Pixels (part 2)

With the information in my blog post entitled Pixels (part 1) [how-many-pixels-part-1], you may conclude that 8x10 inch prints call for a 5 megapixel camera. That's generally true, but not entirely. It is possible to use computer software to artificially increase the number of pixels. Different software programs may perform differently. The general rule of thumb is you can double the number of pixels. So, you might print very good 8x10 prints though your camera only records 3 megapixels.

Maybe you just bought a new DSLR that records images up to 15 megapixels. Those 15 megapixel images may require three times more storage than 5 megapixel images. If you shoot
some pictures on Tuesday, intended for 11x14 prints, set your camera to record large size. If you shoot some pictures on Wedensday, intended for computer screens only, set your camera to record small size.

Pixels get a little weird when you move to television display. Pixels on a computer are square. A digital image that is 400x400 pixels will display square on your computer monitor. Not so on a television screen. Television pixels (defined by either NTSC or PAL standards) are not square.

Consider an picture aspect ratio of 4:3, meaning the picture is 25% wider than it is tall. Standard television (not HD) has a 4:3 picture aspect ratio. Yet the resolution is 720x480 pixels, which numerically seems to be 50% wider than it is tall. But rest assured it is 4:3 because each television pixel is taller than it is wide. (Take a very close look at a television and you can see this is so. It is more difficult to see on a HD television.) While pixels in a computer or in your camera are equally wide as they are tall (often called square pixels), pixels in television are taller than they are wide.

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.