The terms definition and picture size often cause confusion and misunderstandings, because, in practice, they have more than one meaning. One must discern between:
The image size of a graphic file is given in pixels. This image size gives the measurements (width x height) of a digital photo i.e. 4608x3072px. The width is usually listed first.
A digital image on the computer will never have DPI, cm or inch as the unit. It will only have pixels.
Definition is the product of the width and the height of a digital image in pixels. An image with the size 4608x3072px has a definition of 14155776 pixels, or about 14.2 MegaPixel. Camera manufacturers like to use MegaPixel designation to show off their product.
The image size of a print, for example, a poster or picture will be given in cm. or inch (England, USA). This refers to the size of the paper or canvas. The size of a printed photo can be, for example, 10x15cm, or for a poster 90x60cm.
The definition of a printed image will be given in DPI (dots per inch) or PPI (pixel per inch). This point number means that a printer can print so many points per inch (=2.54cm). The higher the value, the finer the print. A decent print definition is 300dpi, which meets most requirements.
DPI means Dots Per Inch and tells the number of printed points (dots) per inch
PPI means Pixel Per Inch and tells the number of pixels per inch
DPI and PPI are basically the same, except PPI is the more correct designation, and DPI only applies to certain printers and printing methods.
During photo developing, only designating DPI/PPI does not make sense. The DPI/PPI value must always be given with the desired image size in cm/inch. The minimum size in pixels of the digital image can be calculated from these two values.
Example: A poster with the size 90x60cm should be printed with a minimum of 150 PPI.
From this description it can be calculated that the image size of the digital photo must add up to at least 5315x3543 Pixel (~18,8 MegaPixel):
Setting the definition for a graphic file in the Properties window in Windows Explorer or a graphics editing program is really not necessary. These values have little meaning and regularly cause confusion.
The image size of a digital photo will only be given in pixels. Definition in DPI/PPI is only shown when the printing size is decided. The 4608x3072px sized photo in the illustration to the right will only have a 300 DPI definition when the photo is a 39x26 cm print.