Canon Technology | Canon Science Lab | Lenses

The word "lens" owes its origin to the Latin word for lentils, the tiny beans that have from ancient times been an important ingredient in the cuisine of the Mediterranean region. The convex shape of lentils resulted in their Latin name being coined for glass possessing the same shape.

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Because of the way in which lenses refract light that strikes them, they are used to concentrate or disperse light. Light entering a lens can be altered in many different ways according, for example, to the composition, size, thickness, curvature and combination of the lens used. Many different kinds of lenses are manufactured for use in such devices as cameras, telescopes, microscopes and eyeglasses. Copying machines, image scanners, optical fiber transponders and cutting-edge semiconductor production equipment are other more recent devices in which the ability of lenses to diffuse or condense light is put to use.

Lenses may be divided broadly into two main types: convex and concave. Lenses that are thicker at their centers than at their edges are convex, while those that are thicker around their edges are concave. A light beam passing through a convex lens is focused by the lens on a point on the other side of the lens. This point is called the focal point. In the case of concave lenses, which diverge rather than condense light beams, the focal point lies in front of the lens, and is the point on the axis of the incoming light from which the spread light beam through the lens appears to originate.

Concave lenses are used in eyeglasses that correct nearsightedness. Because the distance between the eye's lens and retina in nearsighted people is longer than it should be, such people are unable to make out distant objects clearly. Placing concave lenses in front of a nearsighted eye reduces the refraction of light and lengthens the focal length so that the image is formed on the retina.

Most optical devices make use of not just one lens, but of a combination of convex and concave lenses. For example, combining a single convex lens with a single concave lens enables distant objects to be seen in more detail. This is because the light condensed by the convex lens is once more refracted into parallel light by the concave lens. This arrangement made possible the Galilean telescope, named after its 17th century inventor, Galileo.
Adding a second convex lens to this combination produces a simple telephoto lens, with the front convex and concave lens serving to magnify the image, while the rear convex lens condenses it.

The focused image through a single convex lens is actually very slightly distorted or blurred in a phenomenon known as lens aberration. The reason why camera and microscope lenses combine so many lens elements is to correct this aberration to obtain sharp and faithful images.
One common lens aberration is chromatic aberration. Ordinary light is a mixture of light of many different colors, i.e. wavelengths. Because the refractive index of glass to light differs according to its color or wavelength, the position in which the image is formed differs according to color, creating a blurring of colors. This chromatic aberration can be canceled out by combining convex and concave lenses of different refractive indices.

Special lenses, known as fluorite lenses, and boasting very low dispersion of light, have been developed to resolve the issue of chromatic aberration. Fluorite is actually calcium fluoride (CaF2), crystals of which exist naturally. Towards the end of the s, Canon developed the technology for artificially creating fluorite crystals, and in the latter half of the s we achieved the first UD (Ultra Low Dispersion) lenses incorporating low-dispersion optical glass. In the s, we further improved this technology to create Super UD lenses. A mixture of fluorite, UD and Super UD elements are used in today's EF series telephoto lenses.

There are four other key types of aberration: spherical and coma aberration, astigmatism, curvature of field, and distortion. Together with chromatic aberration, these phenomena make up what are known as Seidel's five aberrations. Spherical aberration refers to the blurring that occurs as a result of light passing through the periphery of the lens converging at a point closer to the lens than light passing through the center. Spherical aberration is unavoidable in a single spherical lens, and so aspherical lenses, whose curvature is slightly modified towards the periphery, were developed to reduce it.

Because light is a wave, when it passes through a small hole, it is diffracted outwards towards shadow areas. This phenomenon can be used to advantage to control the direction of light by making concentric sawtooth-shaped grooves in the surface of a lens. Such lenses are known as diffractive optical elements. These elements are ideal for the small and light lenses that focus the laser beams used in CD and DVD players. Because the lasers used in electronic devices produce light of a single wavelength, a single-layer diffractive optical element is sufficient to achieve accurate light condensation.

Chromatic aberration caused by diffraction on the one hand, and refraction on the other arise in completely opposite ways. Skillful exploitation of this fact enables the creation of small and light telephoto lenses.
Unlike pickup lenses for CD and DVD players, incorporating simple diffractive optical elements into SLR camera lenses results in the generation of stray light. However this problem can be resolved by using laminated diffractive optical elements, in which two diffractive optical elements are aligned within a precision of a few micrometers.

The larger the mirror of an astronomical telescope, the greater will be the telescope's ability to collect light. The primary mirror of the Subaru telescope, built by Japan's National Astronomical Observatory, has a diameter of 8.2 m, making Subaru the world's largest optical telescope, and one that boasts very high resolution, with a diffraction limit of only 0.23 arc seconds. This is good enough resolution to be able to make out a small coin placed on the tip of Mt. Fuji from as far away as Tokyo. Moreover, the Subaru telescope is about 600 million times more sensitive to light than the human eye. Even the largest telescopes until Subaru were unable to observe stars more than about one billion light years away, but Subaru can pick up light from galaxies lying 15 billion light years away. Light from 15 billion light years away and beyond is, in fact, thought to be light produced by the "big bang" that supposedly gave birth to the universe.

