****************************************************************************Mathematics 309 - Camera LensesPROPERTIES OF LENSES, OPTICAL GLASSCompositionGlass is a solid, structureless and amorphous. There are two main group classification of optical glass:
Chemicals are combined to produce new glass types. These new glass types are used to benefit other different types of cameras (such as high-speed minature cameras, black/white cameras, etc). PropertiesThe most important optical characteristics of a glass are its refractive index and its degree of dispersion.Refraction is the phenomenon of a light ray that passes from air to glass or from glass to air, and is deflected from its path when it meets the glass surface at an angle. The glasses magnitude depends on two things: the material of the glass and its wavelength. We can see wavelengths as coloured light from (spectrum colours) red, orange, yellow, green, blue, indigo and violet. An example of this is the refraction of light on a raindrop, where we may see a rainbow.
|
where r is the angle or refraction and i is the angle of incidence
The value of n for air is 1.00, water is 1.33, crown glass is 1.46 to 1.72 and flint glass is 1.55 to 1.80.
In a camera, light is transmitted by the aperture. It forms on the screen a circle of light which is the image of the object point. When the distance of the screen from the aperture is increased, the image will become larger as well as the diameter of the circle of light (image point). The size of the aperature depends on the diameter of the circle of light from the image. The light that passes the aperature is scattered or diffracted. So, if the aperature is too small, the image may become less sharp because of the scatter of light at the aperature opening. A sharp image must have a aperture large enough to reduce the effects of diffraction to a minimum.
The diameter of a lens controls the amount of light collected. A smaller lens does not collect as much light than a larger one shown in this picture:
The aperture number determines the angular aperture
A lens may not be distorted if the scale of reproduction (or magnification) is the same for all image points. The scale of
reproduction must be constant at all inclinations of the principal ray within the angle of view
. The ratio is as follows:
For a pin-cushion distortion the scale of reproduction increases from the centre to
the edges. The image with its sides pushed inwards. This occurs when the lens stop is situated behind the lens.
For a barrel distortion the image magnification decreases with increasing
object size. this occurs when the lens stop is in front of the lens.
Distortion can be eliminated by symmetrically arranging two systems consisting of a front and rear component about a
central stop.
ABBERATIONS AND THEIR CORRECTIONS
Optical abberations is an inaccurate image reproduction associated with the lenses or an error of the aberration of
magnification. As a result, you may get blurred image edges, distortion at the margins of the picture area and a lack of
sharpness in the picture. These aberrations can be eliminated by combining several lenses to form a less complex lens
system.
Chromatic abberations
Chromatic abberations occurs when different coloured focal points are arranged behind each other on the optical
axis, with the blue focal point nearest to the lens. This can also be called an under-corrected lens defect.
Sometimes, the refractive index for all optical glasses is greater for blue light rays of short wavelength than for red
rays of long wavelength. As a result the light passing through a converging lens may not meet in a common focus.
Conversely, with diverging lenses the red rays have the greater focal distance than the blue-violet one, which is called
over-correction. What we learn is that every lens has different image distances and focal lengths for light of
every colour (every light colour has its own image as well).
Pin-cushion distortion and Barrel distortion
An image fault, or distortion, can occur if the image produced by an optical system does not strictly correspond in
geometry to the object it is derived from.
TYPES OF LENSES
Normal Camera Lenses
Most standard cameras today have a 35mm lens. 35mm lens refers to the diameter of the lens. The smaller diameter the
lens makes for better photos from far away (wide-angle lenses) and larger diameter of the lens is better for photos
that you are wanting to take from a distance (zoom lenses). The reason why a 35mm is most common camera is because it
is the most versatile lens for everyday photos.
Wide-angle Lenses
Lenses with great angle are known as wide-angle lenses. At the lens the diagonal of all such lenses therefore
subtends more or less the same angle, the angle of view (is between 40 to 60 degrees). If the focal length is short,
the image plan moves nearer to the lens, and a greater angle of view is required to cover the same picture format.
An average wide-angle lens may cover up to 70 to 80 degrees. Some can even go beyond 180 degrees.
A wide-angle lenses may look like the following:
A wide-angle lens is needed wherever you want to take a picture sufficiently far from the subject. For example, if you
want to take a picture of a large building, narrow street, etc, this is a good lens to use because it takes a greater
angle of the view. The scale of reproduction is of course smaller than with a normal (standard) lens. One problem that
occurs with a wide-angle lens is it usually has a large amount of barrel distortion. This occurs if the angle of view
approaches 180 degrees, the size of film will be infinitely large, therefore difficult to overcome.
Zoom Lenses
Using the equation for magnification, the total focal length becomes shorter as the distance between the two components is
reduced. With multi-component systems it is possible to adjust teh focal length by displacing individual lenses. Lenses
with variable focla length have developed which permit the coninuous adjustment of the image scale, preserving a given
distance setting between the subject and the image. This type of lenses was first made for cine cameras and was later
developed for reflex cameras. The simpliest zoom lens can consist of only a single positive and negative lens. The
positive lens would be used to increase/decrease the focal length and the negative lens would help to focus the image.
An example of a more complicated zoom lenses is the Voigtlander Zoomar:
****************************************************************************
Works Cited
By Debbie Loo
****************************************************************************