Pixels (part 1)

Perhaps the most common question regarding digital photography is ... how many megapixels do I need? The answer depends upon how you intend to use the photo. You need to answer two questions: how large will the photo appear (inches or centimeters) and how many dots per inch (or centimeter).

If you want to share it on a computer screen, one megapixel is more than enough. Why? Computer displays typically have either 72 pixels per inch or 96 dots per inch. So, for example, to display a photo at a size of 6 by 9 inches, you need (6 x 96) x (9 x 96) = 497,664 pixels = 0.497 megapixels.

Computer screen: 72 - 96 dots (pixels) per inch
Photo print: 240 - 300 dots per inch
Magazine: 100 - 200 dots per inch
Poster or banner: 100 - 150 dots per inch
Billboard: 10 - 20 dots per inch

Magazines print fewer dots per inch. If you look real close, you may see the individual dots, but maybe not. It depends upon the exact printing equipment and the tendency of ink dots to blend together. Billboards, viewed from far way, use very low resolution simply because it is not apparent from far away.

So, here is another example. For a photo print 6 x 9 inches, you need (6 x 240) x (9 x 240) = 3,110,400 pixels = 3 megapixels. More pixels in your camera means that you can get larger prints without sacrificing quality. Using the same math, you can easily see that a 20 x 24 inch print needs 27 megapixels.

You may ask: 240 dots per inch? My printer supports 1000 dots per inch.
OK, here is the bottom line. If you print at the higher resolution, can you see a difference in the final print? You might see a tiny difference, but 240 dpi usually provides excellent quality and sharpness. If you are using a photo lab to make your prints, consult their guidelines, but 240dpi or 250 dpi is very common.