DPI alteration

[Canon88]

Hey guys, question:

I save my original .JPG images and a full-resolution edited version as a .TIFF both to my external harddrive.

Right now I'm saving the edited .TIFF at 240dpi - a photoshop default, at least on my CS4.

1) If at a later time (for whatever reason) I need to change these images to 300dpi, would they still print okay?

2) Would I have much trouble making this change to my images, and, should I start saving in 300 dpi from this point onwards?

3) How do I change the photoshop default DPI input value?

Thanks guys.

[srwatters]

Don't get hung up on DPI (or PPI) until you're ready to print and know what size the physical image will be. What matters most is pixel count from the original image and the final crop. For example, my Nikon D3 shoots 4256x2832 pixels in the full image. If I don't crop at all, that translates into the following DPI for these print sizes:
4x6 - 708 DPI
8x12 - 354 DPI
16x24 - 177 DPI

I very often print 11x14 images for my book from this camera and they look great. Just remember when you crop, you will lose pixels and your DPI will be reduced. If you want to upsize allowing PS to interpolate the new pixels, you may need to do some additional sharpening or experiment with the different resizing algorithms. I typically use bicubic or bicubic smoother for upsizing, but it's very dependent on image content and how many hard edges are exist in the shot.

Just to add one more thing, you don't always need to print at 300 DPI. For larger formats that are viewed from a greater distance, very often as little as 150 DPI or less can be used with excellent results. Again it depends on the printing method being used, the viewing distance, size of the print, and content of the image. Think about it this way, a HD TV picture is 1920x1080 pixels. You can view that same image from 12" on your 17" computer desktop display or from 10' away on a 60" TV or from 100 yards away on the Cowboy stadium jumbo-tron. In the end the pixels are bigger, but they still are basically the same pixels. (note to uber geeks - I've simplified this analogy and know there are pixel doublers and other technologies used, but the basic concept is what's important here)