(conversation in #cameratalk on 2015-11-18 & 2015-11-19)
JTJ:
DPI/PPI is important any time you’re crossing the physical/digital boundary – in either direction.
When scanning, you’re going from inches to pixels, and when you’re printing, you’re going from pixels to inches. DPI matters in both circumstances. Or, more specifically PPI, but DPI is commonly used to mean pixels per inch.
Ploafmaster:
Certainly - but when talking about the final resizing for a print, then it really is specifically dots, not pixels.
JTJ:
I was differentiating between PPI and DPI for the semantic pedantics because, while I (and most cameras and apps) normally use the two abbreviations interchangeably, there are some folks who use the traditional definitions, in which case there is a distinction: PPI referring to pixels (whether printed or in digital form) and DPI referring to dots of ink laid down by the printer. The dots of ink are usually much smaller than an image pixel. A printed pixel is square of color that can be any of millions of colors, whereas a dot is usually (depending on the printer) one of only four or so colors.
So, when you’re resizing your image for printing, you’re still specifying pixels per inch.
Ploafmaster:
Okay, I thought I had my head wrapped around it, but now I'm not so sure. If we're talking about exporting an image so it can be printed, are you saying, @jtj, that the dots of ink printed per inch on paper are independent of the DPI resolution specified during export of the image? That is, the print quality is a property/setting of the printer, regardless of the pixels per inch set in your export?
If so, then a) I didn't realize that and b) I think I now finally have my head wrapped back around it.
I guess at least you’d want your export DPI to be at or higher than the printer's set DPI (for a given print job) so you don't lose detail?
JTJ:
Yes, the DPI of the printer is determined by the accuracy of the print head and is independent of the image PPI. You want the DPI of the printer to be much higher than the PPI of the exported image because it takes several dots of ink to render a single pixel of image data. On most printers, it takes at least 4 dots of ink (a cyan dot, a magenta dot, a yellow dot, and a black dot) to print a single pixel.
And if you're printing with a halftone screen instead of a stochastic ink jet printer, you need enough dots to be able to accurately draw each halftone dot. So, for a 300 lpi halftone screen, you need a several thousand DPI printer.