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[hmm, i ‘ll have to break this post up. blurb tells me: "Not so fast, Tiger. You need to use at least 5 characters to make a post." lol] Until you output an image to a hardcopy device (like a printer), the DPI is an irrelevant number. Nothing more than a tag in the file header that tells you what DPI is associated w/ the image file.
A 3000×3000 image is going to contain 9,000,000 pixels of resolution detail, no more, no less, no matter what DPI is associated with the image. Once you want print to your with home color printer, in a program like photoshop, then your DPI setting will matter. At 300 DPI, your printer will pack 300 pixels of your image data into 1 inch on paper. At 72 DPI, 72 pixels in 1 inch will print on your paper. Some simple math will get you your final printed image dimensions – 3000 pixels / 300 dpi = 10 inches or 3000/72 = 41 2/3 inches respectively. But note that this is completely independent of your output device’s DPI capability. My epson printer claims to print color at 720 DPI and 1440 DPI horizontally (on their top quality paper only of course <sup>_</sup> ). So if you could actually see the 720 discrete little inkjet ink dots in a row on the print out, you would see that every 2 or 3 dots would be the same color – assuming there’s no dithering or interpolation going on in order to better approximate what you see on the screen. In essence your 300 pixels per inch would be mapped to the 720 dots per inch of your printer. Printing your 300 dpi image at 1440 dpi could be overkill given that you’re not going to see things in your printed image that were not in the original photo just because of a higher output dpi. Your image may look smoother because of the finer ink dots, if you’re using the right paper. Printing a 72 DPI image on a 720 DPI printer, each pixel from your image will be represented by 10 ink "dots" from your printer. ( Depending on your printer driver, the printer may do quite a bit of "smoothing" with noise and dithering to create the illusion of gradation if you print at a really low dpi. )
Anyway, the same concepts apply to scanning in images into the digital world. Do a little math and you’ll find that scanning a 6×4" photograph at 300 DPI on your flatbed scanner and you should be able to obtain a 1800×1200 pixel image (300dpi x 6", 300dpi x 4"). Is this high enough resolution? Depends on what you are doing with it. If your final output is for the web, this may be just a little large for flickr.. but if you’re printing the image back out at 300 dpi or less it could be just right. But I have a lot of old images from my parents’ childhood and those black and whites often measure 1.5" x 2". If I scan them in at 300 dpi that only gives me 450×500 px of resolution. Not good enough if I’d like to print them back out larger at 300 dpi. If I know my output device is 300 DPI, and I want my final photo to measure 6 inches across in the print, ideally I would like my ditigal image to contain 1800×2400 pixels of resolution (6" x 300dpi, 8" x 300dpi). That would mean scanning in the source image of 1.5"x2" at 1200 DPI ( 1800px/1.5", 2400px/2" ). Not all consumer level scanners can handle that resolution, and for 1200 DPI many scanners may interpolate the incoming data from a lower optical DPI (often 600 DPI optically). Also be aware that your source material may not hold up to that resolution. For example, if you try to scan an art or photo book at 720 DPI and higher you will begin to see the individual halftone dots. Also large physical images scanned at high DPI will create files that can be large and unwieldy without gaining you very much in the end. So the input DPI at which you scan should always be determined by the size of the original and what you intend to do with the final digital file. Some simple math and you should have a good idea of the resolution you’ll need to scan at.
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