Thursday, May 12, 2005

More Than Just Megapixels

More Than Just Megapixels
Total posts: 5

By Daniel Grotta and Sally Wiener

Pushing the old adage that more is better, most manufacturers market their newest digital cameras by touting how many more megapixels they have than previous or competing models. By this logic, a 4MP digital camera can't possibly be as good as one with 5MP, and an 8MP unit has to be better than a 6MP model. Right?

Wrong. Megapixels are a measure of quantity (the amount of data captured), not quality. A digital camera's image quality is based not on a single component, but on an entire system. True, the heart of the system is the image sensor, with however many megapixels, but that's only part of the equation of how image quality is achieved. Here, we explore some of the other factors that go into producing high-quality digital photos.

More Than Just Megapixels


More pixels do not necessarily make a camera better, but their size is another matter. Pixels on an image sensor are like shallow buckets that catch photons of light, which the sensor then converts into an electrical charge (electrons). The bigger and deeper the pixels, the more photons they can capture, and the greater the pixel's ability to record detail in the shadows and highlights (dynamic range).

Most digital cameras use either CCD (charge-coupled device) or CMOS (complementary metal oxide semiconductor) image sensors. Larger sensors generally produce greater dynamic range, higher sensitivity, and better signal-to-noise ratio, primarily because they have room for bigger, more light-sensitive pixels. For example, among 5-megapixel models, the Olympus E-1's CCD is approximately 4.2 times as large as the Leica Digilux 2's CCD, and 6.3 times as big as the HP Photosmart 945's. This allows for correspondingly sized pixels: the Olympus's pixels are 6.8 microns wide, while the Leica's are 3.4 microns and the HP's are 2.8 microns. (For information on an unusually designed image sensor, see,1759,1421879,00.asp.)


Figure 1
A camera's lens is just as important to image quality as its image sensor. Depending on their design, lenses may have different resolving power and varied contrast and color characteristics. Generally, high-speed lenses (maximum apertures of f2 or larger) are better for available-light photography, but tend to lose more data near the picture's edge than slower lenses (f2.8 or below) when shot wide open.

Modern zoom lenses are every bit as good as fixed-focal-length lenses, but pictures shot at wide angles (28mm or 24mm equivalencies) have slightly more barrel distortion (where straight lines bow outward from the image center), while telephoto pictures (135mm-280mm) tend to produce the opposite, pincushion distortion. Extreme zooms (6X or greater) are heavier and harder to hold steady, which is why some better models come equipped with antishake technology.

The first digital-camera lens designs were taken from camcorders or film cameras. Some inexpensive models had plastic lenses, which frequently distorted colors and reduced the clarity and quality of light. Even the better models, which used optical glass, often produced vignetting (underexposure of image edges and corners), because light passed through the lens at oblique angles at the edges of the image sensor. (Light must strike image-sensor pixels head-on for the photons to be collected properly.)

Eventually, optical-glass manufacturers began building lenses specially designed for digital cameras. They made use of aspherical elements, low-dispersion glass, exotic color coatings, and other innovations. Some companies formed strategic alliances with distinguished lens manufacturers—Kodak with Schneider, Panasonic with Leica, and Sony with Zeiss—to add prestige and enhance lens quality. Cameras with brand-name lenses (or from camera companies with a history of superior lenses) almost always rate higher in our image quality tests.

With lenses, cleanliness is almost as critical as quality. Dust, fingerprints, or other surface impurities can significantly reduce image definition and integrity. Stray light can cause flare, chromatic aberrations, and other problems, so a proper lens shade is important.


Though it might come as a surprise, all cameras—film, video, and digital—capture analog, not digital images. The CCD converts each pixel's charge into an analog signal of varying voltage, and then an internal analog-to-digital converter (ADC) changes that signal into digital data (bytes). The better (and more expensive) the ADC, and the higher its bit rate (16-bit or 12-bit, as opposed to 8-bit or 10-bit), the smoother the transitions and the more details in highlight and shadow areas. While you can't (and shouldn't) select your camera based upon the type and bit depth of its ADC, cameras offering higher bit depths may produce richer images. (For more about the inner workings of image sensors, see,1583,a=2036,00.asp.)

File Format

How images are saved can directly affect picture quality. Most users prefer the JPEG file format, which is often the only format offered on consumer digital cameras. A good compromise among speed, image quality, and space savings, JPEG is a compressed format that actually throws away image data to shrink the file to a smaller size. Most digital cameras offer two or three levels of compression. Some inexpensive models typically offer 8:1 to 25:1 compression levels, while better cameras' compression levels range from 2.7:1 to 8:1. Cameras with lower compression ratios tend to have cleaner images.

After testing a number of digital cameras with the ability to save RAW files, we now shoot almost everything in RAW format when image quality is the most important factor. RAW saves all related image data with no in-camera processing, allowing photographers great latitude in how they want their pictures to appear.

The Secret Sauce

Figure 2
Some digital cameras score very well on all our low-level tests, indicating that the lens, image sensor, ADC, and other components are quite good, yet their photos aren't up to our expectations. The key factor is how the manufacturer's color and imaging scientists program the camera to interpret the data that defines the photo. These proprietary algorithms help determine how—and how good—the picture will look. This explains why cameras with the same lens and image sensor (which are often sold as part of a kit) may produce images that look quite different.

If you like the pictures that a camera produces, it's probably because you like the choices the designers made, and those choices will likely be made in most cameras from that manufacturer. When you find a camera brand whose images you like, you will probably do well to stay with it.

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