So, you have a photo and want to scan it. Well, what resolution should you scan it at?
If the photo is just for your computer, e-mail, or the web, then 72 DPI (Dots Per Inch) would be just fine. (72 DPI is kind of the “standard” resolution for computer images). If you scan it too high (say 600 DPI), you’ll end up having an image that’s way too big to work with – both in height / width and in file size. Remember, people don’t like downloading 10 meg picture files.
If you want to print photos from the scan, then the best rule of thumb is 300 DPI – assuming you want to print a picture that’s the same size or smaller than the original.
OK, now it’s going to get a little more confusing (just wanted to warn ya).
What if you have a 3×5 print and you want to enlarge it to a 5×7? In that case, you would need to scan it HIGHER then 300 DPI (like 500 DPI or so). Otherwise, when you go to enlarge it, you won’t maintain the 300 DPI needed for photo quality. Here’s why:
At 300 DPI a 3×5 picture measures 900×1500 pixels. However, a 5×7 at 300 DPI is 1500×2100 pixels. So, unless you add more pixels to the scan, you aren’t going to have enough to enlarge the 3×5 to a 5×7 and still maintain the 300 DPI resolution.
Now, you can do the math and figure out the exact resolution you need to scan at (in this case 420 DPI), but it’s usually easier just guess at a higher, round number (like 500) then re-size it in your imaging software. It’s complex enough as it is, no need to make it harder!
Oh, your software may be able to “force” the image to stay 300 DPI through some kind of dot manipulation, but it isn’t the same. Avoid making your image higher resolution through your software whenever you can. It’s always better to scan it higher, then reduce it if need be.
Now, depending on your printer, you may be able to get away with less than 300 DPI. I have gone down to 225 DPI and still had a really good print, but I’m using a photo quality printer too.
Finally, this is all theory – you may find that you like the results you get when you scan in a photo at 300 DPI, enlarge it, then print it. Everyone accepts different levels of quality when it comes to this stuff and I know that it’s very subjective (I used to be a photographer a career back or so ago).
PS – If you’re wondering what printer I use, it’s a HP 970 series. When used with the HP premium photo paper, it will print photographs better than most photo labs. Plus, it’s nice and quick when it comes to printing out B&W text.