Demystifying Digital Camera Resolution

For many people, image resolution is a very perplexing part of digital photography. Some of the confusion stems from the different meanings that resolution has for cameras, printers, monitors, and scanners. Adding to the problem, a lot of incorrect information has been spread around over the years — you've probably gotten bad advice yourself from well-meaning but misinformed friends and salespeople.

The next few sections give you the straight scoop, starting with the element that's at the heart of the resolution equation, the pixel.

What's a pixel?

Your digital camera builds pictures out of tiny blocks of color known as pixels, similar to the way an artist creates a mosaic using colored tiles. (Pixel is short for picture element, if you care to know.)

Unless you greatly enlarge a digital photo, as in the top of Figure 1, you can't distinguish individual pixels. Instead, your eye perceives the grid of pixels as a continuous image, just as when you view a mosaic from a distance. (The close-up in Figure 1 shows the butterfly's head.)

If you want to inspect the pixels in one of your own images, open the picture in your photo editor and use the program's Zoom tool or View menu commands to magnify the display, as shown in Figure 2. This figure features Adobe Photoshop Elements 2. In this program, as in most, the Zoom tool looks like a little magnifying glass. Just click the tool icon in the toolbox and then click on your image to magnify the display.

Figure 2: To get a close-up look at pixels, zoom in on a picture in your photo editor.

Why pixel count matters

The term image resolution refers to the number of pixels in a digital photo.

Image resolution is sometimes stated in terms of pixel dimensions — the number of pixels tall by the number of pixels wide. Other times, resolution is expressed as the total number of pixels.

For example, one camera maker may describe a camera as offering a top resolution of 1280 x 960 pixels, and another manufacturer may characterize the same resolution as 1.2 megapixels. (A megapixel is 1 million pixels.) Both approaches are valid; they're just different ways of saying the same thing.

Whichever terminology you use, more pixels means larger picture files because the camera must generate a specific amount of data to describe each pixel. Aside from file size, however, pixel count has a different impact depending on whether you print the photo or display it on-screen, as explained in the next two sections.

For top-notch prints, capture pixels aplenty

For print photos, an adequate pixel supply is crucial to good picture quality. A print from a high-resolution image rivals anything you can produce from a film camera, as illustrated by the left photo in Figure 3. But a print from a low-resolution image looks awful, as shown by the right photo. Curved and diagonal lines have a jagged, stair-stepped appearance, and fine details and subtle color transitions are lost — all a result of too few pixels.

Figure 3: A high-resolution image produces excellent print quality (left); an image with too few pixels creates an unacceptable print (right).

How many image pixels you need depends on the size of the print you want to make. As a rule, you need a minimum of 200 pixels for each linear inch of the print. For example, a good 4-x-6-inch print requires 800 pixels horizontally by 1200 pixels vertically, or a total pixel count of 960,000, which is just shy of 1 megapixel.

Pixels per inch is abbreviated as ppi.

With some printers, however, you may get better results with 300 pixels per inch. Check your printer manual for information about the optimum image resolution, or just run your own tests to determine the minimum pixel dimensions that produce acceptable prints on your printer. If you're printing your pictures at a lab, ask the service technician for guidance.

Table 1 offers a general guide to the pixel population needed to print a digital photo at various sizes at a resolution of 200 ppi and 300 ppi. Again, to convert pixel dimensions to megapixels, just multiply the number of horizontal pixels by the number of vertical pixels.

Table 1: Pixels to Prints — Resolution Guidelines

Because the aspect ratio (width relative to height) of a digital photo is 4:3, which is different from the traditional frame sizes shown in Table 1, your camera probably doesn't offer the specific pixel dimensions shown in the table. As long as you set your camera to a pixel count that's equal to or higher than the numbers shown here, you'll be set up for good print quality.

If you're not sure at the time you take a picture what size print you will want to make — which is the case for most of us — you're better off overshooting the pixel mark. You can always eliminate excess pixels, but you can't add them after the fact to gain improved print quality.

For screen and e-mail images, capture fewer pixels

When you want to display your picture on-screen, whether on a television or computer monitor, you need far fewer pixels than you do when printing your photos.

For screen images, pixel count affects the size at which an image displays, but not the image quality. For a quick preview, see Figure 4. The figure shows an e-mail message that has a picture of this author's sprawling Indiana estate attached. The author shot the picture at a resolution of 640 x 480 pixels, which is the lowest-resolution setting on many digital cameras. As you can see, even at this minimal pixel count, the picture is too large to fit entirely on the screen. You can see just the rooftop of the house.

By contrast, check out Figure 5. Before attaching the house picture to this message, the pixel count was cut in half, to 320 x 240 pixels. That size is good for e-mail pictures because most people will be able to see the entire image without scrolling the display, as shown here. Notice, too, that the photo quality isn't affected by the pixel count — just the size at which the picture appears.

Display size isn't the only reason to limit the pixel population of your e-mail pictures. As reported earlier in this broadcast, each pixel adds to the size of the image file, and a larger file takes longer to transmit over the Internet. This caution applies to pictures that you want to use on a Web page, too.

If you want to print your picture as well as share it online, choose the resolution setting appropriate for printing. After downloading the image to your computer, make a copy of the picture file to use for screen purposes. Some digital cameras offer a feature that creates an e-mail copy of your image for you — check your manual to find out whether your model offers this option.