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Optics Tutorial

Optics Tutorial

Here we provide the essential information you need to understand how optical telescopes work. This discussion is oriented toward rifle scopes. If you how understand rifles scopes work, then you will understand how other optical telescopes work, because rifle scopes have the most complex optical designs.

This tutorial discusses image contrast at length, because this is one of the key discriminators between good and mediocre optics. We also explain why it is that some types of shooting applications require good optical performance, while others do not. This information is important because it helps you set priorities when selecting a scope for a specific purpose.

Parts of the scope

In this tutorial, we focus on sporting optics that contain telescopes: rifle scopes, spotting scopes, binoculars, and laser rangefinders. This section covers riflescopes. Sections covering other sport optics will be added later. The major parts of the riflescope are shown in Figure 1. The drawing shows the reticle in the second focal plane (SFP). The reticle may also be located in the first focal plane (FFP).

1 Optics Tutorial Figure 1 Part of scope 

Figure 1. Parts of a rifle scope.

Optical Design

The optical design of a rifle scope can be considered to have four parts: the objective, reticle, erector and eyepiece (see Figure 2). The objective forms an inverted image at the first focal plane (FFP) position. Together, the objective lens and the eyepiece form an “afocal” telescope (i.e., the telescope produces a focused image at infinity). The erector lenses form a focal telescope that provides additional magnification, creates an erect image at the second focal plane (SFP), and provides reticle adjustments (windage and elevation). Without the erector lenses, the objective and eyepiece lenses would otherwise have a small magnification of about 3X, and the image would be upside down.

The reticle can either be mounted to the erector tube at the FFP, as shown in Figure 2, or mounted to the erector at the SFP, which is after the erector lenses. The windage and elevation adjustments move the FFP reticle by tilting the tube containing the erector lenses (see Figure 6). The windage and elevation adjustments do not move the SFP reticle, however, but instead just tilt the erector tube it is held in. The FFP reticle is magnified by the erector tube along with the objects in the scene. Because it is located after the erector tube, the SFP reticle is a constant size, regardless of erector tube magnification.

The objective lens determines the overall field of view and image quality, especially when large reticle adjustments are made. Image features in the center of the field of view are called “on-axis”, while those away from the center of the field of view are called “off-axis”. A large fraction of the optics cost is in the objective lens. A field lens located between the FFP and the erector tube prevents loss of light for off-axis objects (near the edge of the field of view). The lenses in the erector tube also effect image quality. Because they are small, however, their cost is relatively low.

1 Optics Tutorial Figure 2 Rifle Scope Optics 

Figure 2. Rifle scope optics.

Figure 2. A - C show the erector lenses in different positions, depending on the magnification. Note how the erector lenses move a large distance in this scope design, which has a 4:1 magnification range (2.5X to 10X). A translating mechanism, coupled to the power ring, moves these lenses inside the erector tube. The lenses slide back and forth inside an inner tube that has a linear slot parallel to the tube axis. Pins protruding out from the lens mounts through the linear slot engage in a helical slot cut into an outer erector tube, which is connected to the power ring. As the power ring rotates, the intersection of the helical and linear slots moves forward or backward, moving the lens with it. There are two different helical slots, one for each erector lens.

Lenses must move even further inside the erector tube for greater magnification ratios of 5:1 or 6:1. This can be accomplished if the helix angle in the outer tube slot is increased. If the helix angle is too steep, however, the torque required to rotate the power ring is too high. The solution is to use a larger tube diameter, which provides a longer translation for the same helix angle, thereby requiring a low torque on the power ring. For that reason 6:1 magnification ratio scopes usually have 30 mm or larger tubes.

The optical design in Figure 2 does not explicitly show any means of focusing the target. In this design the location of the objective is adjustable, hence the term “adjustable objective.” In “side focus” designs another lens is usually added between the objective lens and the FFP that can be moved by rotating a knob located opposite the windage adjustment.

