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Thursday, November 1, 2007
Pixels and dots

People are very excited about using their email, not only to communicate with friends, business colleagues and those in their particular trade, but also to send documents, presentations and photographs.

However, few people seem to understand the significance of file size when sending emails. What exactly does it mean when someone, for example, sends a picture of 3mb? Well, firstly, it means they are crazy!
Secondly, it means a waste of resources and a waste of time, and means it will probably annoy the recipient. Before you ask, “Why is that?” think for a minute what 3mb actually means.
It means three megabytes, or three million bytes. A byte is enough information to send the letter ‘a’ for example. Since the average word might comprise, say, seven letters, then a 3mb photograph is about the same as sending someone an email 428 000 words long. Is that what you are intending to do? For those still using a 56k modem (that is, 56 000 bits a second) about the fastest they will ever get that photograph, all other things being equal, is about ten minutes with a really good connection (unusual). For those lucky enough to have ADSL the speed is a lot better, but will still take about one and a half minutes.
So what is the answer to better communication?
High quality magazines can work with a photograph that has a resolution of 240 dpi (or dots per inch), provided the original, of course, was not out of focus, was well composed and properly exposed for the light. Note that 240 dpi would be equivalent to 240 pixels per horizontal inch on a computer screen. The quality of an image saved in jpg format should not exceed 300 dpi (allowing for the end user to manipulate the picture before using it in a publication. At 8 cms wide such a picture would be about 300kb in size — in other words ten times smaller than our 3mb picture. Put another way, anyone who sends a 3mb picture is sending 2,7mb, or over 360 000 words of completely redundant information!
So how do you resize your picture before sending it? In most picture programs (eg Picture Publisher or Photoshop) there are options to change the dimensions of a photograph. There are two settings: the physical size (width x height) and pixels (dots per inch). When manipulating a picture first change its physical size so that it approximates the desired end result. To do this you turn ‘resample’ off, so that the pixels are not destroyed. For example, you might have a 96dpi picture (not a high quality resolution) but which is 37 cms wide! By reducing its width to 8cms you actually end up with a 451 dpi picture – simply because the dots have been squeezed into a smaller width. You then turn ‘resample’ on and change the 451 dpi to 300 dpi. The file size will be around 300kb.
Please note that, for the purposes of viewing a photograph (one that will not be used in publishing) you need not exceed 96 dpi, the maximum resolution of a typical computer screen. Obviously the file will be a lot smaller and will transmit a lot faster.
In the examples the picture sizes in JPG format would be from left to right: 152kb, 75kb and 47kb respectively. For the purposes of publication in a printed magazine the files are converted to Tagged Image Format (TIF) - a far larger high quality format used for printing, which ended up as 5198kb, 1308 kb and 543 kb, incidentally. But pictures should never be sent via email in this format.
As an experiment go to Google, select images and search for a subject. It will return pictures of all kinds of sizes. You can download them and see what size and quality they are and so understand better how resolution (dpi) and file size (kb and mb) interrelate. By Nigel Benetton
 

Copyright © Insurance Times and Investments® Vol:20.10 1st November, 2007
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