I have someone who wants to make 30x45" posters out of about 15 shots. I don't go this large often. When I do I'm kind of deliberate. The request for posters came as a bit of a surprise and some of the shots selected were with a D3s and cropped down about 30%. Is this going to work? Suggestions or insights? They were wanted yesterday of course. lol
I've found Adobe's CS6 enlargement pretty good. I had an uncropped file from a D300s that the client wanted very large, 40x60. I was surprised how well it interpolated things. I wish I had the D800 but given the size it's pretty good
If Photoshop enlargement doesn't quite look good enough, then Perfect Resize (formerly, Genuine Fractals), from OnOneSoftware, should be your next try. Good luck!
Keith W.