Optical Performance

Resolution and Contrast: A telescope is also called a “visual optic”, because it operates as an aid to the human eye. Unlike a digital camera, which has an image forming lens and image sensor, a telescope needs to be viewed by a person to function. This makes testing visual optics a challenge, because each person judges image quality differently.

1 Optics Tutorial Consider the Target 

Figure 3.Consider the Target - Atmosphere - Scope - Eye as an integrated optical system. One of them usually limits resolution, and it’s not always the scope. In many situations, the atmosphere determines image resolution and contrast, not the scope.

The human eye is a complex optical instrument. The lens in the eye forms an image of an object, or target, on the retina. The photodetectors in the eye, called rods and cones, convert that image to electrical signals, and the brain processes those signals to form what we perceive to be a color image of the target. Image features have a maximum and minimum brightness level, and ratio of the highest to the lowest brightness is called the “contrast.” To varying degrees, the human eye always degrades the contrast of the target.

Resolution is the smallest size image feature that can be distinguished, or “resolved.” This image feature must have a minimum contrast to be resolved. When we say the eye has a certain resolution, what we really mean is that the image has the minimum acceptable contrast at that resolution. Remember, this “image” is something perceived by the brain. For a high contrast, target in direct sunlight, the human eye has a resolution of about 1 arc minute, or 1 minute of angle (1 MOA).

The eye’s contrast depends on the illumination level. This means that in bright light (direct sunlight, etc.) the eye has the best resolution, while in low light (dawn or dusk, dark shade, etc.,) the eye has degraded resolution. By “degraded”, we mean that the minimum acceptable contrast occurs at a larger feature size. At each illumination level the eye trades contrast for resolution in this way to extract the most information from the scene.

We can improve the eye’s image resolution by using a telescope as a visual aid to make the target features appear to have a larger size. For example, a 10X magnification telescope makes target features appear 10 times larger. The aided eye’s resolution is therefore improved by a factor of 10 (i.e., 0.1 MOA for sunlit targets, compared to 1 MOA for the naked eye).

The purpose of the telescope, therefore, is to magnify the target so that it appears larger, or nearer, to the eye. Variety for reasons, the telescope also degrades resolution and contrast (discussed in detail below).

Contrast and Glare.
No riflescope is perfect and two effects in particular can severely degrade image contrast: glare and diffraction. Glare is probably the least understood of all telescope performance parameters, but is one of the primary factors contributing to a clear or “brilliant” image. Glare degrades the image because “stray” light that does not come from the target scene scatters off interior scope surfaces and ends up falling within the image of the scene. This effect is called “veiling glare” and it degrades the contrast of the image. Contrast is the ratio of highest to the lowest possible intensity of an image of a black and white target. Glare is reduced by the use of light baffling apertures inside the riflescope that block stray light, and flat black coatings on the lens edges and interior metal surfaces.

When you look through a good scope and the image seems to "pop" out at you, it's the absence of scattered light that gives the image such high contrast. There are other sources of scattered light, but glare is often the largest. It’s usually the dominant effect that limits image contrast when the target is marginally illuminated and is surrounded by higher illumination terrain or sky - very common in big game hunting. 

Glare performance is one of the differences between good German optics, for example, and lower quality optics. German optical designers minimize glare by adding light baffles (“glare stops”), applying a flat black coating to interior metal surfaces, and using lenses with blackened edges, for example. They test scopes using an integrating sphere to insure they blocked all major sources of glare. These features increase cost.

Generally speaking, glare increases with increasing objective size and magnification. The higher the magnification range of the scope, the higher the glare at all magnification settings. Also, as the magnification range increases, often so does the objective size, leading to a compound effect that significantly increases glare for higher magnification range scopes. Finally, glare seems to increase dramatically at the edge of the exit pupil, making glare much more of a problem for high magnification scopes, in which the eye pupil is larger than the exit pupil and catches all this glare.

