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Digital Image Basics for non-Photographers • Blaise Tobia
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.Specifications for image submission will often refer - completely unnecessarily - to an image's DPI.
Firstly, DPI is the wrong term. What they mean is PPI.
Dots per inch is a term that has meaning only in terms of the detail rendering capacity of a printer.
Pixels per inch is a parameter of a digital image - indicating how much richness or density of information will be printed.
A reminder: in the following dialog resolution really means print resolution:
not the overall file size but instead the density of information.
The image whose parameters are indicated above would print 6.5 inches across and at good quality.
300 PPI is generally considered the minimum print resolution for a high quality printed image.
(This derives from the original laser printers, the first computer printers that could render a 300 PPI image.)
Secondly, neither DPI nor PPI has any bearing for screen-viewed images.
Images will display in a browser or digital image program at a size determined solely by the number of pixels, not their print resolution.
The image whose parameters are indicated above would inherently display on a web page at 27 inches across
(1950 pixels/72 pixels per inch, the standard number for screen resolution).
It would also display at 27 inches across in Photoshop if set for 100% view.
Note that there is an important new exception: images contained in a PDF document.
Because the PDF document is inherently print-oriented, when viewed onscreen it is a kind of virtual print.
In this case, the print resolution of the image file will be a factor. (More details later.)