GLOSSARY OF TECHNICAL OPTICAL TERMS
USED ON THIS WEBSITE
A number of, possibly, unfamiliar words and terms are used on this website.
To try and help you, this short glossary of the technical optical terms used
has been put together.
Please let us know if you think any other terms should be added or explained.
You might also like to read the
grammatical
note.
Achromatic : not suffering from chromatic aberration (q.v.)
capable of reflecting or refracting light without chromatic aberration or
colour distortion.
Biconcave
Lens : a lens that is thinner in the middle than at the edges.
Biconvex
Lens : a lens that is thicker in the middle than at the edges.
Chromatic Aberration : an optical defect caused by a single
lens splitting up the incoming light (as does a prism) into its constituent
colours and spoiling the image as each colour focuses at a different distance
from the screen.
Coma : distortion towards the edge of the image caused by light
approaching the lens at an oblique angle.
Crown Glass : a type of low density glass.
Dioptre : a unit of measurement based on the reciprocal of
the focal length of the lens expressed in metres.
Doublet : a closely spaced pair of lenses an achromatic (q.v.)
lens of two components made of different types of glass (flint and crown
glass : q.v.). They are close together and may be separated either by air
or a thin layer of oil. A triplet has three components instead of a doublets
two.
Field of View : the width of the view measured in angular degrees
that is visible at any one time on the screen. (The field of view of a normal
pair of binoculars is usually about 5° and the moon is 0.5° across
as seen from Earth).
Flint Glass : a type of high density glass.
Focal Length : the distance from lens to the screen when the
image of a distant object is in focus. This governs how large the image is
- the longer the focal length, the larger the image. (Sometimes referred
to as 'Focal Distance').
Focal Ratio : the number produced when the focal length is
divided by the lens diameter and is the ' F' number familiar to photographers.
The smaller the ' F' number is, the brighter the image.
Lens Blanks : a preformed lens supplied to opticians which
are used (by further work) to make prescription spectacles.
Meniscus
Lens : a lens that is convex on one side and concave on the other.
Panning : rotating the optics horizontally to view around the
camera obscura.
Pinhole Camera : an optical device using a small hole to project
an image into a darkened box or room.
Spherical Aberration : an optical distortion inherent in an
optical surface or lens caused by its spherical shape.
Stopped Down : reducing the effective diameter of a lens by
masking its edges either to reduce aberrations or because the edges are damaged.