For spot and stalk and long range big game hunting, glare performance should be a high priority when selecting a scope. Target contrast and illumination are usually low, making animals difficult to see even with the best optics. Atmospheric turbulence is usually low as well, which minimizes a primary source of blur that would otherwise limit image quality.

Glare performance is not a deal breaker for all applications. For prairie dog and ground squirrel hunting and target shooting at a range, for example, glare is not so important because another effect, called atmospheric turbulence (see below), limits image contrast. Also, target contrast and illumination tend to be good. Sun shades are effective at reducing glare. If you can use one at the range, they are recommended.

1 Optics Tutorial Figure 4 Veiling Glare 

Figure 4. Effect of veiling glare on image contrast.

Diffraction. Diffraction is the second effect that can significantly degrade contrast. The optical resolution of any telescope is fundamentally limited by diffraction. Diffraction occurs whenever light is passes through an aperture (such as the objective lens). The fundamental “diffraction limited” resolution is rarely achieved in consumer optics, because optical aberrations are present. Aberrations are another topic. Here we are concerned mainly with the effect of diffraction on image contrast.

Lens optical surfaces are formed by grinding and polishing glass material. For a lens to have “power” it must have at least one spherical surface. This surface is not perfectly spherical, however, because of minor flaws in the fabrication process. Lens fabricators use a term called “surface figure” to describe the degree to which a lens surface is perfectly spherical – that is, without flaws.

A process called diffraction occurs whenever light passes through any optic whose optical surface is not perfectly spherical. When light is “diffracted” it is diverted away from an image point. The primary effect of diffraction in visual optics is to reduce image contrast.

Somewhere between five and twelve lenses are used in a riflescope, and each one contributes to diffraction. Increasing the polishing time improves the surface figure of the lens and thereby reduces diffraction. Increasing the optical polishing time increases fabrication cost. The use of high quality polishing processes is another reason that German optics have a reputation for providing a brilliantly clear (that is, high contrast) image.

Atmospheric Turbulence: The atmosphere between the target and the telescope almost always degrades image resolution. Whenever there are temperature variations in the air, light rays don't actually travel in a straight line, but rather are bent, or “refracted”, a tiny amount as they pass through the air. There are always temperature variations in the air between you and an object that is far enough away that a telescope is useful. This effect causes the image to lose resolution and contrast (Figure 5), and is called turbulence-induced blur.

1 Optics Tutorial Figure 5 Turbulence 

Figure 5.Turbulence-induced blur.

Most shooters are familiar with "mirage", which has the appearance of wavy lines moving across the target image. This effect is due to the turbulent air moving across the field of view. Turbulence-induced blur is a somewhat different effect.

The amount of turbulence-induced blur is determined by the length of the optical path (i.e., the range to target), the “strength” of the turbulence, and the size of the aperture. Therefore, turbulence-induced blur is worse for:

  • Longer ranges,

  • Larger scope apertures,

  • Hotter ground temperatures,

  • Flatter terrain (light rays closer to ground)

  • Target or observer close to the ground.

High turbulence environments are typically those in which the target is small and the terrain in front of the target is flat and in direct sunlight. Typical examples include prairie dog and ground squirrel hunting, and target shooting at a firing range.

Low turbulence environments are typically where the target is surrounded by shade and elevated above the terrain in front of the target (i.e., on a hillside) and/or the air temperature is low. Typical examples include big game hunting in hilly terrain, at dawn and dusk, or under overcast skies, especially in fall and winter when the sun is low in the sky.

The main point is that the image seen when looking through a scope at targets at a firing range is usually blurred due to turbulence, not due to the optics in one scope vs another. Any difference in image "quality" between two side-by-side scopes under these conditions is probably due to the scopes having different effective aperture sizes. It would be much easier to see differences in optical resolution in a side-by-side test if you looked through the scopes at a target across a small valley or canyon on a cool day under an overcast sky.

Mirage: Air and wind can alter the image of the target in various ways. We’ve already discussed turbulence-induced blur. Another effect is “mirage”.

