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A photographic lens (or more correctly, objective) is
an optical lens used in conjunction with a camera body
and mechanism to make images of objects either on photographic
film or on other media capable of storing an image chemically
or electronically.
There is no difference in principle between a lens used
for a camera, a telescope, a microscope, or other apparatus,
but the detailed design and construction are different.
A lens may be permanently fixed to a camera, or it may
be interchangeable with lenses of different focal lengths
and other properties.
A practical camera lens will often incorporate an aperture
adjustment mechanism, often an iris diaphragm, to regulate
the amount of light that may pass. A shutter, to regulate
the time during which light may pass, may be incorporated
within the lens assembly, or may be within the camera,
or even, rarely, in front of the lens.
The lens may usually be focused by adjusting the distance
from the lens assembly to the image-forming surface, or
by moving elements within the lens assembly.
The lens elements are made of transparent materials. Glass
is the most widely used material due to its good optical
properties and resistance to scratching. Various plastics,
such as acrylic (or PMMA), the material of Plexiglas,
can also be used. Plastics allow the manufacture of strongly
aspherical lens elements which are difficult or impossible
to manufacture in glass, and which simplify or improve
lens manufacture and performance. Plastics are not used
for the outermost elements of all but the cheapest lenses
as they scratch easily. Moulded plastic lenses have been
used for the cheapest disposable cameras for many years,
and have acquired a bad reputation: manufacturers of quality
optics tend to use euphemisms such as "optical resin".
• Enjoy the View - Use a Binocular
The maximum usable aperture of a lens is usually specified,
as the focal ratio or f-number, the focal length divided
by the actual aperture in the same units. The lower the
number, the more light is admitted through the lens. Practical
lens assemblies may also contain mechanisms to do with
measuring light, to hold the aperture open until the instant
of exposure to allow SLR cameras to focus with a bright
image, etc.
The two main optical paramters of a photographic lens
are the focal length and the maximum aperture. The focal
length determines the angle of view, the size of the image
relative to that of the object, and the perspective; the
maximum aperture limits the brightness of the image and
the fastest shutter speed usable.
Focal lengths are usually specifed in millimeters (mm),
but older lenses marked in centimeter (cm) and inches
are still to be found.
For a given film or sensor size, specifed by the length
of the diagonal, may be classified:
* Normal lens: angle of view of the diagonal about 50°,
the same as the human eye: a focal length approximately
equal to the diagonal produces this angle.
* Wide-angle lens: focal length shorter than normal, and
angle of view wider.
* Long-focus or telephoto lens: focal length longer than
normal, and angle of view narrower. A distinction is sometimes
made between a long-focus lens and a true telephoto lens:
the telephoto lens is designed to be physically shorter
than its focal length.The 35mm film format is so prevalent
that a 90mm lens, for example, is always assumed to be
a moderate telephoto; but for the 7x5cm format it is normal,
while on the large 5x4 inch format it is a wide-angle.
The real difference between lenses of different focal
length is not the image size, but the perspective. You
can take photographs of a person stretching out a hand
with a wideangle, a normal lens, and a telephoto, which
contain exactly the same image size by changing your distance
from the subject. But the perspective will be different.
With the wideangle, the hand will be exaggeratedly large
relative to the head; as the focal length increases, the
emphasis on the outstretched hand decreases. However,
if you take pictures from the same distance, and enlarge
and crop them to contain the same view, the pictures will
be truly identical. A moderate long-focus (telephoto)
lens is often recommended for portraiture because the
flatter perspective is considered to look more realistic.
Some lenses, called zoom lenses, have a focal length which
varies as internal elements are moved, typically by rotating
the barrel or pressing a button which activates an electric
motor.. The lens may zoom from moderate wide-angle, through
normal, to moderate telephoto; or from normal to extreme
telephoto. The zoom range is limited by manufacturing
constraints; the ideal of a lens of large maximum aperture
which will zoom from extreme wideangle to extreme telephoto
is not attainable. Zoom lenses are widely used for small-format
cameras of all types: still and cine cameras with fixed
or interchangeable lenses. Bulk and price limit their
use for larger film sizes.
An extreme wideangle lens of large aperture must be of
very complex construction to correct for optical aberrations,
which are worse at the edge of the field and when the
edge of a large lens is used for image-forming. A long-focus
lens of small aperture can be of very simple construction
to attain comparable image quality; a doublet (with two
elements) will often suffice. Some older cameras were
fitted with "convertible" lenses of normal focal
length; the front element could be unscrewed, leaving
a lens of twice the focal length and angle of view, and
half the aperture. The simpler half-lens was of adequate
quality for the narrow angle of view and small relative
aperture. Obviously the bellows had to extend to twice
the normal length.
Good-quality lenses with maximum aperture no greater than
f/2.8 and fixed, normal, focal length need three (triplet)
or four elements (the trade name "Tessar" derives
from the Greek tessera, meaning "four"). The
widest-range zooms often have fifteen or more. The reflection
of light at each of the many interfaces between different
optical media (air, glass, plastic) seriously degraded
the contrast and color saturation of early lenses, zoom
lenses in particular, especially where the lens was directly
illuminated by a light source. The introduction many years
ago of optical coatings, and advances in coating technology
over the years, have resulted in major improvements, and
modern high-quality zoom lenses give images of quite acceptable
contrast.
Special purpose lenses:
* Macro lenses are designed for good performance at close
distances, e.g., for images of the same size as the object.
* Apochromat lenses have extreme correction for aberrations
of colour.
* Process lenses have extreme correction for aberrations
of geometry .Process and apochromat lenses are normally
of small aperture, and are used for extremely accurate
photographs of static objects.
* Enlarger lenses are made to be used with photographic
enlargers (specialised projectors), rather than cameras.
* Lenses for aerial photography
* Fisheye lenses: extreme wide-angle lenses with an angle
of view of 180 degrees, with very noticeable distortion.
* Stereoscopic lenses, to produce pairs of photographs
which give a 3-dimensional effect when viewed with an
appropriate viewer.
* Soft-focus lenses which give a soft, but not out-of-focus,
image for photographing the vain.
Some lens manufacturers :
* Canon
* Cosina
* Konica Minolta
* Leica
* Nikon
* Pentax
* Tamron
* Tokina
* Schneider
* Sigma Corporation
• Zeiss
See also:
* Lens (optics)
* teleconverter
* Lens hood
* Large format lens
• Monocular
• Binoculars
• Telescopes
• Nikon
8x42 Monarch ATB
• Zhumell
7x50 Marine with Compass and Reticle
• Zhumell
Bring em Near Pirate Spyglass 25x30
• Swarovski
10x42 EL Binoculars
• Binocular
Cases
• New
ATN Night Vision Technology
camra, caemra, camrea, amera, camara
This article is licensed under the GNU
Free Documentation License.
It uses material from the Wikipedia
article "Camera Lens".
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Camera Lens
