The latest inkjet printer models have resolution in the 1200 or even 2400 dpi range.
On reading a book on colour correction (Professional Photoshop 5: The Classic Guide to Color Correction, Dan Margulis), the section on printing resolution led me to believe that the DPI rating given for an injet printer is, literally, "dots per inch".
This is the number of fixed sized ink dots the printer can put down on a 1-inch long line on the paper, which is different from the DPI setting in Photoshop, which actually means Pixels Per Inch (PPI).
Since inkjet printers use half-toning to control the intensity of a particular ink colour at a particular area (pixel) on the paper, it requires a pixel area that is equal to or greater than the area taken up by 256 dots in order to be able to deliver 256 levels of intensity of the ink in that pixel, to match up with the 8-bit picture data that is sent to it (disregarding colour profiling).
And area of 16 X 16 dots will provide the printer enough space to deliver 256 levels of intensity. Which means that for the 1200 DPI printer, the printing resolution should be 1200/16 = 75 pixels per inch, if you want a colour resolution of 8-bits per channel per pixel.
Printing at higher ppi may result in better line resolution (sharper prints), but because the pixel area is not enough for the printer to put it 256 dots of ink, the trade-off is in getting lower colour resolution.
For example, if I have a 3000X2000 picture, and I want to generate a 10 by 6.6 inch print on a 1200 dpi printer, which way is better:
- set the "ppi" in photoshop to 300 and keep the resolution at 3000X2000, or
- set the "ppi" in photoshop to 75, and reduce the reolution to 750X500
The first scenario seems to be sharper, in principle, than the second, but since the pixel area is only enough for the printer to put in 4X4 = 16 dots, the colour resolution I get is than 16 levels per channel per pixel.
In a nutshell, my understanding is that if I set the ppi beyond 75 on a 1200 dpi printer, I will sacrifice colour resolution per pixel.
I understand I muct have over-simplified the issue somewhere. For example, the role of the photo cyan and photo magenta inks in 6-colour printers. Would appreciate the feedback from other printing experts here.