That depends on the viewers' distance to the print. What I mean to say is if the print is 12" x 18" and you're standing back 10 feet it'll appear sharp, but it won't bear up under scrutiny close-up. You need to determine how big a print you want versus how close people will be viewing that print in order to determine what the maximum size print your camera will produce that will be "sharp".
I have done group photos for the local high school with my XTi and had Office Depot print them on their Canon color plotter at 30 x 40. And as the previous poster said, when they are framed and a person is standing back 10 feet or so, they look OK.
Bur upon closer inspection they are pretty rough.
You need to remember the intended use of your photo. Also this is a cheap way to get a huge print if you have need and schools don't have much money so you do what you have to. It's all good, right?
Billboards can have an astonishingly low resolution (11 dpi)... but that works because they're intended to be viewed from hundreds of yards away.
You could print at 300dpi with an image size of nearly 18 x 12" at 100% resolution (it's actually just fractionally less). But if you were to reduce the resolution to 150 dpi you could print at double that... approximately 36 x 24" at full resolution.)
The viewer's distance is a big factor here. Also... diffraction limits come into play at high f-stops (e.g. about f/11) but most people don't notice this unless they inspect the image closely compared to an image produced at a much lower f-stop.
Lenses and focal ratio play a big factor in how "sharp" something will look.
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