Changing the curvature of one surface. A lens with a higher angle of curvature will have a shorter focal length because the light waves are refracted at a greater angle with respect to the optical axis of the lens. As the refractive index increases or becomes more curved, the focal length of the lens decreases. Coating the surface of the lens will only change the properties of the surface not the focal length and changing the diameter while keeping the curvature of the surfaces of the lens the same, does not impact the angle of curvature or how light waves are refracted.

Minus lenses correct for near-sightedness or myopia which means a person can see objects close but cannot see far away. Lenses used to correct this are flatter on the front surface and more curved on the backside making the thinnest point in the center of the lens. A lens which has two spherical curved surfaces, concave on one side and convex on the other side is called a meniscus lens. When both surfaces of the lens are concave, therefore having the same focal length, it is called an equiconcave lens. Flat concave lenses have a flat surface on one side and a spherical surface on the other. Equiconvex, flat convex and meniscus are types of plus lenses. A meniscus lens can be either a plus or minus lens however, concave refers to minus lenses while, convex refers to plus lenses.

A meniscus lens has one convex and one concave surface. A negative meniscus is thinner at the center than at the edges and a positive meniscus lens is steeper on the convex side making it thicker at the center and thinner on the edges. Meniscus lenses are used for vision correction in both myopic and hyperopic eyes. Biconcave lenses have two concave surfaces and biconvex have two convex surfaces. A plano-convex lens has one flat surface and one convex surface.

False - The axis of a lens notates the position of the astigmatism power, not the amount of power. The sphere denotes the power of the lens required and the cylinder is the amount of power needed to correct for astigmatism.  

Refraction refers to how light slows and bends when passing from air, a light-density medium into transparent glass or plastic, a high-density medium. When light passes from air through a denser medium such as a lens, the light will bend toward the normal because of the changing velocity at the surface between the two media. Diffusion refers to light being scattered when passing through an irregular surface. Reflection is when light is bounced off a smooth surface, the light will reflect at the same angle as it does when it hits the surface. Dispersion is when white light is split into its component colors: red, orange, yellow, green, blue, and violet, when refracted.

The amount of light entering the eye is controlled by the muscles of the iris. These muscles control the size of the pupil, making it larger (dilated) to allow more light in or smaller (constricted) to restrict the amount of light entering. Under low light conditions, the iris opens making the pupil larger and they constrict in bright light conditions. The crystalline lens is located posterior to the iris and is made up of mostly proteins producing one third of the eye's natural power and focuses light into an image on the retina. In the center of the posterior portion of the retina is a small, flat spot called the fovea, which is responsible for high-acuity vision.

A phoria is defined as a misalignment of the eyes, appearing only when binocular viewing is broken. It is only apparent some of the time and occurs when the eyes are fatigued, and the person cannot compensate for the deviation. Unlike a phoria, a tropia is when the eye never straightens out and is constantly turned in, out, up, or down. Exophoria occurs when the eyes drift outward but is temporary, usually caused when the eyes are tired or stressed, or during close vision tasks. Exotropia is also an outward turning of the eyes; however, it is more frequent and noticeable, even when both eyes are open and trying to work together. Exophoria occurs when the eyes drift outward but is temporary, usually caused when the eyes are tired or stressed, or during close vision tasks. Exotropia is also an outward turning of the eyes, however, it is more frequent and noticeable, even when both eyes are open and trying to work together.

Emmetropia is the medical term for naturally occurring vision. An emmetropic eye does not require vision correction because the light entering the eye focuses directly on the retina. Hyperopia (or farsightedness) occurs when the light focuses behind the retina and Myopia (or nearsightedness) is when the light focuses in front of the retina.

A multifocal height is measured from the lower edge of the lens to the center of the pupil. It is always important to adjust the frame to the person before measuring to allow for accurate placement of the multifocal. Taking this measurement from the lower edge of the frame will add extra millimeters to the measurement and cause the multifocal to be placed too high. Measuring to the lower edge of the frame or lens will most likely cause the multifocal to be set too low. 

When titanium is exposed to air and moisture, it immediately forms a strong protective oxide film making it 100% corrosion resistant. Another advantage of a titanium frame is that the metal is hypoallergenic and will not cause an allergic reaction for those with sensitive skin or have a nickel allergy. The rigidity of the metal allows the frame to hold its shape making it more difficult, not easier to adjust for the dispenser. Memory metal frames, or nickel titanium alloys are not a pure titanium frame; they will return to its original shape after bending and are made up of 50% nickel and 50% titanium.

Retroscopic tilt is used to describe the adjustment made to a frame which brings the bottom edge of the frame front outward from the face. This adjustment is made to prevent a frame from resting on a person's cheek. Pantoscopic tilt will maximize the amount of the bridge surface that rests on the nose and will also raise the ocular center of the lens 1mm for every 2 degrees of tilt. Conversely, retroscopic tilt will lower the ocular center. All frames should have some pantoscopic tilt to optimize vision by balancing the vertex in the 90-degree meridian. Periscopic, as in periscopic vision, refers to having distinct vision all around and not only in a distinct or direct line. Retractive is an adjective that describes something that withdraws.

