Tip: What are DPI and PPI?
DPI = Dots Per (linear) Inch
PPI = Pixels Per (linear) Inch
DPI is a real-world measurement, applying to scanners and inkjet printers. In the case of scanners it measures how frequently the scanner samples the original image, in the case of an inkjet printer it measures how small and close together the printer can lay individual dots of ink.
PPI is an abstract measurement that only exists in the computer - 'if we print this file out at size X, then each inch of print will have Y pixels from the original file'. PPI can changed in an image without opening or otherwise altering the rest of the file, and without resampling to a different total number of pixels.
So DPI and PPI have no direct relationship - you can take a 3,000x2,000 pixel image, tell the computer that it is 300ppi and you will get a print 10x6.7", which you can then print at 360dpi, or 2880dpi, depending on what your printer does. Exactly the same file at 200ppi will print out at 15x10", but with each pixel being a bit bigger on the final print. Make the PPI low enough and you will eventually see individual square blocks in the printed image.
An image file on a computer does not generally have a DPI value associated with it, because DPI will be specific to the printer used to print the image. An image purely for screen use will NEVER have a DPI, only PPI.
Unfortunately, even within the industry, people use DPI and PPI for PPI, without realising that there is a difference. Have a look in the image size dialog in Photoshop and try to find DPI - you won't, because it doesn't apply.
LPI = Lines Per Inch
Another term you will come across in the printing industry with offset or four-colour printing is LPI. Offset printers lay down large numbers of parallel lines of ink, varying the thickness/density of that line, rather than laying down individual dots like an inkjet printer. As a general rule, you will need a PPI that is twice the LPI of the output - 150lpi for a good quality glossy magazine usually requires 300ppi images. 75lpi printing for a newspaper will require 150ppi or higher images.
Ian
PPI = Pixels Per (linear) Inch
DPI is a real-world measurement, applying to scanners and inkjet printers. In the case of scanners it measures how frequently the scanner samples the original image, in the case of an inkjet printer it measures how small and close together the printer can lay individual dots of ink.
PPI is an abstract measurement that only exists in the computer - 'if we print this file out at size X, then each inch of print will have Y pixels from the original file'. PPI can changed in an image without opening or otherwise altering the rest of the file, and without resampling to a different total number of pixels.
So DPI and PPI have no direct relationship - you can take a 3,000x2,000 pixel image, tell the computer that it is 300ppi and you will get a print 10x6.7", which you can then print at 360dpi, or 2880dpi, depending on what your printer does. Exactly the same file at 200ppi will print out at 15x10", but with each pixel being a bit bigger on the final print. Make the PPI low enough and you will eventually see individual square blocks in the printed image.
An image file on a computer does not generally have a DPI value associated with it, because DPI will be specific to the printer used to print the image. An image purely for screen use will NEVER have a DPI, only PPI.
Unfortunately, even within the industry, people use DPI and PPI for PPI, without realising that there is a difference. Have a look in the image size dialog in Photoshop and try to find DPI - you won't, because it doesn't apply.
LPI = Lines Per Inch
Another term you will come across in the printing industry with offset or four-colour printing is LPI. Offset printers lay down large numbers of parallel lines of ink, varying the thickness/density of that line, rather than laying down individual dots like an inkjet printer. As a general rule, you will need a PPI that is twice the LPI of the output - 150lpi for a good quality glossy magazine usually requires 300ppi images. 75lpi printing for a newspaper will require 150ppi or higher images.
Ian
G5 2x2GHz, PB 1GHz 15, Mac OS X (10.4.3)
