Photographic Lenses

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Photographic Lenses

• How they work
• How we use them
• Udo von Mulert - LPSNZ

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Lets start at the beginning

• The Pinhole Camera Camera Obscura

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Behaviour of Light

• Light travels in a straight direction (Well
  mostly)
• From any point light travels in all directions

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Lets start at the beginning

• The Pinhole Camera Camera Obscura

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Light in Glass

• When Light enters a medium of different density
  it changes direction

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Light redirected by a prism

• Of importance is the angle at which light
  encounters the density change

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Double the trouble

• Two prism end to end

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Lens and Focus

• With glass in our Camera we introduce focus

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Lens and Focus

• Another look

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A Modern Lens

• Light pass through lens

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Lens and Aperture

• Next thing we need is our pinhole back

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A Modern Lens

• Light pass through lens with small Aperture

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Lens and Aperture

• With a small aperture hole less stray light is
  allowed to enter the camera
• The image is sharper and has a greater depth of
  field

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Type of lenses

• From wide to long an overview
• Focal length and field of view

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Wide Angle Lens
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Mirror Tele Lens
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Focal Length

• Focal length is the distance behind the lens
  where the sharp image is formed when set to
  infinity.

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The most Important DOFDepth of Field

• Only objects at the focus point are at maximum
  sharpness
• The zone between the closest and furthest points
  of acceptably sharp focus is called DOF
• Three factors determine depth of field
• 1) Focal length of the lens
• 2) Aperture setting (f-number)
• 3) Camera to subject distance

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Depth of Field Chart
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Aperture Wide Open f1.8
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Aperture at f8
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Aperture at f16
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Depth of FieldPoints of Interest

• DOF increases with shorter focal length
• DOF increases with higher f-stop number
• DOF increases as the distance between camera and
  subject increases

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Depth of FieldPoints of Interest

• DOF does not begin and end abruptly but changes
  gradually from blur to acceptable sharpness
• Also what is and what is not acceptable is highly
  subjective

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Some Thoughts on Lenses

• Lenses (fixed focal or zoom) are optimised.
• Most Lenses are sharper at f8 than at f2
• Very small aperture f16 and more can result in
  some diffraction causing a fall off in definition

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Special Lenses

• Macro Lenses
• Perspective Correction Lenses
• Medical lenses
• Soft Focus Lenses

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Lens Attachments

• Extension Rings used to change the ability of a
  lens to a closer focus range
• Tele Converterextents the focal range of a lens
  typical 1.5 to 2 times
• Close-up Lens Attachmentadded to the front of
  the lens like a filter
• FiltersPolariser, Neutral Density, Graduated
  Colour or Neutral

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A New Playing Field

• Digital Cameras have a much smaller image area
  than 35mm film. This increases the apparent
  focal lens of each lens. Normally the increase is
  about 1.5 times. A 50mm lens becomes a 75mm and
  so on.
• Think of it as if the image has already been
  cropped in the camera.
• We see in the viewfinder the image as we take it.

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Digital Point and Shoot

• Most digital cameras are not SLR. They can have
  zoom lenses with up 12 times zoom. Often these
  lenses have limitation varying from model to
  model
• They have very good close up ability

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Image Stabilasation

• A new development on the marked!

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Thank you