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DIGITAL IMAGES
I'm not an expert in this field, this is merely my present grasp of the subject,
please do not hesitate to advise me of any misconceptions or improvements to the
spiel.
There are many file formats that a digital image can saved in (e.g. generic
formats such as *.bmp, *.tif, *.gif, *.tga, *.jpg and proprietary formats
developed by the likes of Kodak, Adobe, etc.). Each format has it's advantages
and disadvantages.
The general rule of thumb is that the higher the resolution, the larger the file
size and hence the longer it takes in sending them over the
internet. When scanning an image there is an inclination to scan at the
highest possible resolution to achieve the best quality image but then we find
we have files sizes of many megabytes.
When I scan images I save them at a high resolution which I keep on my hard
disk, I use the *.tif or *.bmp format. I'm informed that the *.tif format lends
itself to better photo editing in programmes such as Adobe Photoshop, etc. but I
also like the *.bmp format because it's then easy to edit out the "red
eye" using the Microsoft Paint program (comes with all the operating
systems).
If I want to send an image to a friend then I'll open the high resolution *.tif
or *.bmp image up in an image viewer/editor and re-save the image in a *.jpg
format with the result that I end up with two image files on my hard disk, one
is the xxx.bmp or xxx.tif (a large file size) and the other is a xxx.jpg (a
small file size), it's the *.jpg format that I attach to the email.
In the following examples, the image (of my "great gran pappy" Sydney
Charles Salmon) on the LHS is a *.bmp image 495 KB in size and the RHS *.jpg
format image that is only 24 KB, a 50:1 reduction. I've attached both these
images for you to spot the difference in resolution. It is difficult to see any
difference, this is, I'm informed, because a computer monitor can't
represent a resolution much greater than 75 dpi (dots per inch) however if
you were to print the images then dependent on the output resolution you want, a
difference will be discernible.
I've used 'thumbnails" of these two prints because it will take a while to
download the LHS image if you decide that you want to look at the full size
version. Click on the images to see them full size, you will notice the
noticeable difference in download speeds between the two.
The
implication of this is that if there is no intention to print out an image then
you may as well set the scanning resolution to 75 dpi regardless of whether the
scanner is physically capable of scanning 300, 600 dpi or whatever with the
result that this will save you a lot of hard disk space. If it is likely that
may want to print the image on a common inkjet printer then set the scan
resolution to 150 dpi.
The *.jpg format, it's what is called a "lossy" format, i.e. every
time you open a *.jpg image and re-save it onto itself (by this I mean
re-save it onto it's original file name), it becomes progressively smaller in
file size with a corresponding reduction in resolution/quality. So in
the circumstance of an image that need to be re-touched in a photo image
viewer/editor, you should go back to the original scan (*.bmp, *.tif or
whatever) edit it then "save as" in the *.jpg format. Experiment - get
a *.jpg image and re-save it several times over and you will notice how the
quality drops off. Merely opening the image and closing it down does not degrade
it.
When you save an image in a *.jpg format there is usually an option to set the
quality and this will determine the size of the *.jpg file and of course, the
viewing quality. In the example of the above RHS *.jpg image the quality level
was set at 75%, had I chosen 100% the file size would have been 139 KB instead
of 24.
To come (maybe):
DIGITAL CAMERAS
SCANNERS
IMAGE VIEWERS/EDITORS
[This page last updated 15/05/2003]