How Many Pixels Do You Need?
Start by taking a good file and making test prints at 360, 300, 240, and 200 ppi.
Choose which resolution looks best on your paper and printer.
Now decide how large you want to print as your standard size. (Hint: For occasional large prints, either send it out to a professional lab, or use an "uprezzing" application such as FixerLab's SizeFixer. Remember that larger prints will be looked at from farther away.)
Multiply each dimension by your chosen resolution, then multiply the two. Voilá. That's how many megapixels you need.
For example: My standard print size is 7 x 10.5 inches. If I want to print at 300 ppi, that's (7 x 300) x (10.5 x 300) = 6,615,000 pixels = 6.615 effective mp.
To find out how big your existing camera can print (optimally), just go the other way.
And here's what a lot of people don't get. If you regularly print smaller than the optimum size for your files, you're just throwing the extra information away. An 8x10 print at 300 ppi needs (8 x 300) x (10 x 300) = 7.2 mp. Call it 8 mp and give yourself a little leeway for cropping. (And remember that that's without NR or uprezzing, and 300 ppi is overkill for many printers.) Once you've got that covered, having a 12 mp, or 16 mp, or 22 mp camera is not going to make that 8x10 look any better. Your printer can't print better than the best it can print; if you're already giving it all the information it can get onto the paper, giving it more won't help.
Posted by: MIKE JOHNSTON