Subaru's primary focus camera boasts a very wide field of view of 30 minutes, which is equivalent to the diameter of the full moon as seen from earth, enabling Subaru to make not only very precise, but also speedy observations of the heavens. The only telescope in the world equipped with a glass primary mirror of 8 m in diameter, Subaru is a powerful aid to research on the birth of galaxies and the structure of the universe. Previously, structural considerations prevented heavy optical systems from being placed on top of the primary focus of large reflecting telescopes. This problem was overcome by the development of a smaller and lighter prime focus corrector lens optical system, comprising seven large lens elements in five groups.

In recent years, the transition from glass to plastic lenses has revolutionized the way we see.

In the past, eyeglass lenses were made of glass. However, in recent years, lenses have transitioned to a durable plastic, that is lighter in weight, and less prone to breaking— providing a more comfortable experience.

What is the difference between a lens for farsightedness and a lens for nearsightedness?

If you are farsighted, your lenses will be thicker in the center and thinner on the periphery. A farsighted prescription is written with a “plus” sign, such as +2.00 diopters (D).

If you are nearsighted, your lenses will be thinner at the center and thicker on the periphery. A nearsighted prescription is written with a “minus” sign, such as -2.00 diopters (D).

Different types of lenses

Single vision lenses are the most basic, and least expensive type of lenses. These lenses are designed for correcting vision for either near or distant clarity.

Bifocal lenses contain two optical powers to accommodate clear vision for both near and far. The lens is divided into two segments, the top of the lens contains the distance vision prescription, while the bottom of the lens contains the near vision prescription. Some bifocals contain a bisecting horizontal line between the two lens powers.

Trifocal lenses contain three optical powers— distance, intermediate and near vision, with two segmenting lines to delineate the powers. The intermediate segment is located above the segment for near vision and is helpful for viewing objects at arm’s length, such as a computer, or car dashboard.

Progressive lenses, also called multifocal lenses are similar to bifocals and trifocals, but contain multiple lens powers to provide vision at all distances— close up, intermediate, distance, and any other lens power necessary for vision clarity. Multifocals gradually blend the lens powers together without a bisecting line, making them more attractive than bifocals. The downside of multifocals is that the clear zone for each part of the lens may be limited.

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Polycarbonate and trivex lenses are impact-resistant, thin, and lightweight. These lenses are ideal for playing sports, as they are less prone to damage. They are also designed with built-in UV protection.

High-index plastic lenses are thinner, lighter, and more comfortable than regular lenses. These lenses are ideal for those with higher prescriptions, specifically those with strong farsightedness.

Aspheric lenses contain various degrees of curvature from the center of the lens to its periphery. A flatter curve allows for thinner and flatter lenses, and reduces eye magnification for farsighted prescriptions. Aspheric lenses may also improve the clarity of your peripheral vision.

Computer lenses filter out blue light— reducing eye strain, fatigue, and headaches— and protecting your overall ocular health. Computer glasses may be purchased without a prescription. If you already wear prescription glasses, your lenses can be treated with a blue light-blocking coating.

Photochromic lenses also known as transition lenses, are clear indoors and darken when exposed to sunlight. These lenses eliminate the need for sunglasses and are more convenient for those who wear glasses full time to protect their eyes from UV radiation. Photochromic lenses are technically a treatment that can be added to your prescription lenses of any type.

Polarized sunglasses reduce glare from surfaces such as water and snow.  They are an ideal choice for driving and playing sports.

Schedule an eye exam with an eye doctor to find out which lenses you need for clear and comfortable vision.

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Why are high index lenses so popular?

High-index lenses are recommended if you have a high optical prescription for nearsightedness, farsightedness, or astigmatism. Lenses that contain a higher prescription, are generally quite thick and heavy. This is because the lens needs to be a certain thickness to accurately bend light to correct the refractive error.

While both regular and high index lenses function in the same way, high index lenses are designed to bend light in the most effective way for more natural vision, and to eliminate the need for thick lenses.

High index lenses are thinner and lighter than regular lenses. These lenses are recommended for all types of refractive errors, but are ideal for those with strong farsightedness prescriptions— a thicker lens for this type of prescription generally produces a stronger magnification of the eyes. With the aspheric design of high index lenses, the center of the lens can be made thinner and create a flatter curvature— reducing the magnification of the eyes.

The following types of lenses are available in high-index materials: 

• Aspheric
• Progressive
• Polycarbonate
• Photochromic
• Polarized

High index lenses are generally scratch resistant and can fit into almost any frame. They can also be treated with special coatings to facilitate improved vision. Many people opt for the following coatings:

• Anti-scratch coatings increase lens durability.
• Anti-reflective coatings prevent light from reflecting off the lenses, glare, and halos around lights.
• UV-protection coatings protect your eyes from the sun’s harmful rays.

Why are high index lenses thinner than regular plastic lenses?

High-index lenses are thinner than regular plastic lenses because they have a higher refractive index than plastic. Plastic lenses contain a refractive index of 1.50, while high-index lenses can range from 1.53 to 1.74.

The higher the index, the thinner the lens— a lens with an index of 1.74 could be up to 50 percent thinner than a regular lens with an index of 1.5, with the same prescription.

The image below compares different types of hi-index lenses:

Which type of lens is right for me?

When ordering new glasses, be sure to bring a recent optical prescription from your eye doctor. Your prescription will help to determine the most appropriate choice of lenses for your specific needs.

That being said, it is always a good idea to do some research before purchasing a new pair of glasses to better understand the type of glasses you will be wearing and ensure optimal satisfaction.

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