option. However, the click value is listed at “1/4 inch”. Apparently, the Terminator scope, as launched, has one reticle choice, a new design, the “MP-8 EXTREME MOA-X1″. Below is a photo of the reticle taken by dealer Citadel Gun & Safe which has Terminators in stock for $3599.00. The reticle has 1 MOA stadia (hash marks), alternating in line length (taller vs. shorter). We’re informed that the markings are 1 MOA at 26X, so they’d be 1/2 MOA at 52X. It also has four sets of hold-over lines (set in +5 MOA intervals), with 1 MOA stadia running out from center. This will let you hold-off for windage, while simultaneously holding-over for elevation correction. We think this reticle is too “busy”, but it may appeal to some folks. Frankly, we are surprised that the front objective is only 56mm. This means that the exit pupil will be a tiny 1.08mm at 52 power. That’s really quite marginal. A 50+ power scope should have a larger front objective. CLICK HERE for More Photos (scroll to bottom of linked page).

IOR Valdada Terminator Scope 12-52x56mm

Story Tip by EdLongrange. We welcome reader submissions.
Permalink New Product, Optics 22 Comments »
January 14th, 2010

March Introduces 8-80X56mm ultra-high-power Zoom Scope

We thought we’d seen the upper limit of riflescope magnification with the new 10-50x60mm Sightron SIII, the 12-50x56mm Schmidt & Bender PMII, and the 10-60×52mm March variable. Now March has raised the magnification bar — with a zoom scope boasting Eighty times (80X) magnification at max power.

That’s a lot of magnification for a spotting scope, much less an optic designed to sit on top of a rifle. We wonder how useful the 80X max power will really be. At 80-power the exit pupil is a tiny 0.7 mm — what we consider the very lower range of usability. A very small exit pupil makes the viewer’s head position ultra-critical; you’ll need to get your head aligned just perfectly. Still, the extra power may prove useful for ultra-long-range shooting or at middle distances when conditions are ideal (no mirage).

Mach 8-80 scope

The new March 8-80x56mm Tactical/Long Range scope features 1/8 MOA clicks, a 34mm maintube, and 60 MOA of elevation. Weight is a reasonable 29.8 ounces (845 grams), while scope length is 16.0″ (shorter than many 40-power scopes). Five (5) different reticles are offered: 1/8 dot, 3/32 dot, 1/16 dot, MTR-1, and MTR-2.

In addition to the new 8-80x56mm, March will offer another tactical scope with a 10 times zoom range. The new March 5-50x56mm also has 1/8 MOA clicks, 60 MOA of travel, and comes with the same five reticle choices. At 15.67″ OAL and 29.3 ounces, the 5-50X is slightly smaller than its 80-power big brother.

Mach 8-80 scope.

We’ll learn more about the new March ultra-zooms at SHOT Show next week, when we visit the Kelbly’s booth. We’ll try to get a good look at the 8-80×56 and let you know how all that power works in the real world. Stay tuned for our SHOT Show reports staring January 19th.

CLICK HERE for March 8-80x56mm and 5-50x56mm Specifications Sheet.

Permalink New Product, News, Optics 7 Comments »