In this second article in our series on digital photography, we’ll look at digital photos in relation to viewing them on a computer screen, and printing them with your printer.

The size of a displayed photo on a computer screen is dependent on the size of the screen, the resolution of the screen, and the size of the photo.
Computers allow you to select different screen resolutions through a Control Panel in the operating system. Let’s assume you have a 17” screen. If you select a low screen resolution of 640 x 480 pixels, you’ll notice that icons, text, and buttons are quite large and perhaps a little blurry. That’s because the pixels (those little dots of color that make up digital images) actually get bigger, because you’re asking relatively few of them to fill up your 17” screen. As you increase the resolution up to 1024 x 768 pixels, the objects on the screen become smaller and crisper. The pixels actually get smaller, as you are now filling up the same space with almost double the number of pixels. As more pixels are squeezed into the same area, the perceived sharpness of the image increases. Another way to look at it: at a higher resolution for a constant screen size, there are more pixels per inch. Larger computer screens are designed to operate at higher resolutions to maintain a sharp picture. As a rule of thumb, all combinations of size and resolution result in about 72 pixels per inch. A 1600 x 1200 digital photo, at 72 pixels per inch, results in a screen size of around 22 x 17 inches. On a 17” screen at 1024 x 768 pixel resolution, it will obviously require some scrolling to see the whole picture. 72 pixels per inch on a computer screen gives you a great looking picture, even with a photo taken at your camera’s lowest resolution 640 x 480 size. However, printing that 640 x 480 photo is a different story.

Printer resolution is defined in dots per inch (dpi), which is basically the same as pixels per inch. Today’s affordable photo-quality inkjet printers boast resolutions of up to 2400 dpi. To get a good quality photo print, the image needs to have at least 300 dots of color per inch. To determine how big your digital photo will be when printed on an inkjet printer, divide the photo size by the dpi you’re going to print at. That 1600 x 1200 photo, at 300 dpi, will result in a 5.3” x 4” print, about the size of a conventional print from a film camera.

Now look at the 640 x 480 photo. At 300 dpi, the printed photo will be 2” x 1.5”. That 640 x 480 photo that took up half your computer screen will only generate a quality print the size of a large postage stamp – not really suitable for preserving the moment in pictures. If you try to use computer photo software to make it bigger, you will be trying to put the same amount of pixels in a bigger space, making each pixel bigger, resulting in a blurry image. If you print at a lower dpi resolution, you’ll get a bigger print, but lesser quality. As you can see, to print a high quality, 300 dpi, 8” x 10” photo, your original has got to be about 2400 x 3000 pixels.

Again, I recommend always shooting at the highest resolution and lowest compression settings your camera and recording media can handle.

Next time we’ll delve deeper into taking quality photos and using the many available software tools to make them look even better.

© 2003 Peter F. Zimowski