The absence of the crystalline lens is called aphakia. This condition can either be from birth or caused by injury. The crystalline lens is most often removed by an Ophthalmologist during cataract surgery and an artificial lens is put in its place to correct vision. Accommodation refers to the eyes' diminished ability to focus between distances. The presence of a cataract or a refractive error may result in the removal of the crystalline lens during cataract surgery, however, it is not the definition of aphakia. 

The cornea is the clear outer layer of the eye with the thickest part in the middle layer made up of water and a protein called collagen. The collagen helps keep the cornea's regular round shape and makes it flexible and strong. When either the front surface of the eye, the cornea, or the lens inside the eye have mismatched or misshapen curves, astigmatism occurs causing blurred vision at all distances. When the cornea is cloudy, it is usually a sign of scarring, infection, an abrasion or swelling of the eye.

Most of the refraction is at the cornea-air surface of the eye. The rays pass from the cornea into the aqueous humor (the watery fluid filling the space in front of the eyeball between the lens and the cornea), which has a refractive index of approximately 1.336, making the refractive index of the cornea about 1.37.

Metal frames include materials such as Monel which is a mixture of a broad range of metals and is the most used material to manufacture eyeglass frames. Other metals include titanium and beta-titanium which offer light weight and durable options. Beryllium is a steel-gray metal that is light weight, very strong and flexible and is available in a variety of colors. Zyl is a plastic material that is very cost-effective and extremely light weight. Nylon is a material that is used primarily for sport or performance frames due to its resistance to hot and cold, and are more flexible, yet rigid enough to hold its shape.

Spreading the pads further apart will increase the space between them and in turn will lower the frame front. Bending the pad arms down and bringing the pads closer together will raise the frame front. Adding pantoscopic tilt which brings the bottom of the frame edge towards the cheeks, will raise the frame front and the ocular center of the lenses.

Cylinder power indicates the amount of lens power needed to correct for astigmatism and must include an axis between 1 and 180 indicating the orientation of the astigmatism. Minus and plus powers are the powers needed to correct for near or farsightedness and are called the sphere power. Prism correction is used in eyeglasses to correct for double vision allowing the person to align two images so only one image is seen. Prism must include a direction, either in or out, or up or down depending on orientation of the doubled image. 

Distometer. Vertex distance is the distance between the front surface of the cornea and the backside of a corrective lens mounted in a frame to be worn by the patient. Measuring the vertex distance with a distometer for prescriptions greater than 4 diopters ensures the accuracy of the necessary power. When plus lenses are moved toward the eye, perceived power is lost; when moved away from the eye, perceived power is gained. For minus lenses, it is the opposite: when lenses move toward the eye, perceived power is gained and when they move away from the eye, perceived power is lost. A lensometer is an instrument used in dispensaries to determine the diopter powers of a lens. A vertometer also measures the diopter power of a lens but is more precise and is used in laboratories to locate and mark the optical center of a lens for edging and mounting. A polariscope is used during the lens inspection process to detect stresses or defects in different types of lens materials.

Crown Glass has a low index of refraction of 1.52 and an Abbe value of 59, which means that light travels faster through the material, making crown glass the leading choice for the sharpest optics. Higher Abbe numbers mean less dispersion, higher visual quality and reduced chromatic aberration.  Crown Glass is also the most scratch-resistant material. Lenses with a higher refractive index are denser making them thinner and more attractive but they have more aberrations, decreasing the visual quality of the lens.

True - A polariscope used in optical laboratories detects internal lens stress to ensure the quality of the product. It is made up of a light source and two crossed polarized lenses to detect any imperfections in a lens caused by manufacturing.

False - Mono meaning one, makes this confusing, however a monocular pupillary distance means each eye is assessed separately, resulting in two numbers, one for each eye. When a binocular pupillary distance is taken, the PD for each eye is added together, resulting in one number.

Upper Lid. Ptosis occurs when the upper eyelid is not functioning properly or has grown weak and struggles to open and close on its own. A Ptosis crutch is installed onto the pair of glasses aiding the lid to function properly without surgery. The drooping lid is supported by the crutch which is a bar attached to the inside of the eyewire frame. A fold above the eye is created by the crutch raising it above the pupil.

Polarized lenses reduce the glare of surfaces, such as pavement, snow or water with a chemical filter that absorbs the incoming horizontal light and allows only vertical light to pass through the lens. This is often confused with Anti-Reflective lenses which reduce reflections on the surfaces of the coated lens and does not reduce glare on external surfaces. Dark tint and Photochromic lenses will allow less light to pass through the lens making it easier to see in bright sunlight however, neither will reduce the glare on surfaces.

CR-39 (Columbia Resin #39) is the material used to manufacture the most basic lens material available and is often offered with no added features. Cellulose acetate is a hypoallergenic plant-based plastic used in frame manufacturing. Conventional hard contact lenses are made from a material called polymethyl methacrylate or PMMA.
 

The American National Standards Institute (ANSI) is a private agency that oversees the standards for products, services, processes, and personnel in the U.S. All optical lenses must abide by the standards set in place by ANSI and must fall within the tolerances for sphere, cylinder, and axis powers.  ASTM is the international organization that overseas global standards. ACSII stands for American Standard Code for Information Interchange and has nothing to do with optical standards.