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LENSES |
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What is a lens? |
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©thangavelu |
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Light must be controlled if our eyes or our cameras are to form images of
objects. You can’t simply place a square of sensitized film in front of a
subject and hope that an image of it will appear on film/sensor. The rays
reflecting out of the subject would hit the film in a random jumble,
resulting not in a picture but in a uniform exposure over the entire surface
of the film.
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For
simplicity’s sake, the drawing shows only a few rays coming from only two
points on the man, his pipe and his coat tip, but their random distribution
over the entire film makes it clear that they are not going to produce a
useful image. The rays from
the pipe, for example, will hit the film all over its surface, never
creating in any one place an image of the pipe. What is needed is some sort
of light-control device in front of the film that will select and aim the
rays, putting the pipe rays where they belong and the coat rays where they
belong, resulting in a clear picture. All photographic lenses do the same
basic job: they collect light rays coming from a scene in front of the
camera and project them as images onto a piece of film at the back. This
chapter explains how this happens and how you can use lens focal length
(which controls the magnification of a scene), lens focus (which controls
the sharpest part of an image), lens aperture, and subject distance to make
the kinds of pictures you want. |
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Why do we need a lens?
Although all the light rays reflected from an object cannot produce
an image on a flat surface, a selection of rays can. Suppose there is a
barrier with a small hole in it, like that in the drawing. All but few rays
from each point are deflected by the barrier. Those few rays that do get
through, traveling in a straight lines from object to film, can make an
image.
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For
example, the few rays from the man’s pipe that get through the hole all fall
on a certain spot near the bottom of the film. Only that one spot on the
film registers an image of a pipe. Similarly, rays from the coat, the shoes,
the ear, the hat brim- from every point on the man- travel to
other specific points on the film. Together they form a complete
image, but one that is inverted. Everything that was at the top of the man
appears at the bottom of his image on the film and everything at the bottom
appears at the top. Similarly, left becomes right and right becomes left. |
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The image-making ability of the pin-hole was first put to use- long before
the invention of photography – as part of the camera
obscura, a darkened room whose only light source is a small hole
in one wall. Light rays coming through the hole form on the opposite wall an
image of the scene outside. The camera obscura is, in fact, a room-size
primitive camera. Shrink the room down to a shoebox size, reduce the hole to
1/50 inch ( 0.5mm) in diameter, place a piece of film at the end opposite
the hole, and it will make a recognizable picture; the photograph was made
with such a camera.
The trouble with this pinhole camera is its
tiny opening, which admits so little light that very long exposures are
needed to register an image on the film. If the hole is enlarged
appreciably the picture it makes becomes much less sharp, like the
photograph , on this page. Why this happens is explained in the two
drawings.
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To take this photograph of a fence and barn, photographer
Ansel Adams replaced the lens of an ordinary camera with a thin
metal disk pierced by a pinhole with 1/50th inch
diameter. The film was exposed for 6 seconds. The way the pinhole produced
an image is illustrated in the diagram. |
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Only a few rays of light from each point on the subject can get through the
tiny opening, and theses strike the film in such tight clusters that bluring
is reduced to a minimum. The result is a soft but acceptably clear
photograph. |
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For a second photograph of the same scene, Adams increased the size of the
opening to 1/8 inch, which meant reducing the exposure time 1/5 sec. The
result is an extremely out-of focus picture. The larger hole permits a great
number of rays from each point on the subject to enter the camera. Theses
rays spread before reaching the film and are recorded as large circles.
Because of their size, theses circles tend to run into one another, creating
an unclear image. |
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The image in the drawing is actually composed of a great many tiny circles
known as circles of confusion. This is because
the hole, small though it is, actually admits a cluster of
light rays, coming at slightly different angles, these rays continue
through the hole in different directions. They fan out, and when they hit
the film they cover a small circular area on it. If the hole in the barrier
is made larger, a larger cluster of light rays will get through to the film
and cover a wider circle. The larger the circles are, the more they will
overlap their neighbors and the less clear the picture will be. |
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To get sharp pictures, the circles of confusion should be as small as
possible. But the only way to achieve that with a pinhole camera is to use
a very small opening, which admits little light and requires long exposure
times. To admit more light and to make a sharper picture, a different
method of image formation is needed. That is what a lens provides.
Photographed This time through a convex lens the image is as good as- or
better than the one shot with the pinhole camera. Its exposure time,
instead of being 6 sec. Was only 1/100 sec. This is because the lens is
much bigger than a pinhole and thus admits far more light. The lens
handles all this light by collecting many rays reflected from a single
point and redirecting them to a coresponding single point on the film or
focal plane.
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Four centuries ago a Venetian noble man named Daniele Barbaro found a new
and better way to convert light rays into images in the camera obscura of
his day. He enlarged the opening of his room-size camera obscura and
fitted into it a convex lens taken from the spectacles of a
farsighted man. To his delight, the lens projected images superior to
those previously supplied by the simple pinhole opening. Several hundred
years later, with the advent of photography, this discovery proved even
more significant. For not only can a camera with a lens provide sharper
images than a pinhole camera, but it admits enough light to take a picture
in a fraction of second.Most modern photographic lenses are based on the
convex lens, the type used by Barbaro. Thicker in the middle than the
edges, the convex lens can collect a large number of light rays from a
single point on an object and refract, or bend, them toward each other so
that they converge at a single point. This point of convergence, the
focal point, falls on a surface called the
focal plane. |
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In a camera, a strip of film is stretched across the focal plane, which is
now the film plane. The film records an image
formed by light rays from an infinite number of points on the object,
focused by the lens into an infinite number of tiny circles of confusion.
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Classification of lenses
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