What can you do if an image has an actual ppi of 300 but an effective ppi of say 115 - 240? At what level will it be safe for commercial printing?
240 effective ppi is probably adequate for almost any commercial printing other than fine art reproduction. 115 could be pushing your luck, but there are so many variables that it's impossible to say anything absolute. What is the intended use and the intended viewing distance? What's the printing method?
You might want to take a look at http://forums.adobe.com/message/2042202#2042202
Thanks, I am using the images for posters for conferences so people can walk right up and look at it in the booth. I have had some luck sourcing higher res images.
Posters for booths are usually printed using inkjet plotters, I think (that's how we used to do it where I worked on my second job in this business), and though you can walk right up to some of them, they really are intended for viewing from some distance away in most cases (but yours might be full of 12 pt type and other stuff that can't be read from more than 18"). For that sort of work, 150 ppi is usually all that's requested, and you can easliy get away with less most of the time.