Shooters often use the word “mirage” to describe the dynamic effect that moving air at different temperatures has on the target image. This effect is most pronounced when looking at straight edges of objects, such as target boards, poles, etc. These edges appear to have moving ripples or waves. In many cases, the image of objects, such as people and animals, looks distorted, as though they were standing in front of a “fun house” mirror. This image distortion changes rapidly, again reminiscent of waves. The direction of the waves is usually upward at an angle, but in strong wind can be horizontal. Shooters use mirage to estimate wind speed and correct for light deflection in bench rest competition.

Outside the shooting community, mirage is a different optical phenomenon in which light rays are bent to produce a displaced mirror image of distant objects. In this tutorial, we are discussing a different effect altogether, which is the loss of optical resolution due to thermal gradients in the air. This effect happens whether or not wind is present, so it is not a wind-related phenomenon.

Aberrations and Resolution. It is important to have at least a 1 degree field of view from the objective lens that is nearly free from optical aberrations. This is because the reticle may be positioned there due to either a large elevation adjustment or a boresight error. Also, the image of the surrounding scene must be clear enough to insure that the shot is safe to take. Whenever a telescope does not have good resolution across a wide field of view, it is usually due to optical phenomena called “optical aberrations”. Aberrations can degrade contrast, but they have a bigger effect on image resolution. Basically, they cause the image to become blurred, and this blur is especially evident for off-axis image features.

Optical aberrations result from the use of simple spherical glass lens surfaces, which are used because they are the easiest to fabricate. There are different aberration effects called spherical, chromatic, coma, astigmatism, distortion and field curvature. Image features in the center of the field of view are called “on-axis”, while those away from the center of the field of view are called “off-axis”. The chromatic aberration and coma occur for either “on-axis” or “off-axis” image features, while the last four aberrations are limited to “off-axis” image features. A detailed description of aberrations is found on Wikipedia.

Optical aberrations are relatively easy to eliminate or “correct” for on-axis image features. Aberrations are more difficult to correct for “off-axis” image features. Minimizing off-axis optical aberrations requires more expensive optics.

Distortion changes the shape of objects but does not degrade image resolution. Field curvature does not necessarily degrade resolution because the eye can accommodate small changes in focus. That leaves the spherical, chromatic, coma and astigmatism.

Even off-axis aberrations can be nearly eliminated from an optical design by adding more lenses, which increases cost. Therefore, when aberrations are present, they result from compromises in the optical design that are motivated by the need to reduce cost. Aberrations usually cause the image to be blurred as you look away from the center of the field of view. In a really good optical design these aberrations are "corrected" and the image has high resolution across the entire field of view.

The image blurring effects of off-axis aberrations are more noticeable at higher magnification levels, which are needed when the target is further away and therefore smaller. Therefore, a proper scope installation minimizes the elevation adjustment needed to aim at a long range target. Sometimes that means having an even larger elevation adjustment in the opposite direction to aim at a target at short range. Also, the blurring effects of simultaneous large windage and elevation adjustments are compounded. Therefore, a good scope installation should also minimize the windage adjustment needed to hit a short range target in low wind.

Technical: The erector telescope and eyepiece have a field of view somewhat independent of the objective lens field of view. The erector field of view depends on the magnification. It is also determined by the eye relief and the eyepiece diameter. All other things being equal, the longer the eye relief the smaller the field of view, and the smaller the eyepiece, the smaller the field of view. Finally, the field of view may be reduced by a field stop in the design. A field stop is simply a round aperture located near one of the focal planes that limits the field of view – usually to block image blur that occurs near the extreme edge of the field of view. A field stop could be located on the reticle mount, or at the other focal plane position (i.e., at the FFP, if the reticle is at the SFP).

