And you also need to be clear about the difference between printer DPI (dots per inch) and printing files to PPI (pixels per inch) sizes which are often used interchangeably but are different things entirely.
Indeed. It can be confusing. Let's try to clear it up a bit.
The key thing to think about first, always, is pixels. For the sake of this exercise let's pretend your image came from a 24MP Nikon DSLR, because it makes the numbers easier to play with.
Your original image would be 6000x4000 pixels. Obviously it will print at 60"x40" with a resolution of 100 pixels per inch (PPI), or 30"x20" with a resolution of 200 PPI, or 24"x16" with a resolution of 250 PPI, and do on. The image file might well have a PPI figure embedded in its metadata, but that's pretty irrelevant because printers ignore it. They just print at the size you request and work out all the PPI stuff on the fly.
Now most printers work at a native resolution of around 300-350 PPI, though it varies between manufacturers. (I think my Epson is 360 PPI, for instance.) This 300 figure arises because it's about the maximum resolution that the human eye can manage. So if you image has 300 PPI when it's printed, then it will be critically sharp even at close inspection. (Assuming that the lens was good enough to capture all that detail in the first place. And assuming that your focus was spot on. And assuming that you haven't lost any detail in post processing. And so on.) But 200 PPI will still look very good on the wall, and canvas prints are only around 100 PPI. (Most computer monitors display around 100 PPI, so that gives you an immediate feel for what 100 PPI looks like in terms of quality.)
Now suppose you want to make that 30"x20" print at 200 PPI, but the printer runs at 300 PPI. The printer needs an image which is 9000x6000 pixels, but all you have is 6000x4000 pixels. What happens? Well, the image needs to be enlarged. You can do that yourself in Photoshop, or you can just let the printer work it out. If you're getting the print done commercially, it's probably best to let the printer do it, especially because you won't necessarily know exactly what PPI resolution they need. So just send them your full size file (6000x4000) and they'll take care of it.
Same idea with making small prints. If you want to make a 15"x10" print, you have 400 PPI which is more than the printer needs. Just let the printer handle it.
Up until now I've always said PPI instead of DPI. That's because we've been talking about the number of pixels. Some people and some computer programs say DPI instead of PPI, but that just confuses things. The one place where DPI is definitely correct is when you're talking about how the printer actually works. Printers typically create each pixel using a number of smaller dots of ink. So for example my Epson creates each pixel from a 4x4 grid of tiny dots of ink, and of course it's the way the different inks are used in that grid which creates the full pallette of colours. So the printer runs at 360 PPI, but 360x4=1440 DPI. For most practical purposes though, you never really need to know or care what the printer's DPI capability is. Just PPI.
Does that help, a bit?
