By Cristina Quinn

1
The Camera

• By Cristina Quinn
• March 25, 2004

2
Introduction

• There are two types of film cameras on the
  market Single Lens Reflex (SLR) cameras and
  point and shoot cameras.
• The main difference is the mechanics of the
  viewfinder.
• In point and shoot cameras, the viewfinder is
  simply a window that gives an idea of what is in
  view.
• SLR camera viewfinders allow for the actual
  image, as seen by the film, to be viewed.
• This is accomplished by using a slanted mirror in
  front of the shutter and a prism to invert the
  image to its proper orientation.

• When the shutter button is pressed, the mirror
  flips out of the way so that the image is
  directed onto the film.

3
Light Control

• The amount of light exposed to the film is
  controlled by both how much light passes through
  the lens and how long the film is exposed.
• The iris diaphragm opens and closes to control
  the diameter of the lens.
• The shutter controls the light flow by the amount
  of time it remains open.
• Most SLR cameras have a focal plane shutter.
• The length of time which the shutter remains open
  can also be adjusted to capture fast motion.

The Iris Diaphragm
The focal plane shutter.
4
Image Size

• The farther away the object is from the lens the
  smaller the image will be.
• The shape of the lens also affects the size of
  the image.
• A lens with a rounder shape will curve the light
  more and form an image closer to the lens.
• A flatter shaped lens will not bend the light as
  much and the image will form farther away.
• Increasing the distance between the lens and the
  real image will increase the size of the image.

5
Focus

• The distance of the object from the lens will
  affect the focal length. i.e. The closer the
  object, the farther away the image will form.
• If the image is not formed on the film, the image
  will appear blurry.
• To focus a lens, it is moved either closer or
  farther away from the film surface.

6
Autofocus

• The two types of autofocus systems are active
  and passive
• More expensive cameras with interchangeable
  lenses use the passive system while less
  expensive cameras use an active system.
• Active systems
• These systems project infrared pulses towards the
  subject and measure its distance from the lens by
  either triangulation (angle of reflection), time,
  or the amount of IR light reflected from the
  subject.
• Problems associated with IR sensing
• Other sources of IR light can confuse sensor
  (i.e. candle flame, fire)
• A black surface will absorb the reflecting IR
  beam.
• The IR beam may reflect off an object other than
  the subject of interest (i.e. a window).
• This system can be used regardless of lighting.

7
Passive Autofocus

• Passive autofocus systems use computer analysis
  of the image contrast to determine the distance.
• A charge coupled device (CCD) is often the sensor
  used which contains 100-200 pixels.
• The microprocessor moves the lens and obtains
  information from the CCD in search of the
  distance which has the maximum intensity
  difference between the pixels.
• This system can be used to focus on an object
  behind glass but requires light and some degree
  of detail.

8
Interesting Features

• Camera zoom lenses contain several lenses in a
  single unit. These lenses correct for chromatic
  aberration i.e. different colours of light bend
  differently and therefore do not line up
  correctly. By changing the distances between
  these lenses, the overall magnification can be
  adjusted.
• f-number a pinhole of light from the focal
  length of the lens is projected from the lens as
  a collimated beam. The f-number is equal to the
  focal length of the beam divided by diameter of
  the beam.
• Many cameras have a "red eye reduction" feature.
  Red eye occurs when light reflects off of the
  blood vessels at the back of the retina. Cameras
  which reduce the effects of red eye do so by
  having the flash go off twice. The first flash
  functions to stimulate the pupils to contract.
  The second flash occurs when the picture is
  actually taken.
• Automatic cameras are controlled by a central
  microprocessor rather than the user. It receives
  information from the autofocus system and the
  light meter and then adjusts the lens and
  aperture accordingly.

9
References

• GoldBerg, Norman. Camera Technology The Dark
  Side of the Lens. Academic Press, Inc., San
  Diego, 1992.
• http//entertainment.howstuffworks.com/camera.htm/
  printable..
• http//science.howstuffworks.com/question51.htm.
• http//travel.howstuffworks.com/autofocus.htm
• The Camera. Time Life Books, 1981.