ppi and dpi
You are now the proud owner of both a film scanner or a digital camera and an inkjet printer. But may have wondered what is the best resolution -or pixel density- of a digitized image to allow for the production of a good print in a given size.
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A printer's software translates pixels into ink dots, that's why printer's resolutions are expressed in dpi, dots per inch. How many dots per inch is a printer capable of producing on paper is a measure of its quality. In general, the more dpi, the smoother is the tonal gradation in the print, the finer the definition and the wider the color gamut.
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On the other hand, a digital image from a scanner should be stated in ppi or pixels per inch. For example, specs for my Nikon CoolScan IV ED LS-40 state it is a 2900dpi scanner. Looking into the effective reading resolution with the MA-20 adapter for slides, clearance of 25.1 x 36.8mm, the scanner yields images 2,870 x 4,203 pixels in size, or 12Mpixels. Roughly the equivalent to a 6Mpixels digital camera image, as it seems from experience.
Doing calculations in the width side: 2870pixels/25.1mm = 114.3426 pixels per mm or 114.3426 pixels per mm x 25.4mm per inch = 2904 pixels per inch, approximately 2900ppi.
Calculations on the length side of the image scanned, reach the same result (4203pixelsx25.4mm per inch/36.8mm=2901ppi, pixels per inch).
So it is ppi, not dpi. This seems to be a common (translation?) mistake. My flatbed Epson scanner also talks about dpi when it should be ppi. Minolta, Hewlett Packard and other brand scanners they all make the same misuse of the term. Strange.
A CoolScan 4000 ED will yield 3,946 x 5,782 pixels from a 35mm negative or unmounted slide, or 22.8Mpixels, roughly the equivalent to a 11.4Mpixel digital camera image.
Digital images sizes from digital cameras are also stated in pixels, like VGA, 640x480.
IMAGE RESOLUTION FOR PRINTING
Minimum resolution for magazine-quality printing is 300ppi, so a VGA image of 640x480 will only allow for a decent print of 3.2 x 1.6 inches (640pixels/300ppi=2.1 inches by 480pixels/300ppi=1.6 inches). 640x480=0.3Mpixels.
If you want to make a good 8x10 print under the above standards, it will be best to have a 300ppi image with a size of 2400 by 3000 (8x300 by 10x300), a 7.2Mpixel scan. So now you know exactly the why of the quest for higher Mpixel rating from scanners (and digital cameras), even when interpolation may acceptably invent pixels where there are none.