The colored dashed lines in Figure 6. A and B show the ray paths for on- and off-axis rays at 10X magnification. Light travels these paths through the scope. The diagram shows how rays are refracted at each lens surface. The green dashed lines show rays originating from an object in the center of the field of view. When the erector tube is centered (Figure 6.A), the green rays stay mostly near the optical axis of the scope after they enter the erector tube. Those rays are called “on-axis” because they originate from an object at the center of the field of objective view and stay close to the optical center line all the way through the scope.

1 Optics Tutorial Figure 6 Parts of the rifle Scope 

Figure 6. Rifle scope optics - Off-axis rays.

When rays are refracted small angles, the image tends to be sharp. When rays are refracted large angles, however, the image tends to become blurred due to something called “off-axis optical aberrations.” The red dashed lines in Figure 6.A show rays originating from an object away from the center of the objective field of view. These rays are also away from the center of the erector tube field of view. The red rays are more strongly refracted inside the erector tube and the eyepiece than the green rays are, causing them to blur. The objective lens also introduces off-axis aberrations because the image is not on the center of the optical axis. Therefore, even with the erector tube centered, off-axis aberrations cause the image away from the center of the field of view to blur.

For most rifle scope objective lenses, off-axis aberrations become noticeable at even 0.5° incidence angle. An angle of 0.5° corresponds to only 30 MOA. Highly corrected very low-dispersion or apochromatic lenses are used by a few manufacturers that increase that angle to about 1.0° (60 MOA).

When the erector tube is tilted (Figure 6.B), the green rays are refracted larger angles because they become off-axis after they enter the erector tube. Therefore, tilting the erector tube to achieve a large elevation adjustment causes the image in the center of the field of view to blur.

The red dashed lines show rays originating from an object near the edge of the erector tube field of view. Comparing Figure 6.A and 6.B, it is apparent that the red rays become more strongly refracted when the erector tube is tilted. In Figure 6.B the red rays are even further off-axis in the objective lens, compared to Figure 6.A, which further increases the off-axis aberrations in the objective lens. Therefore, tilting the erector tube to achieve a large elevation adjustment causes the image at the edge of the field of view to blur even worse than for the image at the center of the field of view.

Magnification and Exit Pupil. Telescopic magnification allows the shooter to resolve target features at long distances. As magnification increases, however, the field of view decreases, which reduces the shooter’s situational awareness. It also reduces the exit pupil size. In most telescopes the exit pupil is equal to the entrance pupil (i.e., objective diameter) divided by the magnification:

Exit Pupil = Objective Diameter / Magnification

For low magnification variable scopes (M=3 or less), however, the exit pupil is often smaller than this equation would indicate. That is because an aperture stop somewhere in the erector tube limits the effective pupil at low magnification settings.

The human eye protects the retina from overload by automatically adjusting its pupil size in response to illumination levels. The minimum pupil diameter is about 1.5 mm. The eye has the best resolution at a pupil size of 2-4 mm. The maximum pupil size during dawn and dusk is about 5 mm. After long periods of near total darkness, the pupil can increase to about 7 mm, but only for relatively young people. People over about 50 years of age have a maximum pupil size of about 5 mm.

If the eye can accept all the light in the exit pupil, the telescope image is nearly as bright as the image seen by the unaided eye. When the exit pupil becomes smaller than the eye’s pupil, however, the image brightness starts to decrease. This reduction in brightness can be significant. A small exit pupil is also more difficult to align with the shooter’s eye. As we discussed earlier, glare is worse for such small exit pupils.

At low illumination (before dawn and after dusk) there is an optimum scope magnification that produces an exit pupil size equal to the eye pupil. This optimum magnification increases with the objective diameter. A magnification below this value reduces the target size. A higher magnification reduces the pupil size, reducing image brightness and contrast. The size of the scope tube has nothing to do with image brightness.

For big game hunting, we recommend only buying as much magnification as you really need, and no more. A good rule of thumb is 1.5X magnification for every 100yds of range. At a range of 600 yds a magnification of 9X or 10X is usually adequate. Even at 1,000 yds a magnification of 14X to 16X is fine for big game hunting.

