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Megapixel Revolution  - or

Differences Between Screen Resolution and Print Resolution

category: digital photography and image editing

Digital cameras are everywhere, and, with six plus megapixel cameras priced for the consumer market, a lot of people out there are learning to edit images and wondering what these huge images are all about. In a nutshell, these huge images are about print detail, the kind of detail that produces a printed image approaching film print quality. The operative idea to consider is print resolution. On screen presentation of images, on the other hand, is concerned with display resolution. Display resolution, size expressed as pixel dimensions, is entirely different than print resolution.

Display resolution is simply the height and width of your image when viewed on the computer screen at 100% of actual size. If your monitor is set to a resolution of 800 by 600, the screen is formatted 800 pixels wide and 600 pixels high. If an image is sized at 400 pixels wide by 300 pixels high, its resolution is 400 by 300, and it will fill exactly half the screen. Display resolution is the only thing that counts in digital presentation of images.

Print resolution, on the other hand, is concerned with defining the desired size of the printed image and the number of dots per inch that the printer will print in reproducing the image. Display resolution, or size expressed in pixel dimensions, comes into play only when computing the maximum number of dots per inch available to the printer at one pixel per dot. The greater the number of pixels available to produce a given size of printed image, the greater the number of dots per inch, at one pixel per dot, the printer can use to define the detail within a given size print.

This is not rocket science, but it seems like it is when you're looking at the Photoshop Image Size dialogue box. Let me see, you may muse, if my image is 400 by 300 pixels, but the resolution is 600 ppi (pixels per inch), will it take longer to download from a web page than the same image at a resolution of 72 ppi? No, but if you print your 600 ppi image, you will be lucky to find it on the paper. Pixels per inch in the Photoshop Image Size dialogue box means the same as dpi (dots per inch) on a printer or a scanner.

If you scan a 5 by 7 inch print at 600 dpi, and open it in Photoshop, your image will be 4443 pixels wide and 3129 pixels high; if you print it, you will get a 5 by 7 inch print, but it will look like a 5 by 7 inch print on screen only when viewed at 12.5% of actual size. OK, you say, since I only need 300 ppi for my printer, I changed the resolution in the Image Size dialogue box and the pixel dimensions halved to 2222 ppi by 1565, but I can still print a 5 by 7 inch print. What gives? Ah, this is the next major source of confusion after ppi vs dpi, the Resample Image check box. You see, Photoshop is your friend. As long as the Resample Image check box is checked, Photoshop believes you're resizing your image and is trying to help you by extrapolating all the other image size numbers based on whatever number you change. Handy only if you know that resample image means physically resize the image file along with its output document.

If you uncheck the Resample Image check box, the physical dimensions of the image in pixels greys out, and you're left with numbers in the Document Size section which means you can change the printed output document only.

He said on 03.07.05 @ 01:57 PM CST