Check this out
http://www.rideau-info.com/photos/mythdpi.htmlThe Horrible DPI Mistake
Here's the scenario - a print shop/graphics designer/magazine asks a client for a photo at 300 dpi. They wish to print it out at 5" x 7". The client already has a beautiful digital photo with pixel dimensions of 2048 x 1536. The client notices that the photo editing software is showing that the photo is set to 72 dpi. So, following orders, the client types in 300 to reset the dpi to 300. In doing so the image is resampled and is enlarged over 4 times to pixel dimensions of 8533 x 6400. The client sends this enlarged 300 dpi photo. The print shop/graphics designer/magazine reject it (too grainy, too colour blotched). The client is crushed. The sad thing is that the client already had the perfect photo (2048 x 1536 @ 72 dpi) which would have been beautifully printed at 5" x 7" (at 292.6 PPI). The print shop/graphics designer/magazine didn't know what they really wanted - see What Print Shops Really Want and the client didn't know how to change the DPI without resizing the image to give the print shop what they mistakenly think they need -see how to change the DPI.