Dave wrote:
John J, very confused, asked:
Colin’s answer should be FAQ.
But since when is 300dpi "high quality"? What happened to 1200?
1200 what? Certainly not dpi.
Like the topic of the thread.
Ok, a little more clarification …
an image is measured in so many pixels, like 2000 x 3000 gives a 6 megapixel image.
Pixels in this sense are dimensionless, i.e. they are not related to any physical size. (I know the sensor has a physical size, but the image it produces is dimensionless.)
When it comes to printing or displaying an image, the size it prints/displays at is set by the pixels-per-inch (ppi) parameter. A 2000×3000 pixel image printed at 300 ppi will yield a 6.66 x 10 inch print. Why? because 2000 pixels at 300 ppi is 2000/300 = 6.67 inches, and 3000/300 = 10 inches.
Change the ppi and the print size changes proportionally – simple arithmetic.
It is generally accepted that an image printed at 300 ppi is adequately high quality. But printers are said to print at 2,880 or even 4,800 dpi, so how does that work?
Well, modern printers have what is called a native resolution. For Canon printers it is 600 ppi, and for Epson it is 720 ppi. If you feed your printer with a 300 ppi image, the printer driver will resample the image to 600 or 720 ppi depending on the printer. But a Canon (for example) printer will print at 4,800 x 2400 dpi – dots per inch. That is ink dots laid down by the printer, not pixels. How come? because the printer lays down many dots to make up one pixel – in Canon’s case, 32 ink dots in an 8 x 4 matrix to make one pixel of the image.
This many-dot system has two advantages; it makes for smoother tonal gradations, and it allows mixing of different-colored inks within the 32-dot pixel to produce exact colors, impossible to do if only one dot was used to print one pixel.
So, recapping:
an image is measured in pixels – no size implied.
An image is viewed or printed at so many pixels per inch – ppi, providing a size to the image.
An image is printed by the printer at so many dots per inch, which is fixed by the printer specs – dpi.
The printer uses many dots of ink to make one pixel.
Have a new respect for your printer. An 8 x 12 inch image is fed to the printer by the printer driver at 600 ppi (regardless of the actual ppi fed to the driver). That’s 8*12*600^2, = 34.560 megapixels. Each of those pixels is made up of 32 ink dots, so that’s 34.560*32, = 1,105.92 megadots – well over a billion dots, every one calculated to produce the right color for every pixel.
I hope that was interesting.
Colin D.