Optical lens; Lens (optical)
An optical lens is a single, optically transparent device shaped/designed to allow the transmission and refraction of light to create a specific and controlled optical outcome.
An optical lens creates either:
- A Converging beam – light is focused to a point (between “near” and infinity).
- A Diverging beam – The light is spread out from the lens and does not focus to a point.
Types of optical lens
Lenses come in a broad range of types. However, in general…
- A converging lens is called a convex lens.
- A diverging lens is called a concave lens.
More complex configurations of these two different types can be seen below…
Refraction – changing the shape of the image
Lenses work by bending rays of light to meet at a focus point. An optical lens is able to bend light by a phenomenon called refraction. When a light wave changes from one medium to another (eg. from air to glass) the light wave will change direction at the contact surface. When the rays of light bend by refraction this changes the characteristics of the image created by the lens. The apparent size and shape of the image can be changed in this way.
Lens diagrams show lines (rays) following the possible scenarios for the way light passes though the lens. Each shows the refraction route of that ray. The diagram also shows how the focal point is derived.
The optical lens has two focus points. The principle focus point is where the light focuses to a point for a converging lens. In the case of a concave (diverging) lens the principle focus point is a virtual (or back-projected) point shown on the diagram before the lens (see the diverging lens diagram further on). This is shown as a dotted line since it is not an actual ray path.
In the case of a converging lens (convex) the rays all end up at the ‘real’ image in the same place. The total sum of the light entering the lens ends up projected onto the image plane. For a diverging lens (concave) the rays are refracted out wider than the incoming rays and never focus to a point.
A simple optical lens
A simple optical lens is one optical element. An example of a simple lens is a magnifying glass . All the different types of optical lenses can act individually as a simple lens. A basic lens diagram showing the main features of a generalised convex simple lens is shown below…
In the case of a diverging optical lens the light is not focussed to a point, but separated apart. The principle rays would be refracted to a wider area than the incoming light. The diagram below shows a lens diagram for a diverging optical lens…
A simple lens may also focus various forms of electromagnetic radiation like ultra-violet light and infra-red light and X-rays. These cannot normally be seen by humans but conform to the same physical laws as light. Ultra-violet light and infra-red light can also be detected by a digital image sensor and captured in an image (shown by colours or tones). However, these wavelengths of light are normally filtered out of consumer DSLRs. In order to image them using a DSLR special filters or adaptations are required.
Compound optical lenses
A compound optical lens is a more complex assembly of element groups or single elements. The assembly of elements work in concert using the same optical axis to create an image. The image may be varied/enlarged/reduced by moving the individual elements relative to one another. An example of a compound lens is a photographic lens as shown in this diagram below…
Construction and materials
Lenses are normally, but not necessarily, transparent. Some may be translucent for very specialist use, or transparent to particular forms of electromagnetic radiation but opaque to vision.
Lenses for optical purposes are normally, but not always made of glass. Very cheap disposable cameras may have plastic lenses. Magnifying optical lenses may also be of plastic. Lenses for non-visible forms of electromagnetic radiation may be made out of a variety of non-glass substances. They may also be optically opaque.
Optical glass is a very highly refined material. The basic silica-based glass used in windows and drinking glasses for centuries is optically sufficient for those purposes but is not of high optical quality. Glass is a class of crystalline substance that can derive from a wide range of chemical substances. While most glass has a silica base the other chemicals in glass is usually what gives the glass its optical superiority. The borosilicates (silica and boric acid) are commonly used in optical glass. Other chemicals are also used including: zinc oxide, phosphorus pentoxide, barium oxide and fluorite. Some manufacturers use other chemicals. Most manufacturers have specific mixes they have experimented with over a period of many years for developing their optical lens range.
The different materials that optical glass is made from have different refraction characteristics. So more than one type of glass could be used in one photographic lens. For example, some lenses cause chromatic aberrations (see below). However, a paired lens of a different material can be used to counteract that aberration.
In general optical lens elements with higher quality optical glass have ultra-low light dispersion characteristics. The glass itself tends to be harder than lower quality glass too. These glasses are made from expensive chemicals and the harder they are the more firing (heat treatment) and grinding (shaping and polishing) is involved in creating the final lens. For these reasons high quality glass optical lenses are expensive. The cost goes up significantly for higher quality grades.
Optical lens aberrations
Combining lenses to correct aberrations is one of the reasons for creating compound lenses. Essentially, an aberration on one lens element is off-set by another lens element to counteract it.
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