Jpg files. What, When & Why.
What is a "JPG" or "JPEG" file?
The name JPEG stands for Joint Photographic Experts Group, the name of the joint ISO/CCITT committee which created the jpeg format or, more accurately, jpeg standard.
Without getting too technical, jpg files are image files (usually photographic images, as line drawings and other vector graphics are less suited to the jpeg format) that are compressed to enable large files to be stored or transmitted as smaller files, like dehydrated food or clothes stored in vacuum bags. For possibly more information about the jpg standard than you could possibly want, check out a detailed analysis of what jpg files are on this Wikipedia page: http://en.wikipedia.org/wiki/JPEG
Essentially, what happens to an image file that is saved as a jpg file is that instead of storing information about the colour and density of each pixel (the basic unit of an image file), a file that is saved as a jpg image has a number of adjoining pixels averaged out into one block of colour. So instead of having a file that has, say, a million pixels described as different colours, a jpg file may have only 100,000 or 500,000 descriptions of those pixels making up the image. The resulting file may look essentially the same but will be much smaller in total bytes or megabytes. The physical dimensions of the image are unchanged, just the amount of information which is stored is reduced.
When an image is saved as a jpg file it will be assigned the extension ".jpg" or ".jpeg". There are other extensions as well but in the real world of internet parlance, the ".jpg" extension is usually used.
Will jpg compression damage the image file?
Only in extreme cases where high compression is applied to a low resolution image. There will be a loss of sharpness and possible colour fringing or other "artifacts". Jpg compression is a "lossy" process where information is removed by the process of averaging pixel information.
This
image shows an extreme example of jpg compression. You can see the tones
in the ocean that have been averaged out into a few shades of blue and
are now visible as blocks of colour, giving a chequered appearance to
the image. Also, colour fringing can be seen on the body of the whale.
Such a highly compressed image is barely suitable for the web and of no
use for any reproduction work.
If this same image was saved with very low compression, there would be no visible indication of compression artifacts. Only extreme enlargement of the image would show any pixel averaging which would be invisible in the final printed product.
Are jpg files only used on web pages?
No. Usually when a web page has an photograph displayed it is in the jpg format. However that is not the only use for jpg files.
Are jpg files only "low resolution" and useless for any serious image use?
No. While jpg files found on web pages are essentially useless for anything else (except maybe a powerpoint presentation which most people sleep through anyway) a jpg file may be used for any purpose, as long as the quality of the file is high enough for the intended use.
When can a jpg file be used for press work or reproduction?
When it has enough information to be reproduced without displaying image degradation from the jpg compression process. It also needs to have a higher dpi (dots per inch or pixels per inch) than an image used on the web.
Typically an image used for the web will have 72 dpi, a file intended for a newspaper may have 100 or 150 dpi, a file for a colour print from an inkjet or similar printer may need 150-250 dpi or even 300 dpi while an image for a magazine or book will need 300 dpi. Some high grade printing might use 600 dpi.
How compressed should a jpg be for my use?
As little as possible. When a file is saved in jpg format, for example in Photoshop, part of the saving process allows you to stipulate how much compression to apply. An image used on a web page may be sized at 600 x 400 pixels at 72 dpi (a fairly large image for a web page) and when saved as a jpg it may be saved at a high compression level which translates to a Quality setting of 3 or 4 as shown below. It may be a little confusing but a highly compressed file has a low compression number, such as "3" shown below, which relates to the quality of the saved image file. In this case you can see that the "3" level is regarded as "Low" quality.
The size of the file, saved at quality level "3", that is transmitted over the internet to your computer will be only about 75 kb (depending on the number of colours in the image and other factors).
When used in a magazine or book, the same image will be sized in inches or mm such as 297mm x 210mm (A4 or full magazine page size) at 300 dpi. The full size of that file will be around 24 -25 megabytes, a much, much larger file. While downloading a file that size may be fine if you have a fast broadband connection, the space taken up on the server and the time taken to upload and download will be much reduced by applying jpg compression, especially when more than one or many files need to be transmitted.
Such a file would be saved in jpg format at level 10-12. The higher numbers denote a much smaller amount of compression. The A4 file would be reduced, when saved at level 10, to a 1-2 megabyte file, much quicker to upload and download, not to mention using less storage space on the server. At level 12 the same file may be around 3-5 megabytes, still much smaller than the original file and with no discernable reduction in image quality.
Any compression "artifacts" or blurring of detail or colour smudging would be invisible or non existent at those low compression levels.
So what's the bottom line?
Let's liken files saved in jpg compression format to cars. A low resolution (72 dpi) file saved at a high compression, low quality rate could be like a rusty old four cylinder car limping along the road. The same file saved at 300 dpi with the lowest compression setting is like a formula 1 racing car. They are both cars, but you know which one will win the race.
Suffice to say, all Marine Themes image files intended for reproduction which are delivered as jpg files are saved at very low compression levels.