PPI wise, a lot of it depends on the print size combined with assumed viewing distance. Assuming a 1/100th of an inch circle of confusion, anything above 100ppi will be perceived by the average person to be photo quality at the normal viewing distance for that size. The higher the ppi, the shorter the viewing distance and the sharper under close scrutiny. For professional portrait and wedding work, 250ppi is perfectly acceptable. This translates to:
4" x 6" print requires a minimum 1500 x 1000 image size
5" x 7" requires 1750 x 1250
8" x 12" requires 3000 x 2000
11" x 17" requires 4250 x 2750
Note that at 8x10, we have just reached the 6mp limit. That's because the 250ppi figure is conservative on the safe side, and depending on your printer and the quality of it's output, you may indeed be able to use far less ppi to make large prints. For example, the continuous tone commercial process printers by Fuji and Noritsu do wonderful 8x10s every day at the 5x7 resolution in the above chart.
My best advice? You've got a printer, right? Take an excellent image and resize it to various ppi ratios starting at 100ppi and then print 8x10s. The point at which you can see obvious degradation of the print is your lowest threshold. Above that is gravy, so to speak. I can state without reservation that I've seen outstanding 11x14s from 3mp DSLRs.
So there ya go....
David
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