Matching pixels and dots
Regardless of the number of pixels in your image file, the number of ink dots that will be printed on the photo paper is determined by the printer's resolution, and the size of the print you require.
Suppose you are using a Canon (or HP) printer whose 'native' print resolution 300 dots per inch.1 Suppose further that you wish to make a 12" X 8" print. Such a print will be made up of 3600 X 2400 ink dots.2 We can't change that. The number of dots is fixed by the printer resolution and the print size. If you were using an Epson printer, which prints at 3603 dots per inch, a 12" x 8" print will consist of 4320 X 2880 dots.
For maximum quality, you need to be able to control the content of every ink dot: you want to be able to match each ink dot with one carefully coloured and placed pixel!
But the number of image pixels in your image file is determined by the sensor size of your digital camera and whether or not you have chosen to crop the image.
It is most unlikely that the number of pixels in your raw image will be exactly the number of dots you require in your print. The number of pixels in your image will need to be increased (in the case of a large print) or decreased (in the case of a smaller print.)
For example, suppose you have an image taken with a 18 MP digital camera, which produces images that consist of 6000 x 3000 pixels. Suppose further, that you want to make a 24" X 16" print of that image on a Canon printer which makes 300 dots per inch.
The 24" X 16" print will be made up from 7200 X 4800 dots. To maintain maximum quality we want to make 1 pixel ➝ 1 ink dot. The 6000 x 3000 pixel image needs to be resampled so that (as near as possible) the same image is reproduced in 7200 x 4800 pixels. The 6000 X 3000 pixel image would need to be upsampled — redrawn using more pixels.
If you take no action at all, and send the 6000 X 3000 pixel image to the printer, telling the driver that you want a 12" x 8" print, the printer driver will take over and resample your image for you to fit your image data into print dots because that's the only way it can work. If it is a Canon or HP printer it will probably print a 24" X 16"" image as 7200 X 4800 dots. The problem is that printer drivers don't necessarily make a very good job of resampling.
What if you want to make a 12" X 8" print from the same image? A 12" x 8" print will be made up of 3600 X 2400 ink dots. To maintain maximum quality we want to make 1 pixel ➝ 1 ink dot. The 6000 x 3000 pixel image needs to be resampled so that (as near as possible) the same image is reproduced in 3600 x 2400 pixels. In this case, the image needs to be downsampled — that is, redrawn using fewer pixels1.
Again, if you take no action, the printer driver will do the job for you, and make a print with the right number of dots. But it may not do it well.
To retain maximum image quality, you should take action to control the resampling. Why?