An Introduction to Digital Photography Session 2

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An Introduction to Digital Photography Session 2

• By Paul Emecz

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How a digital camera works

• Lens
• Aperture
• Shutter
• Image sensor

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Lens

• Brings light from the scene into focus to expose
  an image
• This camera has a
  10x zoom lens, from
  38mm (wide) to 380mm
  (tele)
• This camera has a 37mm
  fixed lens

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Aperture

• A hole that can be made smaller or larger to
  control the amount of light entering the camera
• This also controls depth of field

Shallow depth of field makes part of an image
stand out
Great depth of field keeps everything sharp
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Shutter

• Can be opened and closed at different speeds to
  control the amount of time that light enters
• For moving objects, use a faster shutter speed to
  avoid blurring
• Playing with aperture and shutter speeds can give
  special effects
• Shutter speed controls the length of time a
  subject is exposed to light aperture controls
  the brightness of the light

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Image Sensor

• The image sensor in digital photography is a
  charge coupled device (CCD)

• A matrix of hundreds of thousands of photocells
  creates a digital image.

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Exposure
When you press the shutter release button of a
digital camera, a metering cell measures the
light coming through the lens and sets the
aperture and shutter speed for the correct
exposure. Each pixel on the image sensor records
the brightness of the light that falls on it.
When the shutter closes to end the exposure,
the charge from each pixel is measured and
converted into a digital number. The series of
numbers can then be used to reconstruct the image
by setting the colour and brightness of matching
pixels.
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Colour

• An individual photocell can only measure black
  and white the digital camera uses colour
  filters to work out what colour images are.

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What is a Digital Photograph?

• Vector Images
• These are drawn from
• scratch following a set
• of simple instructions.
• Raster Images
• These are images made
• up of tiny squares of
• different colours, created
• by cameras and scanners.

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Vector Images

• Clip-art are usually vector images. The picture
  stays the same size even when you enlarge it,
  because it is only a set of instructions to draw
  the picture.

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Raster Images

• Raster images are made up of pixels (picture
  elements)

• Each pixel has its own unique location, and is a
  specific colour.

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Colour

• Hue the colour of the light (its wavelength)
• Saturation the intensity of the waves
• Brightness the amount of light reflected

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Pixels

• To draw the vector graphic, you need a few
  hundred instructions where to draw each line,
  what colour to make each section of the picture
  etc. Vector images have a cartoon quality,
  looking simple and clear.
• Drawing a raster image requires thousands of
  pieces of information, which makes raster image
  files much larger in size than vector image files
  of the same picture.

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Spatial and brightness resolution
Spatial resolution is about how small the squares
are and therefore how many of them you have.
In essence, the more squares you have, the bigger
the file size and the better the picture.
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Brightness resolution
1-bit pictures have only one piece of information
black or white. Grey is an effect created by
grouping black and white
8-bit pictures have 256 (28) shades of grey.
12-bit pictures provide over 4000 shades of
brightness
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24-bit colour
A 24-bit image has three 8-bit counterparts for
blue, green and red
This gives over 16 million colours!
Each pixel in a 24-bit image has one of 256
values for blue, green and red.
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Image size

• With all that information to store, digital
  cameras produce large images! When choosing
  settings on your camera, think about how you will
  be using your photo
• Emails smaller file sizes are more popular, but
  will they be printing the pictures?
• Digital processing Kodak say that a 5x7 photo
  requires a 1MP image file to be photo realistic
  8x10 2MP 11x14 3MP
• DTP depends, but remember that your printer
  will limit the amount of resolution you actually
  get
• Webpages smaller files make pages load more
  quickly thumbnails allow the best of both
  worlds

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Comparing sizes
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Compression

• Compression allows you to reduce the size of your
  picture by storing less information. For
  example, if a section of sky is all the same
  colour, why not save this information only once?
  There are two types of compression
• Lossless TIFF, PNG
• Lossy JPEG

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Lossless compression

• Lossless compression decompresses an image so
  that there is no loss at all.
• The problem with lossless compression is that it
  saves very little space the file size is only
  slightly reduced.
• Some digital cameras save larger files in TIFF or
  RAW file formats.

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Lossy compression

• Lossy compression is far more useful, allowing
  you to create files of smaller sizes.
• The more you compress a file in this way, the
  poorer the resolution.
• Once you have compressed a file, you cant
  decompress it.
• There are different ways of compressing files
  reducing the range of colours or removing unused
  colours reducing depth of colour etc.

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JPEG

• Joint Photographic Experts Groups
• Most cameras save into this format automatically.
• You get to choose the level of compression if
  you want to take hundreds of photos, or are only
  using them for the web, this could be ideal!

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Changing sizes
When you shrink the size of an image, you reduce
the quality of the image. This is irreparable
you cannot reverse the process!
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Aspect Ratio
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Aspect Ratio
If you process a photo that is 1024x768 pixels,
and it is being printed as a 6x4, you will
either receive unwanted white space or lose part
of your photo