All about Megapixels

TIP OF THE DAY

All about Megapixels

A Shutterbug's Guide to Meting Out the Megapixels

By IVAN BERGER

Published: October 30, 2003

THE more megapixels the merrier - or so you'd gather from digital camera prices. The higher the number of megapixels, the more expensive the camera will be in comparison with others with similar features. But what are megapixels? How many do you need? Is more always better?

Pixels (known as megapixels when you count them by the million) are picture elements, the tiny spots of data that make up a digital image. All photos are made up of tiny elements: from the ink dots in newspaper photos to the grains of silver or particles of color dye in film photography. A good photo usually has millions of these elements. The more there are, the sharper and more detailed the picture is, and the harder it is to distinguish the elements from one another.

A digital camera's effective pixel count is its horizontal resolution multiplied by its vertical resolution. An image 2,048 pixels across and 1,536 high has just over 3.1 megapixels; a 2,560 x 1,920 image is just over 4.9 megapixels.

Digital cameras made for amateurs usually have between two and five megapixels, though cameras with eight megapixels or more will be available soon.

Those numbers are no match for 35-millimeter film, which has a resolution equivalent to 20 or 30 megapixels, but digital cameras can nonetheless produce excellent images.

How many megapixels you will need depends on how you plan to use your pictures. For e-mail, an image size of 640 by 480 pixels (0.3 megapixel) is usually best: large enough to look sharp on a computer screen but small enough to upload or download quickly. For prints, more resolution is required, and the bigger the print, the greater the difference the pixel count makes.

For prints measuring up to 8 by 10 inches, the difference between shots with two megapixels and five megapixels can be hard to discern. This was not always true, but current digital cameras do a better job of processing the raw data from their image sensors into image files on their memory cards. ("Most people will never, ever need something above three megapixels," said Jon Sienkiewicz, the vice president for marketing at Minolta. "I'll make 8-by-10's all day long from that.")

Cropping and Zooming

In prints larger than 8 by 10 inches, differences in pixel counts become more noticeable. Few amateurs make prints that big, but another reason to go for a higher pixel count is the ability to crop. Pictures that looked good when you shot them may contain distracting elements; cropping allows you to prune those elements away and make the picture stronger. Crop out 40 percent of your picture, though, and you lose 40 percent of its pixels. That might be a worthwhile tradeoff if it reduces a five-megapixel image to three megapixels, but not so if the image goes from two megapixels to a paltry 1.2.

It is also possible to crop within the camera, zeroing in on an important subject area so that it fills as much of the frame as you want. A zoom lens does this by narrowing its view to exclude some subject areas while magnifying whatever is left within the frame. The picture area contains just as many pixels as before, but with more of them now devoted to the subject area you want, its details are clearer. Don't confuse this process, optical zoom, with so-called digital zoom, a purely electronic process that selects a small subject area by throwing away the surrounding pixels: the pixel count of the area you select with digital zoom is the same as before, so you don't gain anything but a tighter composition, and the picture may look fuzzier. It's like cropping your picture in your computer, only with less time to select your composition and no chance to change your cropping if you don't like the result.

"It's not as valuable for cameras that have zoom lenses as it is for entry-level cameras that don't," said Chuck Westfall, director of technical information for camera products at Canon, referring to digital zoom. Entry-level cameras are also, alas, likely to have lower pixel counts to start with. On the other hand, such cameras are mainly used for snapshots, "and for snapshot-sized photos, sometimes digital zoom isn't too bad," said Sally Smith Clemens, a product manager at Olympus.

Whether you crop in your computer or with your camera's digital zoom, the quality of your results will depend on how many pixels your camera used to make the image and how much of that image you crop away.

Memory and Formats

The downside of pixel-rich pictures is the way they fill a camera's memory. Most cameras come with a skimpy 16-megabyte memory card, enough to hold a handful of two-megapixel images but perhaps only one five-megapixel image. A larger memory card is a good investment, but it can be costly: Depending on the type, a 64-megabyte card can cost $20 to $40, a 128-megabyte card costs $30 to $60, and a 256-megabyte card costs $50 to $150.

Large images can fill even a big memory card quickly - for example, a 128-megabyte card might hold as few as eight five-megapixel images. For that reason, cameras give you a choice of resolution settings, letting you use a five-megapixel camera's full resolution for important shots, but throttle it back to three or two megapixels for snapshots. The camera's display will then tell you how many shots will fit on the remaining memory.

Digital cameras offer a choice of file formats; your choice affects the number and quality of images a card can hold. Just about all digital cameras can save images as JPEG (or JPG) files, which compress the image data to save memory space but lose some picture quality in the process. (The format is named after the Joint Photographic Experts Group, which devised it.) Both the memory savings and quality loss vary depending on the amount of compression, so most cameras offer a choice of compression levels, typically described as small, medium and large, or fine, standard and economy. A 16-megabyte memory card, for example, might hold 9 three-megapixel images in Fine mode, 17 in Standard and 32 in Economy.

Saving an image as a JPEG file has surprisingly little effect on its sharpness and detail. But once a JPEG image has been modified by cropping, resizing, sharpening, lightening, darkening or altering its color balance, it should not be saved as a JPEG again. Doing so would compress the file further, and data and picture details would be discarded. Instead of using the Save command, use Save As, and store the picture in an uncompressed format, like TIFF (Tagged Image File Format), which preserves all remaining details.

Many cameras allow you save images as TIFF files as you shoot. These give you maximum image quality but take up a lot of space (about 14 megabytes for a five-megapixel image in TIFF form, versus about 2 megabytes or less for a JPEG) and take longer to store on the memory card (a problem if you're shooting action).

Some cameras also offer a RAW format, which contains all the details the image sensor picked up, unaffected by the camera's settings for white balance, exposure compensation and other factors. These files require a bit more work but give you more creative control over the result. For example, in JPEG's, "overexposed highlights are just lost," Mr. Westfall of Canon said. "In RAW, you have a bit of wiggle room in those areas, and even more capability for bringing out detail in the shadow areas." Unlike TIFF, RAW is a nonstandard file format, differing from one make of camera to another. To edit RAW files, you may need the editing software supplied with the camera, or plug-ins for programs like Adobe Photoshop.

Experts suggest buying a camera with the highest megapixel count you can afford, and saving your photos as JPEG files unless you plan to edit them. That will cut the time your camera spends storing one image before shooting the next, and will leave room for more shots on its memory card. A high megapixel count and compressed files will help ensure that you'll be ready when you see something worth shooting. If you start to run out of memory before you run out of picture opportunities, using fewer pixels or a coarser JPEG setting will let you slip a few more pictures in. Better to get the shot but lose a little quality than to miss the shot entirely.

"It's not about pixels alone," said Ms. Clemens of Olympus. "It's about pictures."