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Optimising Images

   

Files

The following tables are a rough guide to the relative sizes of different file formats and the relative capacity of different storage media:

File Formats

Thumbs up Save as file type:

Thumbs down Avoid saving as file type:
GIF [.gif] > good for line art and blocks of colour, so suits most graphics, supports 256 colours BITMAP [.bmp] > neither 16, 24 nor 256 Colour Bitmaps
JPEG [.jpg] > good for photos and images with subtle tonal changes, supports 16 million colours.  
PNG [.png] > good for colour depth, but not yet widely used as some browsers can’t read it  

 

THESE FORMATS COMPRESS FILES

THESE FORMATS ARE VERY LARGE

 

Average storage capacity

Bytes
Hard Drive  100+ GB 1 GB  1000 MB
DVD     4.7 GB 1 MB  1000 KB (approx.)
CD-ROM    700 MB 1 KB  1000 BYTES (approx.)
Zip Disk    100 MB 1 BYTE   8 BITS
Floppy Disk   1.44 MB    

Scanning 

  • Select a resolution > 300 dpi (dots per inch) is adequate for most purposes. The higher the resolution the bigger the file size.
  • For print media the resolution can be anywhere between 72 and 1200+ dpi  [1200+ dpi is professional print quality for publishing]. Unless you are using a professional printer and high quality printing paper, keep to around 300 dpi. A basic printer only needs 150 dpi as the quality is not high enough to merit higher resolution scans.
  • On many printers the choice is between Draft, Normal and Best. Use Best sparingly

To keep files small change the scale from 100% to 50% or less if you are scanning larger than A6. [An A4 scan at 100% and at a high resolution will use so much memory if you are working on it in an image manipulation program like Photoshop that you may use up all the available RAM  - Random Access Memory: this is what the computer uses to operate programs that are open - and any formatting will take ages to apply].


Editing Images

  • If an image you are working with is using up too much RAM, check the size:

    In     Photoshop go to Image > Image Size (make sure the Constrain Size Proportions box is checked) and reduce either the width or height.

    In     Paint Shop Pro click on Image > Image Information. If you want to reduce the proportions, go to Image > Canvas Size or Image Resize [this allows you to change the print size].  
  • While creating or editing an image it is advisable to save it as a .psp [Paint Shop Pro] or .psd, .pdd [Photoshop] file. These are what' are termed lossless formats i.e. repeated opening will not cause the image to deteriorate noticeably. JPEG is a lossy format and would cause some deterioration after repeated opening. 
  • However, .psp and .psd files are larger, so when the image is finished, compress it by saving as a .jpg or .gif

 

Web

In order to avoid long download times images may need to be compressed. Some detail may be lost, but not significant amounts for web viewing. 

  • Size: web graphics should be a maximum of 800 x 600 pixels
  • Resolution: no more than 72 ppi (pixels per inch)
  • Most monitor screens are 1024 pixels wide by 768 pixels high  
  • The GIF format supports transparency so that the colour of your web page can show through the background etc. This is not possible with JPEGs

Photo-manipulation programs like Adobe Photoshop and Jasc Paint Shop Pro allow users to preview the effect of compressing your image in the above file formats and of viewing them in different browsers. If possible, check your files across different platforms and in different browsers before uploading..

  • PC to Mac
    Preview what your image will look like on a Mac if you’re using a PC:
    choose View > Preview > Standard Macintosh Color,
    or in Photoshop select View > Proof Setup > Macintosh RGB.

  • Mac to PC
    Conversely, on a Mac go to View > Preview > Standard Windows Color,
    or in Photoshop choose View > Proof Setup > Windows RGB.

  • Paint Shop Pro
    In Paint Shop Pro select Export > JPEG Optimiser for photos or Export > GIF Optimiser for vector drawings and try out different compression values between 1 and 100. You can check out the download times and preview the effect of the compression before making a choice.  

© RKM 2002 - 2004

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