One of the things that can be the hardest to get your head around when you start down the digital path is resolution. This confusion is compounded by the fact that there are two different kinds of resolution in the digital world. One is dots per inch, and the other is pixels per inch.
Dots per inch (dpi)-
This refers to the number of dots per inch that an inkjet printer is capable of applying to a sheet of paper. It can range from 720 to 2800 and more. The closer these dots are to each other, the more they blend together forming continuous tones on the print. Naturally, this depends on the paper that’s being printed on. If the paper is porous watercolor paper, the dots soak in and blend just fine at lower dpi settings like 720. On glossy photo papers, a higher setting—such as 1440—is needed because the ink dries on the surface. About the only time people discuss dpi is when they are talking about a printer.
Every pixel output is made up of different colored inks (usually 4-6 colors, although many printers use more now). Because of the small number of colors, the printer needs to be able to mix these inks to make up all the colors of the image. So each pixel of the image is created by a series of tiny dots (you could think of them as sub-pixels). Generally, the higher the DPI, the better the tonality of the image, colors should look better and blends between colors should be smoother. You’ll also use more ink and the print job will be slower.
So a 1200 dpi printer uses 1200 dots of ink in every inch to make up the colors. If you were printing a 300 PPI image, then every pixel would be made up of 16 smaller ink dots (1200 DPI x 1200 DPI / 300 PPI x 300 PPI). A lower DPI would have fewer ink dots making up each pixel, which would make the color look worse. A higher DPI would have more ink dots for each pixel and should give more accurate color (especially under close examination).
Pixels per inch (ppi)-
This is what is usually discussed when talking about resolution in digital photography. It refers to the distance between the pixels that make up digital images. Pixels per inch is an important setting because it determines what digital images look like when they’re displayed and printed. Sometimes a lower value is desirable, and other times a higher value is preferred.
You can specify the interpolation method in the Image Size dialog box.
The default that appears in the dialog box is based on the interpolation method you specified in your General Preferences dialog box. Here are your seven choices:
Nearest Neighbor-Fast, but not very precise; best for illustrations with edges that are not anti-aliased rather than photos. It preserves hard edges.
Bilinear-Medium quality results with most types of images.
Bicubic-Slower but more precise; produces smoother graduations than the two previous methods.
Bicubic Smoother-Based on Bicubic Interpolation, but designed for enlarging images.
Bicubic Sharper-Based on Bicubic Interpolation, but designed for reducing image size because it maintains the detail of the original image.
Preserve Details-Fast,precise best for enlargement with reduce noise option.
Automatic-It detects automatically.