Anyone who has ever agonized over whether to buy a 3-megapixel or 4-megapixel digital camera might be surprised to learn that Spirit's stunningly detailed images of Mars are made with a 1-megapixel model, a palm-sized 9-ounce marvel that would be coveted in any geek's shirt pocket.

Intuitively, more pixels means higher resolution. That's generally true on a display screen. But when capturing images, where a pixel is more properly called a sensor, the count is just one of many factors that control quality.

The technology used to make Spirit's Panoramic Camera, or Pancam, is essentially the same as what goes into a Casio or Pentax digital camera.

But the Pancam's lenses -- there are two, which provides stereo imaging capability -- are crafted more finely than anything you'd probably want to plunk down a Visa for. And the light-capturing chunk of silicon, called a charged coupled device, or CCD, was manufactured with no tolerance for the minor flaws that are inherent in mass-produced consumer cameras.

Perhaps most important, the sensors on Spirit's CCDs are bigger, explained Patrick Myles, director of corporate communication at the Dalsa Corporation, which built the CCDs for all of the rover's cameras (Spirit has nine altogether, including hazard avoidance cameras and a microscopic imager).

A Sony DSC-F717, with a street price of around $600, has 5.2 million sensors (or 5 megapixels) on a chip that is 8.8 by 6.6 millimeters (or .35 by .26 inches). The Pancam has just a million sensors spread across a chip that's 12 by 12 millimeters -- nearly a half-inch square.

Each tiny Pancam sensor, measured in microns, is nearly four times as big as those on the Sony.

In the consumer market, which Dalsa does not target, 5-megapixel cameras often use the same size CCD as a 3-megapixel camera. More pixels are simply crammed onto the same-size chip.

"The pixels themselves get smaller," Myles said. "This has an impact on image quality."

Why? For one thing, smaller pixels are less light-sensitive.

Also, the lens quality might not support the additional pixels. As the receptors get smaller, a higher quality lens is needed to properly focus light onto each pixel. So where each pixel ought to capture different light information -- say perhaps a subtle shading change on the subject's cheek -- the same information can get spread across several pixels after passing through a lower quality lens.

The Pancam was conceived at Cornell University and built at NASA's Jet Propulsion Laboratory. Dalsa, based in Waterloo, Ontario, Canada, makes cinema-quality video components and other high-end imaging devices and was called on to make the CCDs for the Pancam and the other cameras on Spirit and its twin rover, Opportunity.

"They are the world's highest performing chips in terms of light sensitivity and chip quality," Myles said in a telephone interview earlier this week.

Overall, how does a Pancam stack up to the typical 5-megapixel camera you might purchase at Best Buy?

"There really isn't any comparison," Myles said.

Experts argue endlessly about what the human eye can actually see, however. Comparing human vision to what a camera captures "is really up to great speculation," Myles said.

NASA's analogy, Myles explained, is "probably a bit of marketing spin. It helps people visualize the quality." The height and breadth of a Pancam image is roughly equal to what a person would see, taking into account peripheral vision. And the Pancam has a human perspective. It sits atop a mast on the rover, 5 feet (1.4 meters) above the surface.

In the most basic application of this process, three images are gathered of a scene, one each recording red, green and blue light. Those are then put together with special software to create a color picture.

A consumer digital camera uses a single coated filter to make the transition from photon reality to electrons and then digital information.

Spirit's pictures are said to be three times sharper than those of the 1997 Mars Pathfinder mission or the 1970s Viking landers.

The Viking missions, as well as the Voyager missions to the outer planets, used technology similar to antiquated television vacuum tubes. CCD technology was first developed in 1969, but it took decades before arrays were big enough to be useful.

Much of the research that ultimately led to today's commercial digital cameras was funded by NASA. A first major step was in developing an 800 by 800 pixel array -- less than a megapixel -- which is what's in the Hubble telescope.

The Pancam results so far have mission managers ecstatic. Cornell astronomer James Bell, who led the development of the camera, called the first Spirit pictures "absolutely spectacular."

Nobody has argued with him.