In terms of target angles, the size of a typical reticle line is roughly 0.1 MOA, and the size of a reticle dot is 0.125 MOA (fine target dot) to 0.9 MOA (USMC mildot). The vital zone diameter is about 8 inches on most big game. At a range of 1,000 yds, an 8 inch diameter subtends 0.80 MOA. At 15X magnification, the vital zone is magnified to 12 MOA.

This means that the shooter must position the reticle crosshair or dot on a 12 MOA aim point. Recall that the eye has a resolution of about 1 MOA. The angular dimensions of the vital zone is ten times the resolution limit of the eye. The animal’s chest is at about 24” tall, which equals about 36 MOA at 15X magnification, or 36 times the resolution of the eye. This aiming task is fairly easy for an experienced shooter to perform.

Increasing the magnification from 15X to 24X makes the entire sight picture appear 60% larger, but doesn’t make the aiming challenge fundamentally easier. Increasing the upper magnification limit from 15X to 24X, however, significantly increases glare at all magnification settings. For a 50 mm objective, the exit pupil decreases from 3.3 mm to 2.1 mm, reducing both image brightness and eyebox size, and increasing glare for larger eye pupils.

High magnification is useful when the shooter has vision acuity problems that are not corrected. Many shooters cannot wear corrective lenses when they shoot. The scope diopter adjustment will correct for myopia or hyperopia. Astigmatism, however, cannot be corrected using the dioptier adjustment. In this case, higher magnification will improve image resolution, but at the same time degrade contrast (as discussed above).

Eye Relief

The eye relief is the distance behind the scope ocular lens that the exit pupil is in focus. This location is where the eye should optimally be located when aiming the rifle. The eye relief value should be as high as possible for heavy recoiling rifles to prevent the ocular rim from impacting the shooter’s forehead when firing the rifle. Eye relief is measured from the eyepiece lens, which is misleading, because the ocular bell sticks out further and is the first part of the scope that impacts the forehead under excessive recoil.

Typical values of eye relief are 3-4 inches, depending on magnification setting. For high recoiling rifles, the eye relief should be at least 3.5 inches, and the rifle scope should be mounted on the rifle so that the shooter has to move his/her head forward by 0.25-0.5 inches for the eye to be at the eye relief distance. This set-up minimizes the chances that the shooter’s head will be too close and suffer a bruised or even lacerated forehead after shooting the rifle from a typical field position. If the shooter intends to fire the rifle frequently from the prone position, then this shooting position should be used when deciding where to locate the scope in the rings.

Some scopes have a more forgiving eye relief than others, such that the eye can be placed closer or further away than the exact focus position, and yet still see an unobstructed sight picture. The exit pupil area times the length of this usable eye relief is called the eybox. This cylindrical volume encompasses the positions of the eye that will allow the shooter to see the full field of view. Moving the eye closer or further away, or to the left or right, will result in a partially occluded field of view. Again for fast target acquisition under stressful situations, such as hunting dangerous game or close quarters battle, the eyebox should be a large as possible.

Transmission: Fifty years ago, before multilayer dielectric thin film coatings were used in sport optics, scope transmission was as low as 50-60%. As this coating technology became widely used in consumer products, the transmission gradually increased to the 90-98% range that we have today for high quality optics. The difference between 90% and 95% transmission is so small, that it’s no longer much of a discriminator in optical performance. As long as all the lenses (not just the first and last) are multi-layer dielectric coated, the difference between one vendor’s coating and another these days is mainly just marketing.

Hydrophobic coatings: These coatings are applied to the first and last lens surfaces, and cause water to bead up into small droplets on the lens surface. In very wet environments, these coatings can improve the image significantly. The presence of lots of small droplets will significantly increase scattered light and reduce image contrast, however. The coating is preferred for these environments.

