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Photo File Sizes

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  • Doon The Toon
    Hi adrian.gray@pharm.ox.ac.uk. You said:- ... Picture files that I receive for my website are very often larger (in every sense) than they need to be. First of
    Message 1 of 1 , Jun 2, 2001
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      Hi adrian.gray@....
      You said:-

      >If anyone can explain why
      >and what to do I will remove it and replace with a faster loading

      Picture files that I receive for my website are very often larger (in
      every sense) than they need to be.

      First of all, remember that most pc users will have their monitor set to
      display 800x600 pixels. ("pixel" = "picture element")
      This includes window borders and stuff.

      It follows that if a photo's size is greater than this, then viewers will
      have to use their scroll bars to see everything in the photo. So the
      first thing I do is change the photo size.

      Every application has its own way of doing this; in Photoshop, you go to
      the "Image" menu, then "Image size". This gives you a panel where you can
      change the size and resolution of the photo. You have to make sure that
      the box "Constrain proportions" is checked. You then enter a new size for
      the height or width and the other is automatically changed to keep the
      correct proportions.
      For a landscape format photo, I keep the width to not greater than around
      600 pixels. This usually gives a height of around 400 pixels.

      Portrait format is more awkward, because our monitors are landscape
      format. If you keep the height to around 400 pixels (so you don't have to
      scroll up and down to see the whole photo) then the width is going to be
      around 300 pixels. One option is to use two portait format photos side by
      side in one composite file, which, obviously, will be in landscape format.

      Secondly, there is no point in having the photo's resolution any higher
      than 72 dots (pixels) per inch, as this is the resolution of our monitor
      screens. Any higher resolution is just wasted - the monitors CANNOT
      display it. Higher file resolutions are only required for printing and
      result in a huge increase in file size.
      If anyone wants a copy of the file to print out, you should send it to
      them privately, rather than using up a lot of space on the web for print
      quality that may never be required.

      Thirdly, the resultant file should be saved as JPEG. Normally, you will
      have some choice of compression - more compression = smaller file sizes
      but = lower quality. I find most photos display ok compressed to 40-50%
      of their original file size. This is a "try then taste" process. The
      photos of the Morgan fire at my website were saved at a compression of
      50-70% as JPEG doesn't really like large expanses of solid colour (black,
      in the case of the Morgan photos) and can display "banding" instead of
      indiscernable graduation. You may see this on the "sky" in some of the
      photos but I had to compromise between file size and quality.

      Finally, ALWAYS work on a copy of your photo. Each copy can be tailored
      for its eventual purpose, print or web. I usually scan photos, etc at a
      resolution to suit my printer, then work on copies of this file. However,
      I scan stuff solely for the web at 72 pixels per inch.

      Do not assume that everybody uses the same platform (eg Mac, Windows,
      Amiga) as you do. The internet does not belong to micro$oft, no matter
      how much they try to create that impression! Files should be in standard
      cross platform formats, eg TIFF (for printing) or JPEG (for web use). BMP
      is a micro$oft-only format; some of us don't use micro$oft products!

      For printing, higher resolution is better; the resolution of the eventual
      printer is the constraint here. Ask the intended recipient what
      resolution they prefer; a printer which can print at 600 dots per inch
      should be happy with a resolution of half that, ie 300 dots per inch.
      ("pixels" are usually used in connection with viewing on screen; "dots"
      are used for printing)
      But remember - the higher the resolution or eventual printed size, the
      greater the file size thus the longer the file will take to upload and

      If you go to my website at:-


      you will find a page of "Home Computing" links. There are links on this
      page which you may find useful.


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