I tried to keep it as breef as possible, but there was so much info that i couldn't possibly post it all at once.
Part 2 will be post soon, (and breefer i hope...)
Feel free to comment, ask, and request.
-----What is DPI?-----
DPI, or dots per inch, relates to the concentration of color dots used anywhere an image is displayed in pixels or dots. Essentially, a higher DPI yields a higher image resolution. DPI is also important in retaining quality when resizing an image. When imaging documents, DPI becomes important for the following reasons:
You should get picky with DPI if you're scanning images that will need to be printed in a high quality later on. For simple text documents, a lower DPI will be just enough, but for scans from magazines or photographs, a high DPI will be essential to the print quality of those images. A high DPI is also necessary to make your document text-searchable. DPIs of 200 or higher make text recognition available.
If high print resolution isn't a priority, the concerning effect of DPI is file size. The lower the DPI, the lower the size of your digital document. If your scanned documents are just text, you can be less concerned with image quality, and more concerned with document size to save time and disk space. Also, if your document is going to be viewed on a computer screen and not re-sized, there is no need to scan at a DPI any higher than a computer screen (average computer screens are typically 96 DPI or 76 with older models). Lower file sizes are easier to send over networks and can be uploaded quickly.
-----72 / 100 -----
Computer screen viewing only
Low Quality Printing
Small File Size
Text Document Sharing (*not text-searchable)
Good compromise between file size and resolution.
Medium quality printing
Max necessary DPI
Near exact copy of original image
High Resolution Photo-Printing
DPI is NOT THE SAME as PPI
Even though "dots per inch" (dpi) and "pixels per inch" (ppi) are used interchangeably by many, they are not the same thing. Traditional printing methods use patterns of dots to render photographic images on a printed page. While pixels on a monitor are square and in contact with the adjacent pixels, printed dots have space between them to make white, or no space between them to make black. Color photographs are printed using four inks, Cyan, Magenta, Yellow, and Black (CMYK), and four separate dot patterns, one for each ink. Dots per inch (dpi) refers to printed dots and the space between them, while pixels per inch (ppi) refers to the square pixels in a digital image.
/////Keep in mind that many companies will ask for images at 300dpi when they really mean 300ppi./////
Why do I need 300ppi for a "photo quality" image?
First, some background information is necessary. A digital image is what it is. It is however many pixels wide by however many pixels tall. If you divide each dimension by 300, you will have the size of the image at 300ppi. Now think about 300 pixels in an inch of space. Each pixel could be black, white, or any other color, but they are all next to each other with no spaces between them. When a digital image is prepared for reproduction on a printing press, pixels are converted to dots. Dots have spaces between them. 300 pixels become 150 dots and spaces, so 300ppi becomes roughly 150dpi. 150dpi is the accepted standard for printing photographic quality images.
Printers usually refer to the number of rows or lines per inch (LPI). 150 lines per inch is simply 150 rows of 150 dots per inch. 150 LPI and 133 LPI have long been the established standards for the best quality reproduction of photographs in books and magazines. Newspapers traditionally use 85 LPI for photographs and detail is lost because the dots are plainly visible.
Viewing distance changes everything!
Everything is relative to viewing distance. 150dpi (or 300ppi) is accepted as photo quality because the average person cannot see the "dots" at a few inches away. A real photograph made from film in a darkroom has no dots or pixels and therefore is the standard by which "photo quality" is judged. When you move the viewer further away from the printed material, lower dpi is acceptable. A huge billboard might be printed at only 40dpi but no one notices because everyone is 50 yards away from it.
I always see "72dpi". Where is that used?
72dpi should really be 72ppi because most likely, it has nothing to do with printing. Most of the time, 72dpi refers to output on a computer monitor. A 72 pixel by 72 pixel image should take up about one inch of space on the screen. This of course depends on the size of the monitor and what resolution it is set to. When creating images for web sites, we've found it's best to determine what screen resolutions will be used most often by that site's visitors, instead of thinking about inches at all.