Glass: In the sport optics industry, the product chain looks like this:

1 Optics Tutorial Products Chain 

The supplier of the glass blanks is really of secondary importance, because most of the cost in lens fabrication is in the process of grinding and polishing glass material into lenses. All the major glass suppliers, like Schott, Hoya, O'Hara, etc., make equivalent quality glasses and distribute them internationally to the companies that fabricate the lenses. In particular, Schott is only a glass material supplier, not a lens fabricator.

For riflescopes and other high end sport optics, it's typically the optical design and lens fabrication process that determine the optical quality of the end product. The optical product manufacturer usually designs the product and sets the lens specifications. In the lens fabrication process, it's primarily surface figure that determines optical performance and cost. Surface figure describes the very small deviations from a true spherical surface on the lens surface. Better optical performance requires a better surface figure which takes more time and care in the polishing process, and therefore raises the cost of the end product.

It's very likely that all the companies that manufacture scopes in the US buy their lenses from the same lens suppliers, who in turn use both on and offshore factories to fabricate the lenses. In many cases, the lenses are fabricated offshore and then cut to a final diameter either on or offshore. Regardless of where the lens is fabricated, surface figure is what matters.

Color Filters. Some photography filters will fit some riflescope objectives. Of the various filter types available, a yellow filter is probably the most useful during daytime shooting. It should be removed if hunting during dusk and dawn, however.

Technical: There are three considerations for a yellow filter. First, the yellow filter cuts off the violet and some of the blue parts of the spectrum, which includes most of the light scattered from particles in the atmosphere. So using the filter increases image contrast during bright daylight. Chromatic aberration and atmospheric turbulence are also worse at these shorter wavelength colors, so the filter is very likely to help reduce image blur as well. This is all good.

At dusk and dawn, however, atmospheric turbulence and scatter of sunlight are less of an issue, so the benefits of using the yellow filter are diminished.

Second, during daytime the color of peak sensitivity of the eye is green, which exactly matches the spectrum of the sun. As the eye adapts to darkness the peak sensitivity shifts to blue. It takes a long time for the eye to fully adapt to darkness, so it may not happen often during legal hunting hours. However, hunting in the woods in a canyon under a moonless, overcast sky, the eye probably has enough time to fully adapt to darkness before legal hunting ends. Likewise, if one were in a spotting position before dawn, then the eye probably would still be dark adapted when legal hunting time starts. In that case the filter would reduce brightness - a lot.

Third, the filter may reduce the contrast of big game animals against vegetation. The animals are various shades of brown, which is a mix of all primary colors, with an extra dose of red. Vegetation is mostly green, with some brown. The yellow filter cuts out some blue, which is part of the animal's fur color and not part of green vegetation. The increase in daytime image contrast discussed above more than compensates for any small loss of contrast for brown fur against green vegetation.

Scope Testing. Sometimes people try to compare scopes by looking at high contrast "resolution" targets (bar targets, dollar bills, etc.) at a long range. Because of the way they are set up, these tests will usually fail to reveal significant differences between scopes, even in side-by-side comparisons. For a high contrast target under bright daytime conditions, and especially over flat terrain, most scopes will appear to have roughly similar resolution and contrast at a given magnification.

This is because illumination and target contrast are high, and turbulence usually limits resolution under these conditions. High contrast targets can be "resolved" with relatively poor quality optics. On axis optical aberrations are easy to correct. Also, when a manufacturer uses inexpensive lenses that have poor “wavefront” quality, it’s the image contrast, rather than resolution, that suffers first. For a well-illuminated, high contrast target, contrast effects are not readily apparent in the image.

A live fire range is one of the worse places to judge the optical quality of a scope. There are a few reasons why, but the main one is that turbulence in the air is strong at most ranges. The image seen when looking through a scope at targets at the shooting range is usually blurred due to this turbulence, not due to the optics in one scope vs another. Any difference in image "quality" between two side-by-side scopes under these conditions is probably due to the scopes having different aperture sizes. It would be much easier to see differences in optical resolution in a side-by-side test if you looked through the scopes at a target across a small valley or canyon on a cool day under an overcast sky.

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