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An Introduction to Digital Photography – Session 2. By Paul Emecz. How a digital camera works. Lens Aperture Shutter Image sensor. Lens. Brings light from the scene into focus to expose an image

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how a digital camera works
How a digital camera works
  • Lens
  • Aperture
  • Shutter
  • Image sensor
  • 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
  • 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

  • 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
image sensor
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.

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.

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

vector images
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.
raster images
Raster Images
  • Raster images are made up of pixels (picture elements)
  • Each pixel has its own unique location, and is a specific colour.
  • Hue – the colour of the light (it’s wavelength)
  • Saturation – the intensity of the waves
  • Brightness – the amount of light reflected
  • 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.
spatial and brightness resolution
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.

brightness resolution
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

24 bit colour
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.

image size
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 5”x7” photo requires a 1MP image file to be photo realistic; 8”x10” – 2MP; 11”x14” – 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
comparing sizes



Total Pixels

Colour TV

320 x 525


Human eye

11,000 x 11,000

120 megapixels

35-mm slide

The "Economist" magazine says it has 20 million or more. CMOS Imaging News says 5 to 10 million depending on the film. Another source says about 80 million pixels. Robert Caspe at SoundVision states that colour negative film has 1000 pixels per inch while color positive film has 2000 pixels per inch.

Your camera

? Megapixels

Comparing sizes
  • 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
lossless compression
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.
lossy compression
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 can’t ‘decompress’ it.
  • There are different ways of compressing files – reducing the range of colours or removing unused colours; reducing depth of colour etc.
  • 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!
changing sizes
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!

aspect ratio


Width x height

Aspect Ratio

35 mm film

36 x 24 mm


Display monitor

1024 x 768 (or 800 x 600)


Digital camera

1600 x 1200


Photo paper

6 x 4 inches


Photo paper

10 x 8 inches


Aspect Ratio
aspect ratio25
Aspect Ratio

If you process a photo that is 1024x768 pixels, and it is being printed as a 6”x4”, you will either receive unwanted white space or lose part of your photo